List of poster contributions

In the pursuit of Majorana Bound States (MBS), we consider recent proposals for their realization in a wire of magnetic atoms on the surface of a superconductor. Coupling wires to form ribbons, we study how these sates are enhanced or killed. We compare the equivalency of these systems and those with Zeeman and spin-orbit interaction. We quantify the Majorana Polarization which was shown to be a useful tool for identifying MBS in linear chains with Rashba spin-orbit interaction.

Quantum spin Hall insulators are characterized by topologically protected counter-propagating edge states. Here we study the dynamical response of these helical edge states under a time-dependent flux biasing, in presence of a heat bath. It is shown that the relaxation time of the edge carriers can be determined from a measurement of the dissipative response of topological insulator (TI) disks. The effects of various perturbations, including Zeeman coupling and disorder, are also discussed.

Ref: Doru Sticlet and Jérôme Cayssol, arXiv:1406.3633 

Non-perturbative linked-cluster expansions have become a diverse and
powerful tool for the investigation of quantum lattice models used
to calculate ground-state energies and thermodynamic quantities, to
derive effective low-energy models and to investigate
non-equilibrium physics directly in the thermodynamic limit. A
fundamental challenge of such expansions is the breaking of
translational symmetry on single clusters affecting the
applicability in certain regimes severely.

While materials with 5d4 electronic configuration are generally expected to be non-magnetic, it is proposed that an interplay between Coulomb interaction (U) and spin-orbit-coupling (SOC) can create novel gapped singlet-triplet excitonic magnetism [1,2]. Recently, the double perovskite material Sr2YIrO6 with Ir5+ ions is reported to exhibit an antiferromagnetic long-range order below 1.3 K [3]. This material crystallizes in a monoclinic structure with highly distorted IrO6 octahedra. It is believed that this distortion is the driving force for the magnetism in this compound. On the other hand, the Ba-analog Ba2YIrO6 crystallizes in a cubic double perovskite structure [4] and could be a candidate material to study novel excitonic magnetism. From our magnetization measurement, we found Ba2YIrO6 is paramagnetic in the temperature range 2-300K. Curie-Weiss fitting of susceptibility data gives an effective magnetic moment µeff=0.57µB/Ir and a Weiss constant θCW=-135 K (AF). 89Y NMR measurements confirm presence of spin fluctuations in this material. Our study suggests Ba2YIrO6 is magnetic. However the origin and nature of the magnetism is still not clear. In this presentation, details of crystal growth along with structural, magnetic, thermal, electrical transport and NMR measurement results will be discussed.

[1] G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013) 
[2] O. N. Meetei et. al., arXiv:1311.2823v1
[3] G. Cao et. al., Phys. Rev. Lett. 112, 056402 (2014)
[4] J. H. Choy et. al., J. Am. Chem. Soc., 117, 8557 (1995)

Dynamical correlations of quantum magnets provide valuable information about the systems under study. Especially at finite temperature, many observation, e.g. in
inelastic neutron scattering experiments, are still not quantitatively understood.
Generically, the elementary excitations of quantum magnets are hard-core
bosons because no two of them can be present at the same site.
We present an extension of the diagrammatic Brückner approach to deal
with complicated, interacting models of hard-core bosons at finite temperature.
So far, only the dynamics of particle-conserving models without interactions
beyond the hardcore constraint were addressed. 
Here we combine renormalizing continuous unitary transformations, which yield the effective models, with the diagrammatic approach to calculate the dynamics
at finite temperatures. We employ a mean-field decoupling to include effects of additional interactions and correlated hopping processes. Specifically, we address the development of the static structure factor as function of temperature as well as the thermal broadening of single-particle peaks.

In this contribution, we examine how THz fields can control orbital domain structure. As domains can be thermally stable, domain control is a useful for functional devices. Specifically, we show that THz fields can manipulate the direction of orbital domains in La0.5Sr1.5MnO4 (LSMO). As orbital domains control the optical, electronic and structure properties of the material, our results demonstrate the potential for non contact control of solid properties at high speeds.

