Saturday, 01 December 2018
Presenter:
Jason Gardner (ANSTO, Australia)
Authors:
 Jason Gardner (ANSTO, Australia)
 Dominic Ryan (McGill University)
 Rajib Sarkar (Technische Universität Dresden)
 Tilo Soehnel (University of Auckland)
 Chris Ling (University of Sydney)
Neutron Scattering, (117/119)Sn nuclear magnetic resonance (NMR) and muon spin relaxation data have been used to probe the dynamics of the kagome magnet, Fe_{4}Si_{2}Sn_{7}O_{16}. The longrange spatial correlations and phase transitions were reported earlier from neutron diffraction, Mössbauer spectroscopy and heat capacity measurements [1], these will be reviewed before the new studies of the dynamics and the likelihood of a second transition will be discussed. The only magnetic ions in this compound are layers of highspin Fe^{2+}(d^{6}, S=2) arranged on a perfectly hexagonal kagome lattice. Below
T_{N}=3.5 K, the spins partially order into canted antiferromagnetic chains, separated by paramagnetic spins on the geometrically frustrated lattice. A completely frozen state has not been observed down to the lowest temperatures, although some signatures of a low temperature transition have been observed.
Motivated by the recent findings in the Ba_{3}CuSb_{2}O_{9} system, we have investigated Sr_{3}CuM_{2}O_{9} [M= Sb, Nb] for possible spin liquid behaviour. Due to the different ionic size of Sr compared to Ba, the Srbased systems crystallize in a different structure that the Basystem. The bulk susceptibility of the Srbased compounds shows a CurieWeiss behaviour with a large, AF Θ_{CW} but no sign of ordering down to 1.8 K. Our specific heat data show no anomalies down to 0.3 K and a power law behaviour of the magnetic heat capacity is seen. ^{121}Sb and ^{93}Nb NMR data also do not show any evidence of LRO. These data point towards quantum spin liquid (QSL) behaviour.
The system Y_{2}CuTiO_{6} contains edgeshared triangular planes where the vertices are occupied by magnetic Cu and nonmagnetic Ti atoms in equal proportion. In spite of the large dilution of the triangular magnetic lattice, the magnetic susceptibility shows a large, AF Θ_{CW} (about 240 K) without any sign of ordering down to 1.8 K. The magnetisation in a low field (50 Oe) shows no bifurcation in ZFC/FC curves down to 0.5 K. Likewise, the heat capacity shows no anomalies and the magnetic contribution has a power law behaviour at lowtemperatures. Our muSR measurements down to 50 mK also do not show any signs of ordering. Our ^{89}Y NMR results also do not show any signs ordering. These observations are typical of potential quantum spin liquid systems. In the present case, however, the magnetic heat capacity C_{m} depends on the magnetic field H. We observed that the data could be scaled such that H^{γ}C_{m}/T vs T/H follow a universal curve with γ = 0.6. Such a scaling behaviour has been suggested in systems with random singlet formation. While the randomness of the exchange coupling has been suggested in YbMgGaO_{4} due to Mg/Ga site disorder (with no depletion of the magnetic lattice), in the present case it is surprising that in spite of the large (50%) depletion of the lattice, no spin freezing is observed.
I will present the results of our bulk and local probe measurements on the above systems.
Spin liquids, despite their apparently featureless ground states, are exotic magnetic states which host fractionalised excitations and emergent gauge fields. Interestingly, quenched disorder can nucleate defects with unusual properties and thus reveal the hidden collective excitations of such states. In this talk, I will explain how disorder affects the physics of a prototypical frustrated magnet, dipolar spin ice, both at high and low temperatures and in fact leads to a new phase at low temperature, a "topological spin glass”, which shows signatures of both spin liquidity and glassiness. I will also describe a new cluster Monte Carlo algorithm that allows us to study the continuous phase transition to the topological spin glass and reliably extract the critical temperature and exponents.
Ching Hao Chang^{1} and R. Ganesh^{2}
^{1}Department of Physics, National Cheng Kung University, Taiwan
^{2}The Institute of Mathematical Sciences, HBNI, C I T Campus, Chennai, India
Transition metal oxides are well known for their complex magnetic and electrical properties. When brought together in heterostructure geometries, they show particular promise for spintronics and colossal magnetoresistance applications. In this article, we propose a carrierdriven coupling mechanism in heterostructures composed of itinerant ferromagnetic materials. The coupling is mediated by charge carriers that strive to maximally delocalize through the heterostructure to gain kinetic energy. In doing so, they force a ferromagnetic or antiferromagnetic coupling between the constituent layers. To illustrate this, we focus on heterostructures composed of SrRuO_{3} and La_{1−x}A_{x}MnO_{3} (A = Ca/Sr). Using the minoritycarrier nature of SrRuO_{3}, we provide a simple explanation for antiferromagnetic alignment that is known to occur in multilayers. We present a phenomenological Kondolattice model which reproduces the known magnetization properties of multilayers. In addition, we discuss a quantum well model for heterostructures and argue that the spindependent density of states determines the nature of the coupling. As a smoking gun signature, we propose that bilayers with the same constituents will oscillate between ferromagnetic and antiferromagnetic coupling upon tuning the relative thicknesses of the layers. We present abinitio results that support this prediction.
Reference:
ChingHao Chang et al., Phys. Rev. B 96, 184408 (2017).
