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arXiv2013

Two distinct spin liquid states in a layered cubic lattice

We construct a family of short-range resonating-valence-bond wave functions on a layered cubic lattice, allowing for a tunable anisotropy in the amplitudes assigned to nearest-neighbour valence bonds along one axis. Monte Carlo simulations reveal that four phases are stabilized over the full range of the anisotropy parameter. They are separated from one another by a sequence of continuous quantum phase transitions. An antiferromagnetic phase, centred on the perfect isotropy point, intervenes between two distinct quantum spin liquid states. One of them is continuously deformable to the two-dimensional U(1) spin liquid, which is known to exhibit critical bond correlations. The other has both spin and bond correlations that decay exponentially. The existence of this second phase is proof that, contrary to expectations, neither a bipartite lattice structure nor a conventional Marshall sign rule is an impediment to realizing a fully gapped quantum spin liquid.

cond-mat.str-el↗

arXiv2013

Magnetism in nanoparticle LaCoO3

LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural properties were examined using SQUID magnetometery and neutron diffraction. The nanoparticles exhibit ferromagnetic long-range order beginning at T_C approximately 87K that persists to low temperatures. This behavior is contrasted with the ferromagnetism of bulk LCO, which also starts at T_C approximately 87K but is suppressed below a second transition at T_o approximately 37K, due to a structural phase transition. The ferromagnetism in both systems is attributed to the tensile stress from particle surfaces and impurity phase interfaces. This stress locally increases the Co-O-Co bond angle gamma, and competes with the thermal contraction of the lattice. It has recently been shown that LCO loses long-range ferromagnetic order when gamma decreases below the critical value gamma_c = 162.8 degrees. Consistent with this model, we show that gamma in nanoparticles remains larger than gamma_c at low temperatures, likely a consequence of all spins being in close proximity to surfaces or interfaces.

cond-mat.str-el↗

arXiv2013

Low-Frequency Quantum Oscillations due to Strong Electron Correlations

The normal-state energy spectrum of the two-dimensional $t$-$J$ model in a homogeneous perpendicular magnetic field is investigated. The density of states at the Fermi level as a function of the inverse magnetic field $\frac{1}{B}$ reveals oscillations in the range of hole concentrations $0.08<x<0.18$. The oscillations have both high- and low-frequency components. The former components are connected with large Fermi surfaces, while the latter with van Hove singularities in the Landau subbands, which traverse the Fermi level with changing $B$. The singularities are related to bending the Landau subbands due to strong electron correlations. Frequencies of these components are of the same order of magnitude as quantum oscillation frequencies observed in underdoped cuprates.

cond-mat.str-el↗

arXiv2013

Hybrid Monte-Carlo simulation of interacting tight-binding model of graphene

In this work, results are presented of Hybrid-Monte-Carlo simulations of the tight-binding Hamiltonian of graphene, coupled to an instantaneous long-range two-body potential which is modeled by a Hubbard-Stratonovich auxiliary field. We present an investigation of the spontaneous breaking of the sublattice symmetry, which corresponds to a phase transition from a conducting to an insulating phase and which occurs when the effective fine-structure constant $α$ of the system crosses above a certain threshold $α_C$. Qualitative comparisons to earlier works on the subject (which used larger system sizes and higher statistics) are made and it is established that $α_C$ is of a plausible magnitude in our simulations. Also, we discuss differences between simulations using compact and non-compact variants of the Hubbard field and present a quantitative comparison of distinct discretization schemes of the Euclidean time-like dimension in the Fermion operator.

hep-lat↗

arXiv2013

Long-range string orders and topological quantum phase transitions in the one-dimensional quantum compass model

In order to investigate the quantum phase transition in the one-dimensional quantum compass model, we numerically calculate non-local string correlations, entanglement entropy, and fidelity per lattice site by using the infinite matrix product state representation with the infinite time evolving block decimation method. In the whole range of the interaction parameters, we find that the four distinct string orders characterize the four different Haldane phases and the topological quantum phase transition occurs between the Haldane phases. The critical exponents of the string order parameters β=1/8 and the cental charges c=1/2 at the critical points show that the topological phase transitions between the phases belong to an Ising type of universality classes. In addition to the string order parameters, the singularities of the second derivative of the ground state energies per site, the continuous and singular behaviors of the von Neumann entropy, and the pinch points of the fidelity per lattice site manifest that the phase transitions between the phases are of the second-order, in contrast to the first-order transition suggested in pervious studies.