We show theoretically that periodically driven systems with short range Hubbard interactions offer a feasible platform to experimentally realize fractional Chern insulator states. We exemplify the procedure for both the driven honeycomb and the square lattice, where we derive the effective steady state band structure of the driven system by using the Floquet theory and subsequently study the interacting system with exact numerical diagonalization. The fractional Chern insulator state equivalent to the 1/3 Laughlin state appears at 7/12 total filling (1/6 filling of the upper band). The state also features spontaneous ferromagnetism and is thus an example of the spontaneous breaking of a continuous symmetry along with a topological phase transition. We discuss light-driven graphene and shaken optical lattices as possible experimental systems that can realize such a state.

We consider a model for a one-dimensional quantum wire with Rashba spin-orbit coupling and repulsive interactions, proximity coupled to a conventional s-wave superconductor. Using a combination of Hartree-Fock and density matrix renormalization group calculations, we show that for sufficiently strong interactions in the wire, a time-reversal invariant topological superconducting phase can be stabilized in the absence of an external magnetic field. This phase supports two zero-energy Majorana bound states at each end, which are protected by time-reversal symmetry. The mechanism for the formation of this phase is a reversal of the sign of the effective pair potential in the wire, due to the repulsive interactions. We calculate the differential conductance into the wire and its dependence on an applied magnetic field using the scattering-matrix formalism. The behavior of the zero-bias anomaly as a function of the field direction can serve as a distinct experimental signature of the topological phase.

We present a controlled microscopic study of mobile holes (vacancies) in the spatially anisotropic (abelian) gapped phase of the Kitaev honeycomb model. We address the properties of (i) a single hole - its internal degrees of freedom as well as its hopping properties; (ii) a pair of holes - their (relative) particle statistics and interactions; (iii) the collective state for a finite density of holes. We find that each hole in the doped model has an eight-dimensional internal space, characterized by three internal quantum numbers: the first two 'fractional' quantum numbers describe the binding to the hole of the fractional excitations (fluxes and fermions) of the undoped model, while the third 'spin' quantum number determines the local magnetization around the hole. The 'fractional' quantum numbers also encode fundamentally distinct particle properties, topologically robust against small local perturbations: some holes are free to hop in two dimensions, while others are confined to hop in one dimension only; distinct hole types have different particle statistics, and in particular, some of them exhibit non-trivial (anyonic) relative statistics. These particle properties in turn determine the physical properties of the multi-hole ground state at finite doping, and we identify two distinct ground states with different hole types that are stable for different model parameters.

By using density matrix renormalization group, we study the spin liquid phases in spin-$1/2$ XXZ kagome antiferromagnets. We find that the emergence of spin liquid phase doesn't depend on the anisotropy of XXZ interaction, particularly the two extreme limits---Ising (strong $S^z$ interaction) and XY (zero $S^z$ interaction)---host the same spin liquid phases as the $SU(2)$ Heisenberg model. We have obtained both time-reversal invariant spin liquid and  chiral spin liquid with spontaneous time-reversal symmetry breaking, and we show they will continuously transit into each other when one tunes the second and third neighbor interactions. I will also discuss the possible implication of our results on the nature of spin liquid in nearest neighbor XXZ kagome antiferromagnets, including the most studied nearest neighbor spin-$1/2$ kagome anti-ferromagnetic Heisenberg model.

The physics of strongly correlated electron systems is a very promising field of research for the realization of exotic phases like quantum spin liquids displaying topological order and fractional excitations. Another fascinating topic are topological insulators which typically result from spin-orbit interactions. One important issue is therefore to search for new physics in situations where strong correlations and spin-orbit coupling are present at the same time.
One microscopic model, which combines both features, is the Kane-Mele-Hubbard-Model which we tackle in this work at zero temperature and at half filling. Our aim is to derive effective low-energy spin models about the strong-coupling Mott limit by applying pertubative and graph-based continuous unitary transformations in order to separate spin and charge degrees of freedom.