Materials always contain some amount of disorder (randomness) or impurities, which can significantly alter their properties. While in some cases it is desirable to minimize the amount of disorder, in other cases disorder can be important to achieve certain material properties. Here we discuss a class of insulating quantum magnets in which disorder can induce a completely new kind of state, the properties of which were previously not known. We do this by largescale computer simulations of a model system designed to generate the state in question  the random singlet state  while at the same time being amenable to the most efficient simulation algorithms. Our results show that, upon introduction of disorder, an antiferromagnet can undergo a phase transition at a critical point into the RS state, provided that there are some interactions in the system that favor correlation of singlet pairs (dimerization). The RS state, which is likely realized in many materials that have so far been classified as 'disordered spin liquids', exhibit powerlaw scaling properties of experimentally measurable properties.
The honeycomb lattice iridates A_{2}IrO_{3} (A = Na, Li) were proposed as candidates for the realization of the Kitaev Heisenberg model with hopes of stabilizing Kitaev’s quantum SpinLiquid (QSL). However, both materials were experimentally found to have magnetically ordered ground states. We report single crystal growth of a new layered honeycomb lattice iridate K_{2}IrO_{3} with a different interlayer stacking sequence compared to Na_{2}IrO_{3} and $\alpha$Li_{2}IrO_{3}. From magnetic susceptibility χ versus temperature T data we find $S_{eff} = 1/2$ moments interacting strongly with a Weiss temperature $\theta ≈ 200$~K and no sign of magnetic order or spin freezing down to $T = 1.8$~K. Heat capacity data show a broad maximum around 30 K which is insensitive to magnetic fields. These behaviours are consistent with expectations for a Quantum Spin Liquid.
Several novel quantum spin states will be presented for highly frustrated quantum spin systems such as triangular and honycome lattices. The spinorbital interaction is also considered. In particular，possible spin liquid states will be also discussed for those systems.
We discuss the multipolar ordering in Pr(TM)2(Al,Zn)20 Kondo materials. In Pr ion site, interplay of spin orbit coupling and crystal field splitting leads to non Kramers doublet as a ground state, where magnetic dipole degrees of freedom is absent but quadrupolar and octupolar degrees of freedom exist. Focusing on such unique ground state, we discuss physical properties of multipolar ordering and their phase transitions. We also discuss emergence of topological superconductivity in this cage compounds and their relevance to multipolar ordering.
Sunday, 02 December 2018
Geometrical frustration in the kagome lattice leads to localization of magnetic excitations into resonating hexagons. Recent numerical studies [1,2] of the spin1/2 kagomelattice antiferromagnet in an applied field demonstrated that these easily localized magnons can induce various magnon crystal phases, which result in the appearance of many plateau structures in the magnetization process. We studied how these plateau states melt into nonplateau phases in the kagome antiferromagnet with varying a magnetic field [3]. We examined instabilities of the plateau phases by means of degenerate perturbation theory, and found some emergent supersolid phases below the m=5/9 plateau. In the supersolid states the pattern of resonating hexagons are preserved from the plateau crystal state and the originally polarized spins outside the hexagons dominantly join the superfluid component, or equivalently, participate in the magnetization process.
[1] S. Nishimoto, N. Shibata, and C. Hotta, Nat. Commun. 4, 2287 (2013). [2] S. Capponi, O. Derzhko, A. Honecker, A. M. Läuchli, and J. Richter, Phys. Rev. B 88, 144416 (2013). [3] X. Plat, T. Momoi, and C. Hotta, Phys. Rev. B 98, 014415 (2018).
Quantum effect is expected to dictate the behaviour of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy (TEE) and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z_2 topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of offdiagonal and diagonal quantum processes leading to the quasidegeneracy of states and effectively, the classical degeneracy is restored.
Extracting intrinsic magnetic susceptibility and understanding the origin of complex magnetic ordering set an attractive venue in correlated quantum matter. Nuclear Magnetic Resonance (NMR) is a site selective microscopic technique and is extremely sensitive to low energy spin dynamics defining the ground state properties of quantum materials. NMR spectroscopy offers a unique setting in mapping the tomography of materials under study. NMR enables us to probe topological order and elucidate the nature of enigmatic elementary excitations in quantum magnets by adopting a “perturb to reveal” approach.
ChaoHung Du^{1,2}, K. C Rule^{3}, C.W. Wang^{4}, S. Yano^{4}, F.C. Chou^{5}
^{1}Department of Physics, Tamkang University, Taiwan
^{2}Research Center for Xray Science, Tamkang University, Taiwan
^{3}Australian Center for Neutron Scattering, ANSTO, Australia
^{4}Neutron Group, National Synchrotron Radiation Research Center, Taiwan
^{5}Center for Condensed Matter Sciences, National Taiwan University, Taiwan
The doubleperovskites exhibit many fundamentally interesting chemical and physical properties; they can have electronic structures raging from insulator, metallic, halfmetallic to superconductor; they can also have different magnetic order parameters; or even simultaneously containing the multiple order parameters, such as multiferroicity. The doubleperovskite ferrite YBaCuFeO_{5} (YBCFO) is such a material to show a rich phase diagram. Using a modified floating zone growth method, we are able to grow the highquality single crystal of YBCFO for the detailed studies using neutron scattering. Using neutron powder and single crystal diffraction, YBCFO was observed to show a commensurateincommensurate magnetic transition at T_{N}~ 175 K. This incommensurate phase develops into a spiral spin ordering with a propagating vector along caxis. Further using neutron TOF and tripleaxis spectrometers, we observed that the spiral spin ordering shows a very unusual dispersion behavior as a function of temperature, which could be the consequence of a strong coupling between the spin moments of Fe^{3+} and Cu^{2+} and their d_{z }orbitals. This finding also calls for the analytically modeling for the further understanding the mechanism behind.