cond-mat.str-el↗

arXiv2013

Imaging the Nanoscale Band Structure of Topological Sb

Many promising building blocks of future electronic technology - including non-stoichiometric compounds, strongly correlated oxides, and strained or patterned films - are inhomogeneous on the nanometer length scale. Exploiting the inhomogeneity of such materials to design next-generation nanodevices requires a band structure probe with nanoscale spatial resolution. To address this demand, we report the first simultaneous observation and quantitative reconciliation of two candidate probes - Landau level spectroscopy and quasiparticle interference imaging - which we employ here to reconstruct the multi-component surface state band structure of the topological semimetal antimony(Sb). We thus establish the technique of band structure tunneling microscopy (BSTM), whose unique advantages include nanoscale access to non-rigid band structure deformation, empty state dispersion, and magnetic field dependent states. We use BSTM to elucidate the relationship between bulk conductivity and surface state robustness in topological materials, and to quantify essential metrics for spintronics applications.

cond-mat.mes-hall↗

arXiv2013

Topological lattice field theories from intertwiner dynamics

We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.

gr-qc↗

arXiv2013

Monte-Carlo simulation of graphene in terms of occupation numbers for the excitonic order parameter at hexagonal lattice

We present the results of the Monte-Carlo simulation of graphene-like statistical model in terms of occupation numbers. We study the problem of the phase transition in graphene to an insulating phase. Only antiferromagnetic order parameter was studied at the moment by means of Hybrid Monte-Carlo process on the hexagonal lattice because of the sign problem for excitonic order parameter in fermionic determinant. Therefore we have studied the possibility of the phase transition to an excitonic phase by means of simplified Monte-Carlo process on the hexagonal lattice. We show that this phase, which corresponds to a charge polarization of graphene sublattices, can appear at some values of Coulomb electron-electron potentials. In this work we study the dependence of the phase transition characteristics on the strength of on-site electron-electron interaction. We show that excitonic phase transition is also impossible in suspended graphene.

cond-mat.str-el↗

arXiv2013

Magnetic and Dielectric Properties in Multiferroic Cu3Mo2O9 under High Magnetic Fields

The magnetic and dielectric properties under high magnetic fields are studied in the single crystal of Cu3Mo2O9. This multiferroic compound has distorted tetrahedral spin chains. The effects of the quasi-one dimensionality and the geometrical spin frustration are expected to appear simultaneously. We measure the magnetoelectric current and the differential magnetization under the pulsed magnetic field up to 74 T. We also measure the electric polarization versus the electric field curve/loop under the static field up to 23 T. Dielectric properties change at the magnetic fields where the magnetization jumps are observed in the magnetization curve. Moreover, the magnetization plateaus are found at high magnetic fields.

cond-mat.str-el↗

arXiv2013

Spin-orbit coupling induced semi-metallic state in the 1/3 hole doped hyper-kagome Na3Ir3O8

The complex iridium oxide Na3Ir3O8 with a B-site ordered spinel structure was synthesized in single crystalline form, where the chiral hyper-kagome lattice of Ir atoms, as observed in the spin-liquid candidate Na4Ir3O8, was identified. The average valence of Ir is 4.33+ and, therefore, Na3Ir3O8 can be viewed as a doped analogue of the hyper-kagome spin liquid with Ir4+. The transport measurements showed that Na3Ir3O8 is in fact a semi-metal. The electronic structure calculation demonstrated that the strong spin-orbit coupling of Ir yields the semi-metallic state out of an otherwise band insulating state, which may harbor exotic topological effects embedded in the hyper-kagome lattice.

cond-mat.str-el↗

arXiv2013

Fractional Quantum Hall Matrix Product States For Interacting Conformal Field Theories

Using truncated conformal field theory (CFT), we present the formalism necessary to obtain exact matrix product state (MPS) representations for any fractional quantum hall model state which can be written as an expectation value of primary fields in some conformal field theory. The auxiliary bond space is the Hilbert space of the descendants of primary fields and matrix elements can be calculated using well-known CFT commutation relations. We obtain a site-independent MPS representation on the thin annulus and cylinder, while a site-dependent representation is possible on the disk and sphere geometries. We discuss the complications that arise due to the presence of null vectors in the theory's Hilbert space, and present an array of procedures which optimize the numerical calculation of matrix elements. By truncating the Hilbert space of the CFT, we then build an approximate MPS representation for Laughlin, Moore-Read, Read-Rezayi, Gaffnian, minimal $M(3,r+2)$ models, and a series of superconformal minimal models. We show how to obtain, in the MPS representation, the Orbital, Particle, and Real Space entanglement spectrum in both the finite and infinite systems.

cond-mat.str-el↗

arXiv2013

Experimental Electronic Structure of the Metallic Pyrochlore Iridate Bi2Ir2O7

Angle-resolved photoemission measurements have been performed on Bi2Ir2O7 single crystals, a prototypical example of the pyrochlore iridates. The density of states, the Fermi surface, and the near Fermi level band dispersion in the plane perpendicular to the (111) direction were all measured and found to be in overall agreement with our LDA + SOC density functional calculations. Our observations indicate the general validity of the LDA + SOC-based approach for the electronic structure of pyrochlore iridates, raising the possibility that some of the novel predicted phases such as quantum spin ice or Weyl Fermion states may exist in this family of compounds.