We introduce a variatonal method to derive effective Hamiltonians in
terms of creation and annihilation operators in second quantization. It
is based on the matrix product state formalism and exploits translational
invariance to work directly in the thermodynamic limit. Benchmark results
are presented for the one-particle contributions for the transverse field
Ising model in one dimension and related systems.
The route to two-particle properties is elucidated.

B. Klemke (Helmholtz-
Zentrum Berlin), P. Strehlow (Physikalisch-Technische Bundesanstalt, Berlin),
A. Sokolowski (Helmholtz-Zentrum Berlin), M. Meissner (Helmholtz-Zentrum
Berlin) — Geometrical frustration is a common feature of condensed matter
systems in which the lattice geometry inhibits the formation of a single ground-
state conguration. In order to analyse low temperature heat transport in the
spin ice compound Dy2Ti2O7, we derive thermodynamic field equations that are
based on the kinetic theory of phonons and their interaction with localised mag-
netic excitations [1]. It is shown that the solution of the derived field equations
for given boundary and initial values of heat-pulse experiments well describes
all measured temperature profiles recorded in the temperature range from 0.3
to 15 K and in magnetic fields up to 1.5 T. Thus, the data of both the specific
heat and the thermal conductivity, which were obtained in thermal relaxation
and steady-state heat transport measurements, are in agreement with the ther-
modynamic modelling of heat-pulse experiments. The evaluated temperature
and field dependencies of both the specific heat contributions and their cor-
responding relaxation times indicate that the magnetic excitations above the
ground-state manifold of the spin ice compound Dy2Ti2O7 take the form of
magnetic monopoles.
[1] P. Strehlow, S. Neubert, B. Klemke, M. Meissner, Continuum Mech.
Thermodyn. 24, 347 (2012).
1

In the conventional picture, magnetic excitations are long-lived at low temperatures with decreasing lifetime as temperature is increased. The accepted explanation is that thermally activated excitations collide with each other limiting their lifetimes -an effect observed experimentally as a Lorentzian energy broadening of the lineshape. This picture works well for gapless magnets with long-range magnetic order [1,2]. The basic assumption is that the excitations interact only weakly and the available states cover a big region of phase space, so that as they collide they fluctuate randomly among a large range of different states in an uncorrelated manner. However for some magnets, where there are strong interactions between the excitations and the phase space is limited, this reasoning may not apply. Gapped antiferromagnets such as dimer systems which have a singlet ground state and a band of triplet excitations are potential candidates. 
In this work we introduce our recent investigations of a highly dimerised 1D antiferromagnet BaCu2V2O8 which is a potential candidate for the observation of asymmetric thermal line shape broadening. Indeed, BaCu2V2O8 has tetragonal symmetry (I-42d, a=b=12.66843 Å, c=8.124 Å) where the magnetic Cu2+ ions (spin-1/2) form alternating spiral chains along the c-axis. Our inelastic neutron scattering measurements on a single crystal of BaCu2V2O8 reveal that the magnetic excitations consist of two excitation branches  which have a gap of 36meV and disperse along the L direction over the energy range 36-46 meV. Both modes are dispersionless in the H and K directions implying that the dimers are coupled together one-dimensionally along the c-direction, but with negligible coupling within the a-b plane. Both modes are identical except that they are shifted with respect to each other by half a period along L. The high ratio of gap to bandwidth (=3.6) in BaCu2V2O8 make this a candidate compound for detailed observation for asymmetric thermal lineshape broadening over a wide temperature range. In order to explore the temperature dependence of the line shape of the magnetic excitations we perform the energy scans at the dispersion minima at different temperatures.   The data were fitted well by an asymmetric Lorentzian function [3] and the extracted fit parameters suggest that the line shape becomes increasingly asymmetry with increasing temperature. The results are in agreement with the recently explored temperature dependence of the magnetic excitation spectrum of copper nitrate, which is a spin-1/2 alternating chain with a similar ratio of gap to band width (=3.5) [4].  As for copper nitrate, the asymmetry thermal line shape broadening in BaCu2V2O8 can be attributed to strong correlations between the excitations. This result confirms that excitations in strongly dimerised 1 D magnetic systems behave as a 1D strongly correlated gas of Bosons [4].