References:
 Y.C. Lai, et. al., J. of Physics: Condes. Matt. 29, 145801 (2017)
 W.C. Liu, et. al., J Appl. Cryst. 49, 1721 (2016)
 M. Morin, et. al., Nature Comm. 7, 13758 (2016)
 D. Dey, et. al., Scientific Report 8, 2404 (2018)
We propose manybody invariants for multipolar higherorder topological insulators by generalizing Resta's pioneering work on polarizations. The manybody invariants are designed to measure the distribution of electron charge in unit cells and thus can detect quantized multipole moments purely from the bulk ground state wavefunctions. Using the invariants, we prove the bulkboundary correspondence of the higherorder topological insulators. Further, we show that our invariants can diagnose the corner charge of rotational symmetric crystalline insulators. Application of our invariants to spin systems as well as various other aspects of the manybody invariants will be discussed.
In the last decade, experimental quest for the hypothetical ``quantum spin liquid'' (QSL) state met several candidate materials on geometrically frustrated lattices such as triangular and kagome lattices in 2D, and a pyrochlore lattice in 3D. These spin1/2 compounds exhibit no magnetic ordering nor the spin freezing down to very low temperature, while the measured physical quantities mostly exhibit gapless behaviors. We have argued that these compounds might contain significant amount of (effective) quenched randomness or inhomogeneity of varying origin: either of external origin such as the substitution disorder, or of internal origin dynamically selfgenerated via the coupling to other degrees of freedom in solids such as the charge, the lattice and the orbital. We propose as a minimal model the s =1/2 antiferromagnetic Heisenberg model on various geometrically frustrated lattices with a quenched disorder in the exchange interaction, and demonstrate that, when the randomness exceeds a critical value, the model exhibits a randomnessinduced gapless QSLlike state, a ``randomsinglet state'', accompanied by the Tlinear lowtemperature specific heat widely observed experimentally. The randomsinglet state, where local spin singlets of varying strengths are formed in a hierarchical manner, might be viewed as an ``Andersonlocalized'' RVB (resonatingvalence bond) state. The results are discussed in connection with a variety of experimentally observed QSL materials, which seem to provide a consistent explanation of many recent experimental observations.
References:
[1] K. Watanabe, H. Kawamura, H. Nakano and T. Sakai, J. Phys. Soc. Jpn. 83, 034704 (2014).
[2] H. Kawamura, K. Watanabe and T. Shimokawa, J. Phys. Soc. Jpn. 83, 103704 (2014).
[3] T. Shimokawa, K. Watanabe and H. Kawamura, Phys Rev. B 92, 134407 (2015).
[4] K. Uematsu and H. Kawamura, J. Phys. Soc. Jpn. 85, 113702 (2017).
[5] K. Uematsu and H. Kawamura, Phys. Rev. B 98, 134427 (2018).
Since Haldane conjectured that ground state of onedimensional Heisenberg antiferromagnet has a finite spin excitation gap for integer spins or gapless excitations for halfodd integer spins, it has inspired lots of theoretical and experimental studies on the lowdimensional quantum magnets. Recently, we have found a new S=1 onedimensional chain compound NiTe_{2}O_{5}, in which NiO6 octahedra compose the onedimensional chain with edgesharing. Comprehensive magnetic, elastic, and thermal properties have been studied in single crystalline as well as polycrystalline NiTe_{2}O_{5}. In this presentation, we present physical properties of NiTe_{2}O_{5} compound and discuss thermodynamic behavior associated with the order parameter.
A fundamental motif in frustrated magnetism is the tetrahedral cluster  four spins with each pair separated by the same distance. If the spins are coupled by Heisenberg couplings, they must add to zero to minimize energy. This leads to a simple mathematical criterion for classical ground states  we have four vectors which are constrained to add to zero. We show that this leads to a fivedimensional space of allowed states. Remarkably, this space has 'nonmanifold' structure. It contains 'singular' points about which it appears to be six dimensional. We use this construction to build a semiclassical theory for the tetrahedral cluster. In the lowenergy limit, it takes a very simple form. It decomposes into two independent objects  a rigid rotor (a spinning top) and a free spin. This free spin is perhaps the simplest example of an 'emergent' quantity, arising from angular variables in the spin configuration. This provides an elegant way to understand the energy spectrum and physical properties of tetrahedral molecular magnets.
Monday, 03 December 2018
Spinorbit coupling plays a very important role in several problems of condensed matter physics, especially in the spinorbit entangled jstate. The conventional wisdom in the community is that in order to realize the jphysics one needs to look at compounds with heavier elements like Ir. In this talk, I will challenge this wisdom and demonstrate how some of Cu oxides can host jphysics despite the smaller spinorbit coupling [1].
