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arXiv2014

Anomalies of discrete symmetries in various dimensions and group cohomology

We study 't Hooft anomalies for discrete global symmetries in bosonic theories in 2, 3 and 4 dimensions. We show that such anomalies may arise in gauge theories with topological terms in the action, if the total symmetry group is a nontrivial extension of the global symmetry by the gauge symmetry. Sometimes the 't Hooft anomaly for a d-dimensional theory with a global symmetry G can be canceled by anomaly inflow from a (d+1)-dimensional topological gauge theory with gauge group G. Such d-dimensional theories can live on the surfaces of Symmetry Protected Topological Phases. We also give examples of theories with more severe 't Hooft anomalies which cannot be canceled in this way.

hep-th↗

arXiv2014

First-principles study of Exchange Interaction in Ising-type Multiferroic Ca$_3$CoMnO$_6$

We perform first-principles calculations of multiferroic Ca$_3$CoMnO$_6$ and evaluate the exchange coupling constants using the Green's function method. We clarify the effect of intra-chain and inter-chain exchange interactions on magnetic stability. We find that inter-chain exchange coupling constants are antiferromagnetic and that there are geometrical frustrations in the triangular lattices of magnetic chains in Ca$_3$CoMnO$_6$. The magnetic transition temperature is evaluated using effective Hamiltonian with calculated exchange coupling constants. We obtain the transition temperature 5.80K. The value has the same order as that of experimentally observed.

cond-mat.str-el↗

arXiv2014

Magnetic nano-fluctuations in a frustrated magnet

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.

cond-mat.str-el↗

arXiv2014

Rayleigh scattering in coupled microcavities: Experiment

We experimentally analyze Rayleigh scattering in coupled planar microcavities. We show that the correlations of the disorder in the two cavities lead to inter-branch scattering of polaritons, that would otherwise be forbidden by symmetry. These longitudinal correlations can be inferred from the strength of the inter-branch scattering.

cond-mat.str-el↗

arXiv2014

Transport evidence for the coexistence of the topological surface state and a two-dimensional electron gas in BiSbTe3 topological insulator

Topological insulators (TIs) are new insulating materials with exotic surface states, where the motion of charge carriers is described by the Dirac equations and their spins are locked in a perpendicular direction to their momentum. Recent studies by angle-resolved photoemission spectroscopy have demonstrated that a conventional two-dimensional electron gas can coexist with the topological surface state due to the quantum confinement effect. The coexistence is expected to give rise to exotic transport properties, which, however, have not been explored so far. Here, we report a magneto-transport study on single crystals of the topological insulator BiSbTe3. Besides Shubnikov-de Haas oscillations and weak anti-localization (WAL) from the topological surface state, we also observed a crossover from the weak anti-localization to weak localization (WL) with increasing magnetic field, which is temperature dependent and exhibits two-dimensional features. The crossover is proposed to be the transport manifestation of the coexistence of the topological surface state and two-dimensional electron gas on the surface of TIs.

cond-mat.mes-hall↗

arXiv2014

On the informational completeness of local observables

For a general multipartite quantum state, we formulate a locally checkable condition, under which the expectation values of certain nonlocal observables are completely determined by the expectation values of some local observables. The condition is satisfied by ground states of gapped quantum many-body systems in two spatial dimensions, assuming a widely conjectured form of area law is correct. Its implications on the studies of gapped quantum many-body systems, quantum state tomography, and quantum state verification are discussed. These results are based on a partial characterization of states with small yet nonzero conditional mutual information, which may be of independent interest.

quant-ph↗

arXiv2014

Surface vs bulk electronic structures of a moderately correlated topological insulator YbB6 revealed by ARPES

Systematic angle-resolved photoemission spectroscopy (ARPES) experiments have been carried out to investigate the bulk and (100) surface electronic structures of a topological mixed-valence insulator candidate, YbB6. The bulk states of YbB6 were probed with bulk-sensitive soft X-ray ARPES, which show strong three-dimensionality as required by cubic symmetry. Surprisingly the measured Yb 4f states are located around 1 eV and 2.3 eV below the Fermi level (EF), instead of being near EF as indicated by first principle calculations. The dispersive bands near EF are B 2p states, which hybridize with the 4f states. Using surface-sensitive vacuum-ultraviolet ARPES, we revealed two-dimensional surface states which form three electron-like Fermi surfaces (FSs) with Dirac-cone-like dispersions. The odd number of surface FSs gives the first indication that YbB6 is a moderately correlated topological insulator. The spin-resolved ARPES measurements provide further evidence that these surface states are spin polarized with spin locked to the crystal momentum. We have observed a second set of surface states with different topology (hole-like pockets). Clear folding of the bands suggest their origin from a 1X2 reconstructed surface. The topological property of the reconstructed surface states needs further studies.