References: 
[1] T. Huberman, et al J. Stat. Mech. p. 05017 (2008). 
[2] S. P. Bayrakci, et al, Science 312, 5782 (2006).
[3] D. L. Quintero-Castro, B. Lake, et al Phys. Rev. Lett. 109, 127206 (2012).
[4] D. Tennant, B. Lake, , et al. Phys. Rev. B 85, 014402 (2012).

The formation of fractional quantum-Hall (FQH) states in lattice systems without externally applied magnetic fields -- dubbed fractional Chern insulators (FCI) -- is a recent and promising theoretical proposal that has the potential to render the FQH effect experimentally more accessible. The paradigmatic FCIs are found when interacting electrons with frozen spin degree of freedom populate relatively flat topological bands, with the interaction strength being smaller than the gap to other bands. Strong interactions that mix bands, on the other hand, may give rise to competition between topological and conventional charge order. After discussing this competition, I will introduce a class of states in which FCI topological order is induced by the presence of a charge-density wave and will present numerical evidence for this coexistence. Finally, based on these compositely ordered states, I will provide a recipe for topological order out of a topologically trivial band structure.

     
              R. Kumar, Tusharkanti Dey, A.V. Mahajan
      Department of Physics, IIT Bombay, Powai, Mumbai-400076, India   
                               
Currently, there is a great surge of interest in 5d-based transition metal oxides because of their tendency to mimic Mott insulating behavior in spite of a small on-site Coulomb repulsion (U) and a large spin-orbit coupling (SOC). Among 5d, in particular, iridates have attracted a lot of attention because initially they were expected to be metallic because of a small U. But it has now been recognized in many real materials that in presence of a strong SOC even a small U can give rise to a Mott insulating state; some examples are Sr2IrO4 [1], Na4Ir3O8 [2]. Depending upon the strength of SOC and U, varieties of exotic ground states have been predicted and verified in real 5d-materials [3]. Of these, a very exotic ground state is the quantum spin liquid (QSL) state, and is of special interest in condensed matter physics. The compound Na4Ir3O8 [2] was the first QSL to be discovered belonging to the 5d-family. But recently we have evidenced QSL state in the high pressure cubic phase of Ba3YIr2O9 [4] and in a 2D edge shared triangular network Ba3IrTi2O9 [5]. Here we report the magnetic dilution effect in the 5d-based spin liquid (Jeff = ½ ) compound Ba3IrTi2O9 recently proposed by us. The title compound belongs to a hexagonal class with space group P63mc and structurally both Ir and Ti atoms form 2D triangular motifs adjacent to each other. Due to the comparable ionic size of Ir4+ and Ti4+ ions, significant site disorder (nearly 50%, very close to the percolation threshold for a 2D triangular lattice) is observed and this leads to the formation of depleted triangular planes in which Ir4+ spins possibly reside on the corner shared triangles and consequently spins get frustrated. Bulk susceptibility measurements confirm the absence of long-range magnetic order down to 2K [5]. Since Ba3IrTi2O9 is in the vicinity of the percolation threshold so in order to check the impact of magnetic dilution on QSL state, we diluted the system by replacing 50% Ir4+ (S=1/2) ions with nonmagnetic Ti4+ (S=0) ions. The measured bulk susceptibility on Ba3Ir0.5Ti2.5O9 [6] in the temperature range 2-400K do not evidence any bifurcation in ZFC and FC susceptibility under a small magnetic field 25 Oe. Also, we found a negative θCW ~ -100K with a reduced Curie constant (nearly 2/3 of that expected for spin ½). Heat capacity measurements down to 0.35K support a disordered ground state. Local probe measurement μSR do not see any ordering/freezing down to 20mK. It is surprising that this system is robust against such a heavy magnetic dilution and yet maintains the quantum spin liquid ground state. 