[1] Choong H. Kim, Hwanbeom Cho, Santu Baidya, Vladimir V. Gapontsev, Sergey V. Streltsov, Daniel I. Khomskii, JeGeun Park, Ara Go, Hosub Jin, arXiv:1810.08594
Magnetic systems are characterised by microscopic interaction strengths that couple magnetic moments of different atomic sites. Depending on the magnitudes, signs and ranges of these interactions, various magnetic ground states can be realised, the simplest examples being various collinear magnetic arrangements. In simple systems exhibiting transitions from paramagnetic to ordered (ferro or antiferro) magnetic states with the lowering of temperature, one may obtain an estimate of the net interaction strength from the temperatureintercept (Ɵ_{CW}) of the inversesusceptibility plot as a function of the temperature via the CurieWeiss Law. This magnitude of Ɵ_{CW} is often a good indication of the transition temperature. However, there are many systems where the magnetic ordering temperature is significantly suppressed compared to Ɵ_{CW} due to frustrations in the magnetic interactions. In extreme cases, no magnetic ordering is found down to the lowest temperature probed despite a sizable value of Ɵ_{CW}, prompting one to believe that such systems have an exotic ground state, known as quantum spin liquid, characterised, among other things, by an absence of ordering despite sizable interactions between various magnetic sites. We have been probing several new systems that are likely candidates of this class of compounds. I shall discuss some of these systems in my presentation.
The search for quantum spin (orbital) liquids (QSL) materials where local moments are well formed but continue to fluctuate quantum mechanically down to zero temperature still remains a fundamental challenge in condensed matter physics. In this talk, we shall show that the electronic structure of 6H perovskite type quaternary iridates Ba_{3}MIr_{2}O_{9}, have all the necessary ingredients to host QSL state. In Ba_{3}MIr_{2}O_{9}, Ir ions form structural dimers and nonmagnetic M provides a knob to tailor the valence of Ir leading to emergent quantum phases. As a first example [1], we shall consider the pentavalent (d^{4}) 6H perovskite iridate Ba_{3}ZnIr_{2}O_{9} and argue that the ground state of this system is a realization of novel spinorbital liquid state. Our results reveal that such a system provides a very close realization of the elusive J=0 state where Ir local moments are spontaneously generated due to the comparable energy scales of the singlettriplet splitting driven by spinorbit coupling (SOC) and the superexchange interaction mediated by strong intradimer hopping. While the Ir ions within the structural Ir2O9 dimer prefers to form a spinorbit singlet states(SOS) with no resultant moment, however substantial frustrated interdimer exchange interactions induce quantum fluctuations in the SOS states favoring spinorbital liquid phase at low enough temperature. As a second example [2] we shall consider the d^{4.5} insulator Ba_{3}YIr_{2}O_{9} and explain the origin of the pressure induced magnetic transition to a spinorbital liquid state in this system. We shall also discuss the importance of Kitaev interactions in the realization QSL phases for the d^{5} members of the same family[3]. Finally we shall compare our results with d^{3} [4] and d^{4} [5] Ir based double perovskites, particularly explain the origin of moments and presence of spinorbital singlets in Ba_{2}YIrO_{6 .}
References
[1] A. Nag et. al. Phys. Rev. Lett. 116, 375501 2016
[2] S.K. Panda et. al. Phys. Rev. B 92, 180403 (Rapid Comm.), 2015
[3] S. Bhowal, S. Ganguly and I. Dasgupta, 2018 (Submitted for publication)
[4] S. Bhowal and I. Dasgupta, Phys Rev B 97 , 024406 (2018)
[5] A. Nag et. al., Phys Rev B 98, 014431 (2018)
Recently, Kitaev’s model is drawing considerable attention as a platform to study quantum spin liquid, and several compounds, such as RuCl_{3} and H_{3}LiIr_{2}O_{6}, are proposed as candidates to support this spin liquid phase. Among many nontrivial properties, the Kitaev’s spin liquid phases host unusual excitations: spins are fractionalized into itinerant Majorana fermions and Visons, and the latter behave as abelian/nonabelian anyons. To diagnose the formation of Kitaev’s spin liquid in actual materials, and to understand the nature of its exotic response, the understanding of these elementary excitations is essential.
In this contribution, we present an exact analytical solution of dynamical correlation function for Kitaev’s spin liquid. With this solution, we will address how local disturbance of the system, such as impurity, manifests itself in the thermodynamic and dynamical properties, in association with the nature of fractionalized excitations. In particular, we will focus on the Vison zeroenergy state appearing around the site vacancy, and discuss how to observe it experimentally.
Motivated by the recent discovery of a macroscopically degenerate exactly solvable point of the spin1/2 XXZ model for Jz/J=−1/2 on the kagome lattice [H. J. Changlani et al. Phys. Rev. Lett 120, 117202 (2018)]  a result that holds for arbitrary magnetization  we develop an exact mapping between its exact quantum threecoloring wavefunctions and the characteristic localized and topological magnons. This map, involving resonating twocolor loops, is developed to represent exact manybody ground state wavefunctions for special high magnetizations. Using this map we show that these exact ground state solutions are valid for any Jz/J≥−1/2. This demonstrates the equivalence of the groundstate wavefunction of the Ising, Heisenberg and XY regimes all the way to the Jz/J=−1/2 point for these high magnetization sectors. In the hardcore bosonic language, this means that a certain class of exact manybody solutions, previously argued to hold for purely repulsive interactions (Jz≥0), actually hold for attractive interactions as well, up to a critical interaction strength. For the case of zero magnetization, where the ground state is not exactly known, we perform density matrix renormalization group calculations. Based on the calculation of the ground state energy and measurement of order parameters, we provide evidence for a lack of any qualitative change in the ground state on finite clusters in the Ising (Jz≫J), Heisenberg (Jz=J) and XY (Jz=0) regimes, continuing adiabatically to the vicinity of the macroscopically degenerate Jz/J=−1/2 point. These findings offer a framework for recent results in the literature, and also suggest that the Jz/J=−1/2 point is an unconventional quantum critical point whose vicinity may contain the key to resolving the spin1/2 kagome problem.