cond-mat.str-el↗

arXiv2014

Axial quadrupole phase of a uniaxial spin-1 magnet

Uniaxial phase of the axial quadrupole spin-1 magnet in an external magnetic field is investigated. A case where the system includes single-ion anisotropy and anisotropic biquadratic exchange interaction the most general form, is considered. It is proved, that the molecular field approximation of the relative magnetization does not depend on temperature and increases linearly with the external magnetic field. Two branches of the spin excitation spectrum are derived. Condition of mitigation of the spectrum defines the boundary between the axial quadrupole and angular phases. The critical temperature of the corresponding phase transition depends considerably on the anisotropy constants of the biquadratic exchange interaction.

cond-mat.str-el↗

arXiv2014

Critical behavior of the energy gap and its relation with the Berry phase close to the excited state quantum phase transition in the Lipkin model

In our previous work [Phys. Rev. A \textbf{85}, 044102 (2012)], we have studied the Berry phase of the ground state and exited states in the Lipkin model. In this paper, using Hellmann-Feynman theorem, we derive the relation between the energy gap and the Berry phase close to the excited state quantum phase transition (ESQPT) in the Lipkin model. We find that the energy gap is linearly dependent upon the Berry phase close to the ESQPT. As a result, the critical behavior of the energy gap is similar to that of the Berry phase. In addition, we also perform a semiclassical qualitative analysis about the critical behavior of the energy gap.

cond-mat.str-el↗

arXiv2014

Accurate variational electronic structure calculations with the density matrix renormalization group

During the past 15 years, the density matrix renormalization group (DMRG) has become increasingly important for ab initio quantum chemistry. The underlying matrix product state (MPS) ansatz is a low-rank decomposition of the full configuration interaction tensor. The virtual dimension of the MPS controls the size of the corner of the many-body Hilbert space that can be reached. Whereas the MPS ansatz will only yield an efficient description for noncritical one-dimensional systems, it can still be used as a variational ansatz for other finite-size systems. Rather large virtual dimensions are then required. The two most important aspects to reduce the corresponding computational cost are a proper choice and ordering of the active space orbitals, and the exploitation of the symmetry group of the Hamiltonian. By taking care of both aspects, DMRG becomes an efficient replacement for exact diagonalization in quantum chemistry. DMRG and Hartree-Fock theory have an analogous structure. The former can be interpreted as a self-consistent mean-field theory in the DMRG lattice sites, and the latter in the particles. It is possible to build upon this analogy to introduce post-DMRG methods. Based on an approximate MPS, these methods provide improved ansätze for the ground state, as well as for excitations. Exponentiation of the single-particle (single-site) excitations for a Slater determinant (an MPS with open boundary conditions) leads to the Thouless theorem for Hartree-Fock theory (DMRG), an explicit nonredundant parameterization of the entire manifold of Slater determinants (MPS wavefunctions). This gives rise to the configuration interaction expansion for DMRG. The Hubbard-Stratonovich transformation lies at the basis of auxiliary field quantum Monte Carlo for Slater determinants. An analogous transformation for spin-lattice Hamiltonians allows to formulate a promising variant for MPSs.

cond-mat.str-el↗

arXiv2014

Certain General Constraints on the Many-Body Localization Transition

Isolated quantum systems at strong disorder can display many-body localization (MBL), a remarkable phenomena characterized by an absence of conduction even at finite temperatures. As the ratio of interactions to disorder is increased, one expects that an MBL phase will eventually undergo a dynamical phase transition to a delocalized phase. Here we constrain the nature of such a transition by exploiting the strong subadditivity of entanglement entropy, as applied to the many-body eigenstates close to the transition in general dimensions. In particular, we show that at a putative continuous transition between an MBL and an ergodic delocalized phase, the critical eigenstates are necessarily thermal, and therefore, the critical entanglement entropy equals the thermal entropy. We also explore a qualitatively different continuous localization-delocalization transition, where the delocalized phase is non-ergodic whose volume law entanglement entropy tends to zero as the transition is approached.