References:
[1] B.J. Kim et al., Science 323, 1329 (2009).
[2] Y. Okamoto  et al., Phys. Rev. Lett. 99, 137207 (2007).  
[3] L. Balents et al., arXiv preprint arXiv: 1305.2193 (2013).
[4] Tusharkanti Dey et al., Phys. Rev. B 88, 134425 (2013).
[5] Tusharkanti Dey et al., Phys. Rev.B 86, 140405 (2012).
[6] R. Kumar et al., (present work).

We study theoretically the collective dynamics of rotational excitations of polar molecules loaded into an optical lattice in two dimensions. We explore the collective many-body phases that form following a microwave pulse. We show that, owing to the long-range interactions between molecules and energy conservation in this isolated system, the rotational excitations can form a Bose-Einstein condensate with long-range order, even for the natural (undressed) dipole interactions. This manifests itself as a divergent T2 coherence time of the rotational transition even in the presence of inhomogeneous broadening. The dynamical evolution of a dense gas of rotational excitations shows regimes of nonergodicity, characteristic of many-body localization and localization protected quantum order.

We have explored the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the 'isosceles'-type distortion that has been extensively investigated. We show that suprisingly it is able to stabilize the 120° spin structure (typical of the undistorted triangular antiferromagnet) for a large range of exchange interaction values, with new structures found only for extreme distortions. A physical realization of this model is alpha-CaCr2O4. Despite its highly symmetric 120° spin structure, the magnetic excitation spectrum of alpha-CaCr2O4 is very complex. The unique pattern of nearest-neighbor exchange interactions as well as the substantial next-nearest-neighbor interactions place it close to the phase boundary of the 120° structure as is clearly revealed by the observation in neutron scattering of low energy modes acting as soft modes of the neighboring structure. Indeed, fitting to linear spin-wave theory favors a set of exchange parameters within the nearby multi-k phase in contradiction to the observed 120° order, and quantum fluctuations may be necessary to stabilize alpha-CaCr2O4 within the 120° phase. 

Sb2Te3 is a topological insulator material with a bulk band gap of 0.28 eV, and simple surface states consisting of a single Dirac cone existing in the band gap. The Landau quantization in Sb2Te3 is not sensitive to the intrinsic substitutional defects, so a linear energy dispersion of surface states can be achieved. Thus, Sb2Te3 nanowire provides an ideal system to elucidate the topological sate of matter. Despite recent research progress on topological insulators, the experimental study on Sb2Te3 is unexpectedly rare. The main reason is due to the relatively weak bonding between Sb and Te and the molecular nature of both Sb and Te beams, Sb vacancies and anti-site defects can easily form. Therefore, the as-grown Sb2T3 is heavily p-doped with its Fermi level lying in the bulk valence band. In our work, we have obtained high quality single crystalline Sb2T3 nanowires fabricated via a low pressure catalytic CVD method. SEM and TEM characterizations are performed to verify the Rhombohedral single crytalline structure of space group R3'm. To measure the intrinsic magnetoconduction, multi-electrodes are patterned via ebeam lithography to contact an individual nanowire on top of a dielectric gate electrode. The measurement had shown the Aharonov-Bohm oscillations under perpendicular magnetic field, and a weak antilocalization peak around B=0 T. 