Tuesday, 04 December 2018
The recent discovery of topological semimetals, which possess distinct electronband crossing with nontrivial topological characteristics, has stimulated intense research interest. By extending the notion of symmetryprotected band crossing into one of the simplest magnetic groups, namely by including the symmetry of timereversal followed by spaceinversion, we predict the existence of topological magnonband crossing in threedimensional (3D) antiferromagnets. The crossing takes the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a “spinweb” compound, Cu_{3}TeO_{6}, we demonstrate the presence of Dirac magnons over a wide parameter range using linear spinwave approximation [1]. Inelastic neutron scattering experiments have been carried out to detect the bulk magnonband crossing in a singlecrystal sample. The highly interconnected nature of the spin lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with the linear spinwave calculations. The predicted topological Dirac points are confirmed [2].
[1] K. Li et al., PRL 119, 247202 (2017).
[2] W. Yao et al., Nature Physics 14, 1011 (2018).
The search for topological insulators has been actively promoted in the field of condensed matter physics. Recently, the concept of topologically insulating state and associated edge states have been extended to bosonic quasiparticles, such as magnons in solids [1, 2]. In the talk, we demonstrate that also triplons can construct topological bands in the spin1/2 twodimensional dimerized antiferromagnet Ba_{2}CuSi_{2}O_{6}Cl_{2} [3].
Inelastic neutron scattering experiments were performed by using the coldneutron disk chopper spectrometer AMATERAS [4] installed in MLF, JPARC. Twenty pieces of single crystals were coaligned so that an a* or b*direction for every crystal coincided with each other. A color contour map of the scattering intensities is shown in Fig. a. The two dispersive branches correspond to the band which is dispersive along both H and K directions. In addition, the decrease in intensity was observed at 2.6 meV, irrespective of scattering wave vectors. This can be explained by alternation in interdimer interactions along the adirection: alternation induces an energy gap between the low and high energy bands, and small structure factor of the latter band at high energies results in “splitting” of the triplon branch, as illustrated in Fig. b. The topology of the bands can be understood by regarding Ba_{2}CuSi_{2}O_{6}Cl_{2} as a pseudoonedimensional variant of SuSchriefferHeeger (SSH) model [5]. The correspondence indicates the presence of thermally excited topologically protected edge states induced by a bipartite nature of the lattice [6].
(a) Excitation spectrum at 0.3 K.
(b) Schematic dispersion relations of the triplon bands.
[1] R. Shindou, R. Matsumoto, S. Murakami, J. Ohe, Phys. Rev. B 87 144427 (2013).
[2] L. Zhang, J. Ren, J. S. Wang, and B. Li, Phys. Rev. B 87, 144101 (2013).
[3] M. Okada et al., Phys. Rev. B 94, 094421 (2016).
[4] K. Nakajima et al., J. Phys. Soc. Jpn. 80, SB028 (2011).
[5] W. P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett. 42, 1698 (1979).
[6] K. Nawa et al., condmat arXiv:1810.08931 (2018).
Although the low energy fractional excitations of onedimensional integrable models are often wellunderstood, exploring quantum dynamics in these systems remains challenging in the gapless regime, especially at intermediate and high energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics in the antiferromagnetic spin1/2 XXZ chain with the Ising anisotropy via the formfactor formulae. Various excitations at different energy scales are identified crucial to the dynamic spin structure factors under the guidance of sum rules. At small magnetic polarization, gapless excitations of psinons and antipsinons dominate the low energy spin dynamics. In contrast, spin dynamics at intermediate and high energies is characterized by the two and threestring states. The dynamic spectra of the identified dominant excitations evolve with clear energy separations when tuning the magnetic field, conveying a simple and straightforward way to clearly identify the novel string excitations in proper condensed matter systems. Our predictions have been experimentally confirmed on the quasionedimensional material SrCo2V2O8, where the details of the experimental observations will also be discussed.
Topological spin texture consisting of multipleq spin spiral is of great interest for novel quantum transport phenomena and spintronic functions. A recent interesting example is a magnetic skyrmion, which is a topologically stable, vortexlike spin object discovered in noncentrosymmetric systems allowing the DzyaloshinskiiMoriya (DM) interaction [1,2]. The title compound SrFeO_{3} is a promising candidate of the centrosymmetric compound hosting a novel skyrmion lattice in the absence of the DM interaction. SrFeO_{3} has been known as a rare oxide showing both helimagnetism and metallic conduction while preserving the centrosymmetric cubic lattice. While the magnetic ground state has been believed to be a simple properscrewtype spin order for long time, we have found that the magnetic phase diagram of SrFeO_{3} hosts a rich variety of helimagnetic phases, two of which show novel topological helimagnetic orders [3].