cond-mat.dis-nn↗

arXiv2014

Spin structure factor and quantum phases of frustrated spin-1/2 chains

The static structure factor S(q) of frustrated spin-1/2 chains with isotropic exchange and a singlet ground state (GS) diverges at wave vector q_m when the GS has quasi-long-range order (QLRO) with periodicity 2π/q_m but S(q_m) is finite in bond-order-wave (BOW) phases with finite-range spin correlations. Exact diagonalization and density matrix renormalization group (DMRG) calculations of S(q) indicate a decoupled phase with QLRO and q_m = π/2 in chains with large antiferromagnetic exchange between second neighbors. S(q_m) identifies quantum phase transitions based on GS spin correlations.

cond-mat.str-el↗

arXiv2014

Correlation of magneto-electric coupling with crystal structure in Dy doped hexagonal YMnO3

Coexistence of mutually influencing ferroelectric and magnetic ordering at low temperature in hexagonal phase of rare-earth manganites gives rise to interesting physical phenomena. Here, we report a detailed analysis of powder neutron diffraction (ND) studies on polycrystalline Y1-xDyxMnO3 (x = 0, 0.05) as a function of temperature. Our analysis shows that Dy doping at Y site leads to modification in lattice constants: unit cell shrinks in c direction, but expands in ab plane, unit cell volume increases and effective Mn magnetic moment also reduces. All the Mn-O bond lengths and O-Mn-O bond angles shows significant variations leading to reduction in tilting of MnO5 polyhedra. Dielectric measurements on these samples show that, due to Dy doping, inverse S-shape anomaly shifts to the lower temperature side and magnetoelectric coupling decreases. Magnetic measurements confirm the decrease in the frustration of Mn triangular lattice. Temperature dependent specific heat measurement shows anomaly at Néel temperature (TN) and results indicate a decrease in entropy due to Dy doping. These results are understood in terms of modification in magnetoelectric coupling triggered due to Dy induced strain in Y plane of YMnO3.

cond-mat.mtrl-sci↗

arXiv2014

Mapping the first order magnetic transition in Yb$_2$Ti$_2$O$_7$

The very nature of the ground state of the pyrochlore compound Yb$_2$Ti$_2$O$_7$ is much debated, as experimental results demonstrate evidence for both a disordered or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a Quantum Spin Liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first order ferromagnetic transition at 245 mK and 150 mK in a powder and a single crystal sample respectively. Its first-order character is preserved up to applied fields of $\sim$ 200 Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for Yb$_2$Ti$_2$O$_7$.

cond-mat.str-el↗

arXiv2014

Exciton Condensates in Double Quantum Wells: A Laboratory for nontrivial topologies and unconventional superconductivity

It is shown that exciton condensates exhibit an incredible richness in the role that fundamental symmetries play whether manifest or broken. We investigate the appearance of the singlet and the triplet excitonic condensates under spin-orbit coupling (SOC) and consider realistic couplings to the radiation field as manipulated by the fundamental symmetries related to spin, time, orbital and particle degrees of freedom. The role of the strong SOC in inducing a ground state with coexisting singlet and triplet components is shown. It is also shown that the ground state of an excitonic condensate, in the realistic senario of spontaneous coupling to the radiation field, is dominated by a dark component transparent to photoluminescence.

cond-mat.str-el↗

arXiv2014

Magneto-electric operators in neutron scattering

We succeed in deriving an exact expression for the magnetic interaction of neutrons and electrons including magneto-electric operators, allowed in the absence of a centre of inversion symmetry. Central characters are a spin anapole and an orbital (toroidal) analogue, in addition to familiar parity-even operators. A simulation of neutron diffraction by antiferromagnetic copper oxide makes full use of information inferred from a thorough investigation with resonant x-ray Bragg diffraction.

cond-mat.str-el↗

arXiv2014

Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4)

A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tensors, are absent from the so-called jeff = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr2IrO4. The construct encompasses the jeff = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr2IrO4.

cond-mat.str-el↗

arXiv2014

A Phenomenological Theory of Loop-Current Phases

A phenomenological theory of the loop-current and loop-spin-current phases is proposed. In order to investigate the stability of these phases, a Ginzburg-Landau-Wilson type action is constructed as a functional of the orbital magnetization. From the analysis of this action based on the Landau theory and momentum-shell one-loop renormalization group theory, it is found that the loop-current and loop-spin-current phases are stable if a certain interaction between the orbital magnetizations is sufficiently large. Moreover, these phases are likely to be stable in systems with large orbital susceptibility, for example, Dirac electron systems.

cond-mat.str-el↗