Using angle-resolved photoemission, we studied (111) - oriented epitaxial films of Pb-Sn-Se grown by molecular beam epitaxy. The topological to trivial insulator phase transition [1] is monitored probing the bulk valence band as a function of Sn concentration and temperature between 30 K and room temperature. We have shown two types of Dirac cones centered at Gamma bar and M bar which originate from the band inversion at the L point in the bulk Brillouin zone distinct from the pair of Dirac-cone surface states near the of the surface Brillouin zone on the (001) surface [2]. We discuss the behavior of the band structure at the phase transition in comparison to the conventional Z2 type topological insulator protected by time reversal symmetry.


References
[1] P. Dziawa, B. J. Kowalski, K. Dybko, R. Buczko, A. Szczerbakow, M. Szot, E. L usakowska, T. Balasubramanian, B. M. Wojek, M. H. Berntsen, O. Tjernberg, T. Story, Nature Mat. 11, 1023 (2012).
[2]. Y. Tanaka, T. Shoman, K. Nakayama, S. Souma, T . Sato, T. Takahashi, M. Novak, Kouji Segawa, and Yoichi Ando PHYSICAL REVIEW B 88, 235126 (2013).

Unusual spin structure of double perovskite and re-entrant spin-glass (RSG) compound Lu2MnNiO6 has been investigated using conventional magnetometry and neutron diffraction. As system cools below the ferromagnetic transition temperature (TC), spins cant ferromagnetically with respect to the crystallographic ‘c’ axis, resulting novel critical behaviour. Comprehensive study of temperature dependent neutron diffraction and first principle-based computation divulges that octahedral tilting induced structural transition and on imprinting spin-orbit interaction results canted spin structure below TC, which might be the intrinsic scenario for all RSG compounds.

Fully gapped superconductors with dominant triplet pairing have been shown to be topologically non-trivial in two and three dimensions. Consequently, they allow for linearly dispersive surface-states with Majorana character protected by symmetry. In this study we look at the interplay between edge, bulk, and impurity states on the boundary of topological superconductors without inversion center and discuss their stability under strong disorder in finite lattices of two and three dimensions. We show that a delocalized state at exactly zero energy always exists from weak to strong disorder and study the localization transition of states within the superconducting gap. We see that ingap states are fully coupled and undergo a critical transition for disorder strengths comparable with the system’s bandwidth as the surface density of states is highly enhanced.

In a one-dimensional spinless p-wave superconductor with coherence length xi, disorder induces a phase transition between a topologically nontrivial phase and a trivial insulating phase at the critical mean free path l = xi/2. We show that a multichannel spinless p-wave superconductor goes through an alternation of topologically trivial and nontrivial phases upon increasing the disorder strength, the number of phase transitions being equal to the channel number N. The last phase transition, from a nontrivial phase into the trivial phase, takes place at a mean free path l = xi/(N + 1), parametrically smaller than the critical mean free path in one dimension. Our result is valid in the limit that the wire width W is much smaller than the superconducting coherence length xi. 

Studies of the response of multi-component materials to an ultrashort optical excitation attract attention due to the emergence of new transient states, which could have an impact on the final physical configuration of the system [1, 2]. Ferromagnets are doped to tailor properties such as exchange interactions [3]. Using time-resolved Magneto-Optical Kerr effect in the pump-probe configuration, we evaluate the ultrafast dynamics of two typical ferromagnets: in-plane magnetized Permalloy and perpendicularly magnetized FePt, as a function of the concentration of the dopant (Copper).

In both ferromagnets, the Curie temperature is observed to decrease nearly linearly as a function of the concentration of Copper [4], implying similar physics in the quasi-equilibrium case. However, in the transient femtosecond regime, the demagnetization dynamics are different and contradict expectations based on available theories and experiments.

The study of the transient ferromagnetic state in Cu-doped Permalloy reveals that the demagnetization dynamics gets progressively slower at higher copper concentrations, as would be expected from theories of demagnetization such as M3TM or the µT models [5, 6]. But an anomaly is seen as the concentration of Copper approaches 40%, where the demagnetization gets faster. The results could be explained as being caused due to the stronger suppression of Ni moments than of Fe moments with increased dopant concentration. On the other hand, Cu-doped FePt exhibits a transiently inverted hysteresis loop, which persists through 150 ps, even in the presence of an externally applied restoring magnetic field. 