In this presentation, I will show the topologically nontrivial helimagnetic phases in the simple cubic perovskite SrFeO_{3}, which were discovered by the polarized and unpolarized small angle neutron scattering (SANS) measurements on the single crystalline samples. We found that SrFeO_{3} shows two kinds of multipleq helimagnetic structures:_{}an anisotropic doubleq spin spiral and an isotropic quadrupleq spiral hosting a threedimensional lattice of topological singularities [4]. As a related topic, our recent discovery of a novel helimagnetic phase in the isostructural cubic perovskite Sr_{1x}Ba_{x}CoO_{3} by Sakai et al. will be also presented [5]. These perovskitetype oxides not only diversify the family of SkX host materials, but furthermore provides an experimental missing link between centrosymmetric lattices and topological helimagnetic order.
This work was done in collaboration with T. Nakajima, J. H. Kim, D. S. Inosov, Y. Tokunaga, S. Seki, N. Kanazawa, Y. W. Long, Y. Kaneko, R. George, K. Seemann, J. S. White, J. L. Gavilano, Y. Taguchi, T. Arima, B. Keimer, and Y. Tokura. This work is supported by JST PRESTO Hypernanospace design toward Innovative Functionality (Grant No. JPMJPR1412) and the JSPS GrantinAid for Scientific Research (A) Grant No. 17H01195.
[1] S. Mühlbauer et al., Science 323, 915 (2009).
[2] S. Seki, X. Z. Yu, S. Ishiwata, and Y. Tokura, Science 336, 198 (2012).
[3] S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011).
[4] S. Ishiwata et al., arXiv:1806.02309.
[5] H. Sakai et al., Phys. Rev. Mater. 2, 104412 (2018).
Interface engineering of complex oxides has become a popular approach to realize fascinating collective phenomena, which are very often “hidden” or unattainable in the constituent bulk materials. While the strong interplay among spin, charge, orbital, lattice degrees of freedom facilitate interesting manybody quantum phenomena in correlated oxides, the additional broken symmetries and frustrated couplings across the interface of artificial heterostructures may give rise to new electronic, magnetic states. Geometrical lattice engineering (GLE) has been presented as another potential way in recent times to realize novel topological and quantum manybody states. The key idea behind the GLE is to design fully epitaxial fully epitaxial ultrathin heterostructures with an artificial lattice geometry (e.g. buckled honeycomb lattice, Kagome lattice etc.) generated by stacking of a very specific number of atomic planes along a particular orientation.
As a prototype example of such interface and lattice engineering, I will talk about our ongoing work on rare earth nickelate heterostructures. The results of synchrotron diffraction, xray absorption spectroscopy, resonant xray scattering to elucidate the response of underlying lattice, spin, orbital and charge degrees of freedom combined with strain and quantum confinement will be presented.

S. Middey et al., Annual Review of Materials Research 46, 305334 (2016).

S. Middey et al., Phys. Rev. Lett. 116, 056801 (2016).

S. Middey et al., Phys. Rev. Lett. 120, 156801 (2018); Phys. Rev. B 98, 045115 (2018); Appl. Phys. Lett. 113, 081602 (2018).
The magnetic field response of the Mottinsulating honeycomb iridate Na_{2}IrO_{3} is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peakdip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering (≈ 15K) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a longsoughtafter fieldinduced quantum spin liquid beyond the peakdip structure. Kitaev systems are thus revealed to be excellent candidates for fieldinduced quantum spin liquids, similar physics having been suggested in another Kitaev material α−RuCl3.
Wednesday, 05 December 2018
A simple Néel order can be destroyed by geometrical cancellation of magnetic interactions between nearestneighbor spins on trianglebased networks or by competitions among two or more kinds of interactions. Stabilized alternatively are quantum disordered states called the spin liquid or unusual longrange orders having emergent degrees of freedom such as chirality or topological excitations. Extensive theoretical studies have proposed fascinating states of matter in various classes of models with frustration, and experimental studies have been undertaken to search for related exotic phenomena in real candidate compounds. However, there are numerous obstacles to clarify the frustration physics: the selection of a ground state from macroscopically large number of states existing next to it within a small energy range is subtle and is rather difficult to predict even by the stateoftheart calculation techniques. Moreover, the ground state is often severely influenced by perturbations like anisotropy or additional interactions to the simplest Heisenberg model. Furthermore to experiments, the presence of crystallographic disorder, which does exist more or less in actual compounds, tends to disturb and mask the intrinsic properties of frustrated systems. Therefore, one has to be careful to enjoy the interesting frustration physics.
In my presentation, I will focus on some frustrated spin systems from materials point of view. A particular emphasis will be on two kagometype copper minerals: volborthite [1] and Cdkapellasite (CdK) [2]. The others are socalled J_{1}–J_{2} magnets with first and secondneighbor interactions in 1D chains [NaCuMoO_{4}(OH), 3] and 2D square lattices [AMoOPO_{4}Cl (A = K, Rb), 4]. In addition, I would like to address our recent trials searching for novel magnets in 5d transition metal compounds.
References
[1] H. Ishikawa, M. Yoshida, K. Nawa, M. Jeong, S. Krämer, M. Horvatić, C. Berthier, M. Takigawa, M. Akaki, A. Miyake, M. Tokunaga, K. Kindo, J. Yamaura, Y. Okamoto and Z. Hiroi, Phys. Rev. Lett. 114, 227202 (2015).