cond-mat.str-el↗

arXiv2013

A DFT+Nonhomogeneous DMFT approach for Finite Systems

For reliable and efficient inclusion of electron-electron correlation effects in nanosystems we propose a combined density-functional-theory/nonhomogeneous dynamical-mean-field-theory (DFT + DMFT) approach which employs an approximate Iterative Perturbative Theory (IPT) impurity solver. The validity of the method is demonstrated by successful examination of the size-dependent magnetic properties of iron nanoparticles containing 11-100 atoms. We show that the DFT+ DMFT solution is in very good agreement with experimental data, in particular it does not lead to the overestimation of magnetization that is found with the DFT and DFT+U techniques. More importantly, we demonstrate that DFT+DMFT approach can be used for accurate and realistic description of nanosystems containing about hundred atoms.

cond-mat.str-el↗

arXiv2013

Structural variations and magnetic properties of the quantum antiferromagnets Cs2CuCl4-xBrx

Depending on the crystal growth conditions, an orthorhombic (O-type) or a tetragonal (T-type) structure can be found in the solid solution Cs2CuCl4-xBrx (0 < x < 4). Here we present measurements of the temperature-dependent magnetic susceptibility and isothermal magnetization on the T-type compounds x = 1.6 and 1.8 and compare these results with the magnetic properties recently derived for the O-type variant by Cong et al., Phys. Rev. B 83, 064425 (2011). The systems were found to exhibit quite dissimilar magnetic properties which can be assigned to differences in the Cu coordination in these two structural variants. Whereas the tetragonal compounds can be classified as quasi-2D ferromagnets, characterized by ferromagnetic layers with a weak antiferromagnetic inter-layer coupling, the orthorhombic materials, notably the border compounds x = 0 and 4, are model systems for frustrated 2D Heisenberg antiferromagnets

cond-mat.str-el↗

arXiv2013

Phase structure of topological insulators by lattice strong-coupling expansion

The effect of the strong electron correlation on the topological phase structure of 2-dimensional (2D) and 3D topological insulators is investigated, in terms of lattice gauge theory. The effective model for noninteracting system is constructed similarly to the lattice fermions with the Wilson term, corresponding to the spin-orbit coupling. Introducing the electron-electron interaction as the coupling to the gauge field, we analyze the behavior of emergent orders by the strong coupling expansion methods. We show that there appears a new phase with the in-plane antiferromagnetic order in the 2D topological insulator, which is similar to the so-called "Aoki phase" in lattice QCD with Wilson fermions. In the 3D case, on the other hand, there does not appear such a new phase, and the electron correlation results in the shift of the phase boundary between the topological phase and the normal phase.

cond-mat.str-el↗

arXiv2013

Topological insulator states in quasi-one-dimensional sawtooth chain

We report the discovery of flatten Bloch bands with nontrivial topological numbers in a quasi-one-dimensional sawtooth chain. We present the nearly flat-band with a obvious gap and nonzero Chern number of the modulated sawtooth chain by tuning hoppings and modulation. With the increasing of the strength of the modulation, the system undergos a series of topological phase transitions in the absence of interaction. By adding interaction to the model, we present exact diagonalization results for the system at $1/3$ filling and some interesting connections with fraction quantum Hall states: topological degeneracy, nontrivially topological number and fractional statistic of the quasihole.

cond-mat.str-el↗

arXiv2013

Magnetic properties of single crystalline CeMg$_{12}$

Single crystal of CeMg$_{12}$ is obtained by Bridgman method. CeMg$_{12}$ crystallizes in the tetragonal structure with space group $I4/mmm$ (#139). The Laue pattern confirms the tetragonal crystal structure of CeMg$_{12}$. We have studied the magnetic properties by measuring magnetic susceptibility, magnetization, heat capacity and electrical transport. Specific heat measurement shows that the compound orders at 1.2 K. We have measured the magnetic susceptibility in the temperature range 1.8 to 300 K, and the susceptibility data show that [100] crystallographic direction is the easy axis of magnetization. At high temperature inverse susceptibility varies linearly with temperature, and follows the Curie-Weiss behaviour. The effective moment is close to the free ion value thus indicating Ce is in trivalent state in this compound.

cond-mat.str-el↗

arXiv2013

Complex $φ^4$ Theory at Finite Temperature and Density via Extended Mean Field Theory

We review the Extended Mean Field (EMFT) approximation and apply it to complex, scalar $φ^4$ theory on the lattice. We determine the ($T,μ$) phase diagram and study the critical properties of the Bose condensation transition, at zero and finite temperature. In both cases we obtain results which agree very well with recent Monte Carlo studies. Within our approximation we can reach the thermodynamic limit and can thus obtain results at lattice spacings unreachable by present Monte Carlo simulations. We find that our approximation reproduces accurately many phenomena of the model, like the "Silver Blaze" behavior at zero temperature and dimensional reduction at finite temperature.

hep-lat↗