Thus we demonstrate that there could be a vast difference in the non-equilibrium behavior of physical systems whose quasi-equilibrium behavior is similar.

References
[1] S. Kaiser et al, Phys. Rev. B. 89, 184516 (2014).
[2] S. Alebrand et al, Appl. Phys. Lett. 101, 162408 (2012)
[3] H. H. Hsieh et al, Phys. Rev. B 57, 15204 (1998).
[4] S. Mathias et al, Proc. Nat Acad. Sci 109, 4792 (2012)
[5] B. Koopmans et al, Nat. Mater. 9, 259 (2010)
[6] B. Y. Mueller and B. Rethfeld, arXiv:1403.6885 [cond-mat.mtrl-sci] (2014)

The knowledge of the dynamical response functions is of particular interest in view of experimental applications such as momentum resolved tunneling in quantum wires. However, neglecting curvature in the generic dispersion in 1D - resulting in Luttinger liquid theory - leads to incorrect results for singularities in the dynamical responses. Recently it has been shown that using a mapping to a Luttinger liquid with an additional high frequency mobile impurity and taking the leading irrelevant operators into account nonperturbatively it is possible to determine the exact threshold singularities. 
We present a constructive derivation of a mobile impurity model for the 1D Hubbard model.  Crucially, the Luther-Emery point for both charge and spin fermions constitutes the good starting point for a weak coupling expansion. We have numerically identified this special point in an extended Hubbard model.   

J. Morris, A. Schneidewind, K. Siemensmeyer, A. Tennant 

Suggestions by Conlon and Chalker (CC) [1] on the spin dynamics of a pyrochlore Heisenberg antiferromagnet have inspired this work where we investigate the dynamics of the frustrated fcc- magnet HoB12 with inelastic neutron diffraction in the paramagnetic state. Similar to the prediction of CC we can identify regions in q – space where the dynamics shows different behaviour in the paramagnetic phase: These are so called “pinch points” of the reciprocal lattice which show static spin correlation that diverge only in the limit T-> 0, second, on general lattice points and third along nodal lines that connect reciprocal lattice points. The data are compared with classical MonteCarlo simulations, followed by a simulation of the dynamic behaviour in the ordered [2] and the paramagnetic phase.
  
1. P.H. Conlon, J.T. Chalker; Phys. Rev. Lett. 102 (2009) 237206
2. K. Siemensmeyer et. al.; Jour. Low Temp. Phys. 146 (2007) 581

Quantum spin Hall insulators are characterized by topologically protected counter-propagating edge states. Here we study the dynamical response of these helical edge states under a time-dependent flux biasing, in presence of a heat bath. It is shown that the relaxation time of the edge carriers can be determined from a measurement of the dissipative response of topological insulator disks. The effects of various perturbations, including Zeeman coupling and disorder, are also discussed.

A neutron diffraction and NMR study of the field induced phases of linarite PbCuSO$_4$(OH)$_2$ is presented for temperatures down to 1.7,K and fields applied along the $b$ axis. A spin flop transition in two steps resulting into a collinear structure was observed. Furthermore, an extraordinary sine-wave modulated structure with magnetic moments parallel to the field direction was found, which encloses the other long-range ordered phases at higher fields and temperatures. This phase exhibits a shift of the propagation vector upon changing the magnetic field and is discussed in terms of a 3D spin density wave phase, which tentatively can be described with density waves of bound three-magnons.