[2] R. Okuma, T. Yajima, D. NishioHamane, T. Okubo and Z. Hiroi, Phys. Rev. B 95, 094427 (2017).
Vortex like spin arrangements called magnetic toroidal, magnetic monopole, and magnetic quadrupole moments are known as a source for unique symmetrydependent phenomena such as linear magnetoelectric effects. We report our recent discovery of magnetoelectric responses in the new antiferromagnets A(TiO)Cu_{4}(PO_{4})_{4} (A = Ba, Sr, Pb), whose crystal structure is characterized by an alternating array of Cu_{4}O_{12} spin clusters with convex geometry known as square cupola. We find that each Cu_{4}O_{12} spin cluster in these materials form a magnetic quadrupole type spin arrangement, which is stabilized due to the square cupola geometry. Moreover, in the A = Pb system, these magnetic quadrupole moments order uniformly in all the Cu_{4}O_{12} spin clusters. We demonstrate that this ferroic magnetic quadrupole order gives rise to highly anistropic (DC) linear magnetoelectric effect, induction of electric polarization by an applied magnetic field. As an extension of the magnetoelectric effect into an optical regime, we also observe a nonreciprocal linear dichroism for visible light, i.e., the sign of the linear dichroism is switched between two counterpropagating light beams. These results indicate that the convexshaped spin cluster can be a promising structural unit that hosts unique magnetoelectric responses arising from magnetic quadrupole moments.
Frustration is known to give rise to many interesting physical phenomena. One such example is magnetic fragmentation when original spins decompose into new elementary degrees of freedom. Inspired by Jospehson junctions arrays we propose a model of magnetic fragmentation on kagome lattice for classical planar spins with isotropic nearestneighbor Heisenberg interactions and broken sublattice symmetry. As couplings are varied, the groundstate exhibits a transition from plain ferromagnet to a highly degenerate manifold of states that retain nonzero magnetisation. Importantly this behavior, i.e. fragmentation, persists for a range of couplings. We analyse the properties of these groundstates, like correlations and longwavelength behaviour. As a simpler version we also consider the d=1 version of the model, that also exhibits highly degenerate groundstates in part of the phase diagram, but does not show any magnetic ordering. Finally we discuss how this model can be extended to different types of spins and other 2D and 3D lattices.
The Hida model, defined on honeycomb lattice, is a spin1/2 Heisenberg model of antiferromagnetic hexagons (with nearestneighbour interaction, J_{A}) coupled via ferromagnetic bonds (with exchange interaction, J_{F}). It applies to the spingapped organic materials, mMPYNN · X (for X = I, BF4, ClO4). For J_{F}  ≫ JA, it reduces to the spin1 kagome Heisenberg antiferromagnet (KHA). Motivated by the recent findings of spontaneously trimerized singlet (TS) ground state for the spin1 KHA, we investigate the evolution of the ground state of Hida model from weak to strong J_{F }/J_{A} using triplon analysis and Schwinger boson meanfield theory. Our study of the Hida model shows that its uniform hexagonal singlet (HS) phase for weak J_{F} /J_{A} soon gives way to the dimerized hexagonal singlet (DHS) ground state, which for strong J_{F} /J_{A} approaches the TS state. We further find that the evolution from the uniform HS phase for spin1/2 Hida model to the TS phase for spin1 KHA happens through two quantum phase transitions: (1) the spontaneous dimerization transition at small J_{F} /J_{A} (~ −0.28) from the uniform HS to DHS phase, and (2) the moment formation transition at moderate J_{F} /J_{A} (~ −1.46) across which the pair of spin1/2’s on every FM bond begins to behave as bound moment that tends to spin1 for large negative J_{F}’s. The TS ground state of the spin1 KHA is thus adiabatically connected to the DHS ground state of the Hida model. Our calculations imply that the mMPYNN · X salts realize the DHS phase at low temperatures, which can be ascertained through a distinguishing feature in neutron diffraction.
An impurity in a TomonagaLuttinger liquid leads to a crossover between short and longdistance regime which describes many physical phenomena. However, calculation of the entire crossover of correlation functions over different length scales has been difficult. We develop a powerful numerical method based on infinite DMRG. By utilizing infinite boundary conditions we can obtain correlation functions within a finitesize window that contains the impurity. For the S = 1/2 chain, we demonstrate that the full crossover can be precisely obtained, and that their limiting behaviors show a good agreement with fieldtheory predictions.
The effects of quantum fluctuations in the Heisenberg model on the isotropic and breathing pyrochlore lattices are discussed for a generic spinS. By employing the pseudofermion functional renormalization group method, we find, for S=1/2 and S=1, an extended quantum spin liquid phase. The effects of temperature, quantum fluctuations, breathing anisotropies, and further neighbor Heisenberg couplings, on the nature of the scattering profile, and the pinch points in particular, are analyzed. We place our results in the context of recently experimentally studied Heisenberg pyrochlore magnets, such as the S=1 NaCaNi2F7, a promising candidate spin liquid material, and the breathing pyrochlores, LiInCr4O8, LiGaCr4O8, LiInCr4S8, LiGaCr4S8, CuInCr4S8, and CuInCr4Se8.
Reference: arXiv: 1802.09546, Y. Iqbal, T. Müller, P. Ghosh, M. J. P. Gingras, H. O. Jeschke, S. Rachel, J. Reuther, and R. Thomale (2018)
We study transport across junctions of a Weyl and a multiWeyl semimetal separated by a region of thickness d which has a barrier potential U_0. We show that the tunneling conductance G of such a junction becomes independent of the barrier strength in the thin barrier limit. We show that this independence is a consequence of the change in the topological winding number of the Weyl nodes across the junction and point out that it has no analogue in tunneling conductance of either junctions of twodimensional topological materials (such as graphene or topological insulators) or those made out of Weyl or multiWeyl semimetals with same topological winding numbers. We study this phenomenon both for normalbarriernormal (NBN) and normalbarriersuperconducting (NBS) junctions involving Weyl and multiWeyl semimetals and discuss experiments which can test our theory.
Thursday, 06 December 2018
Weak Mott insulators emerge from 5d electron systems with strong spinorbit interactions. Coulomb repulsion (U) between electrons in 5d orbitals is not strong enough to suppress charge degrees of freedom completely. For example, the charge gap is comparable to the magnonband width for strontium iridates. Such electron systems can be described by the Hubbard model with an intermediate U, which would be in the crossover regime between Slater and Mott insulators. Reliable calculation for the intermediateU region is theoretically challenging because of the lack of a small parameter in the model. We have developed a new numerical approach that enables efficient calculation of dynamical quantities at finite temperatures in the broadU region. The sampling of auxiliary vector fields from the Boltzmann distribution and the real time dynamics of the density matrix are successfully combined in our method.
In the presentation, I will explain our semiclassical approach that fully captures spatial fluctuations of the systems. As an application of the newly developed method, the dynamical spin structure factor and the dielectric constant for the kagome lattice will be demonstrated, where the ground state has an octahedral spin configuration with a chiral symmetry breaking. Importantly, our approach is applicable to various systems without any restrictions on the type of hopping, the type of lattice, and electron filling. The broad U region is accessible up to the temperature comparable to the charge gap.
The spintriplet superconductor by definition should involve spin ordering that gives rise to the spin collective phenomena such as the spin collective modes and the spin supercurrent. However, in the superconducting phase in the bestknown candidate material, Sr2RuO4, only the latter has been observed just for the mesoscopic sample [1]. I will show how the recently fabricated heterostructure of bulk Sr2RuO4 and the ferromagnetic SrRuO3 provides a particularly natural probe for detecting bothtypes of spin collective phenomena. The spin supercurrent that can be injected naturally into Sr2RuO4 from SrRuO3 can be used to obtain an exceptionally highquality spin valve, while applying an AC bias on SrRuO3 can drive the Sr2RuO4 spin collective modes [2].
References:
[1] J Jang, D J Ferguson, V Vakaryuk, R Budakian, SBC, P M Goldbart, and Y Maeno, Science 331 (2011), 186
[2] SBC, S K Kim, K H Lee, and Y Tserkovnyak, Phys. Rev. Lett. 121 (2018), 167001
Breathing pyrochlore magnets, where localized spins are arranged at the edge of alternating small and large tetrahedra with magnetic interaction J and J', respectively, have attracted attention as unique systems that possess both geometrical frustration and bond alternation. The two systems that are spin3/2 magnets LiInCr_{4}O_{8} and LiGaCr_{4}O_{8} and a pseudospin1/2 magnet Ba_{3}Yb_{2}Zn_{5}O_{11} have been intensively studied. In these magnets, J and J' are suggested to be antiferromagnetic. We focus on Asite ordered Cr spinel sulfides LiInCr_{4}S_{8}, LiGaCr_{4}S_{8}, and CuInCr_{4}S_{8}, where Cr^{3+} ions carrying an S = 3/2 spin form a breathing pyrochlore structure [1]. Although these three sulfides are suggested to have both antiferromagnetic J and ferromagnetic J' on their pyrochlore structures, lowtemperature magnetic properties of them differ significantly, as seen in magnetization processes measured up to 72 T. We also found they show different thermal expansion properties including negative thermal expansion and a magneticfield induced volume change at low temperature, indicating that a rich physics exists in this spin system.
[1] Y. Okamoto, M. Mori, N. Katayama, A. Miyake, M. Tokunaga, A. Matsuo, K. Kindo, and K. Takenaka, J. Phys. Soc. Jpn. 87, 034709 (2018).
The competition between Ruderman, Kittel, Kasuya and Yosida (RKKY) interaction and the Kondo effect in Ce and Yb based intermetallic compounds have been studied quite extensively. These studies on the Ce and Ybbased intermetallic compounds have resulted in a plethora of interesting phenomena like heavy fermion, valence fluctuation, unconventional superconductivity, quantum phase transition etc. All these interesting properties occurs due to the hybridization of the localized 4f electron with the conduction electrons. A weak hybridization usually results in a magnetic ground state, while a stronger hybridization results in a weaker localization of the 4f electron pushing the system eventually to a nonmagnetic ground state. The hybridization strength and the effects due to the crystal electric field (CEF) depend on the atomic environment around the Ceatom in the unit cell, which includes the interatomic distances between the Ce and other atoms on different crystallographic sites. Most of the Cecompounds possess a single unique atomic position for the Ce atom in the unit cell. However, there are cerium compounds which host multiple sites for Ce atoms in the unit cell. In this talk, I will present some of our recent results on the crystal growth and anisotropic physical properties of some Cebased intermetallic compounds.