Magnetism in rare earth quasicrystals presents many basic open questions despite the considerable research efforts since the discovery of this material class. Here we take a two-fold theoretical approach, with results that match much of the experimentally observed phenomenology. First, we compute RKKY interactions between localised moments, using tight binding models on two-dimensional quasiperiodic tilings. Second, we study the statistical mechanics of Ising spins coupled via these RKKY interactions. We find the emergence of strongly coupled spin clusters with significantly weaker inter-cluster coupling. Spins freeze in an apparently disordered configuration at low temperatures, but without evidence of the glassiness that is characteristic of conventional spin glasses.

We present numerical results for experimentally relevant finite-temperature spectral functions of one-dimensional strongly correlated quantum systems. As examples, we study the finite-temperature dynamic spin structure factor as well as the electron spin resonance (ESR) intensity of spin-1/2 XXZ Heisenberg chains in magnetic fields. The method is based on a purification of the finite-temperature density operator and works directly in the frequency domain by exploiting a Liouville space formulation of the dynamics. We implemented this via moment expansions of the Green's function in a density-matrix renormalization group (DMRG) framework using matrix product states.

Recently, realizations of chiral spin liquids related to the Laughlin
state were predicted in highly frustated spin models on the kagome lattice
such as model with up to third nearest neighbour Heisenberg interactions or scalar
chirality interactions. Using large scale exact diagonalization techniques, we confirmed their existence
and investigate the extent and stability of these phases. Moreover, we analyze the excitations in these models and relate them to the one of the Laughlin wavefunction, using Brillouin zone mappings developed in the context of Chern/topological insulators. While the chiral characteristic is unambiguously confirmed, the explanation of our results by the simple Kalmeyer-Laughlin picture turns out to be not completely satisfactory.

Recently the pyrochore magnets received an explosion of interests due to the novel phenomena resulting from the geometrical frustration of competing magnetic interactions within the corner-sharing tetrahedra lattice, such as the spin ice state in Dy/Ho titanates and stannates, and spin liquid phases in Tb2Ti2O7 and Er2Sn2O7.[1] Here we studied the magnetic properties and structure of the Ising-type Nd2Zr2O7 (NZO) and the Heisenberg-type Gd2Zr2O7 (GZO) rare earth zirconates by magnetization measurement and neutron diffraction. Polycrystalline and single crystal samples were prepared by using solid state reaction and optical floating technique, respectively. Both compounds exhibit a negative Curie-Weiss temperature (-5.7K for GZO) indicating antiferromagnetic interactions. NZO shows a local (111) easy axial anisotropy while GZO is isotropic evidenced by the magnetization measurements of single crystals at 1.8K. The Neutron diffraction experiments indicate that NZO forms an ‘all-in-all-out’ antiferromagnetic order while GZO remains disorder and only shows diffuse scattering at 0.05K. According to the previous heat capacity reports, both compounds order magnetically at 0.37K and 0.7K, respectively. [2-3] The disagreement for GZO may be because there is a certain degree of disordering between Gd and Zr sublattices as has been found commonly in those compounds.[4] Further investigations are in progress.
Reference:
[1] J. S. Gardner, et al. Reviews of Modern Physics 82, 53 (2010).
[2] S. Lutique, et al., The Journal of Chemical Thermodynamics 35, 955 (2003).
[3] M. D. Alice, et al, Journal of Physics: Condensed Matter 20, 235208 (2008).
[4] T. Moriga, et al, Solid State Ionics 31, 319 (1989).

Kitaev's compass model on the honeycomb lattice realizes a spin liquid whose emergent excitations are gapless Majorana fermions and static Z$_2$ gauge fluxes. We discuss the proper selection of physical states for finite-size simulations in the Majorana representation, based on a recent paper by Pedrocchi, Chesi, and Loss [prb {bf 84}, 165414 (2011)]. Physical observables acquire large finite-size effects, in particular if the ground state is {em not} fermion-free, which generically applies to the system with isotropic couplings.

To illustrate our findings, we compute the static and dynamic spin susceptibilities for finite-size systems. Specifically, we consider random-bond disorder which preserves the solubility of the model, calculate the distribution of local susceptibilities, and extract the NMR lineshape.