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arXiv2013

Non-trivial order parameter in Sr$_{2}$IrO$_{4}$

Electronic structure calculations obtained with an approach with density functional theory with an enhanced local Coulomb interaction, DFT+$U$, are presented for the relativistic magnetic insulator Sr$_{2}$IrO$_{4}$. The results are in accordance with experiments with a band gap and a small moment anti-ferromagnetic ground state. This solution is thereafter thoroughly analyzed in terms of Landau theory where it is found that the ordered magnetic moments only form a secondary order parameter while the primary order parameter is a higher order magnetic multipole of rank five. It is further observed that the electronic structure in the presence of this order parameter is related to the earlier proposed $j_\mathrm{eff}=1/2$ model, but in contrast to that model the present picture can naturally explain the small magnitude of the ordered magnetic moments.

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arXiv2013

Non-Fermi liquid behavior in Bose-Fermi mixtures at two dimensions

In this paper we study the low temperature behaviors of a system of Bose-Fermi mixtures at two dimensions. Within a self-consistent ladder diagram approximation, we show that at nonzero temperatures $T\rightarrow0$ the fermions exhibit non-fermi liquid behavior. We propose that this is a general feature of Bose-Fermi mixtures at two dimensions. An experimental signature of this new state is proposed.

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arXiv2013

Itinerant and high-energy localized single-particle spectra in large-U$_d$ high-Tc cuprates

A theory of the underlying metallic state of large-U$_d$ high-Tc cuprates is presented starting from the covalent Cu-2O 3-band model associated with a sufficiently small copper occupancy. U$_d=\infty$ is dealt by the slave fermion approach. Diagrammatic low order NCA theory in terms of the Cu-O hopping is supplemented with two independent slave particle chemical potentials which implement the U(1) local gauge invariance "at average", thus avoiding the mean-field approximations. The resulting hole spectra consist of itinerant and localized states. The itinerant states close to the Fermi level are coherent. They exhibit dichotomies, the copper/oxygen in real and the nodal/antinodal in reciprocal space. The localized states, related to random Cu-O "mixed valence" fluctuations within the CuO$_2$ unit cells, are incoherent and fall well away from the Fermi level. Consequently, Luttinger's band sum rule for the conduction band is broken. Comparison with IPES, ARPES and NQR data on NCCO, LSCO, Bi2201 and Bi2212 - in order of increasing correlations - leads to remarkable agreements considering that only three bare band parameters are involved.

cond-mat.supr-con↗

arXiv2013

Quasiparticle Theory of Resonant Inelastic X-ray Scattering in high-T$_c$ cuprates

We develop a formalism for calculating resonant inelastic x-ray scattering (RIXS) spectra in systems of itinerant electrons with arbitrary band structures, accounting for the effect of the positively-charged core hole exactly. We apply this formalism to the cuprate superconductors and obtain quantitative agreement with experimental data over a wide range of dopings. We reproduce the dispersing peaks and non-trivial polarization dependence found in RIXS experiments on several materials. Thus we demonstrate that features previously attributed to collective magnetic modes can be explained by band structure alone.

cond-mat.supr-con↗

arXiv2013

Features of the surface tension for the metal--insulator boundary in the vicinity of MI phase transition in the presence of external magnetic field

The self-consistent equations for MI phase transition are formulated. We assume two order parameters which describe the phase transition. The first one is the density distribution at MI boundary $ρ(\vec r)$. The second one is a two component complex vector in spin space $Ψ(\vec r)$. It determines electron density in metallic or semimetallic phase in the presence of external magnetic field. Two different components of the vector describe possible spin states of electrons inserted in the external magnetic field. The first order type MI phase transition determined by the variation of the density distribution is considered by means of the gradient expansion of Cahn and Hillard type \cite{CahnHillard}. The second order type transition of electron density beside MI boundary is described by Ginzburg -- Landau expansion \cite{LandLif2}. The interaction between these two parameters is assumed to be linear as a function of electron density with a coefficient which depends on metallic density (cf. \cite{JinwuYe_Lubensky}). The obtained nonlinear equations are exactly solved in the case of MI boundary in the presence of the parallel to the boundary or perpendicular to it uniform magnetic field. The surface tension $Σ_{mi}$ at the MI boundary is calculated. It is shown that $Σ_{mi}$ is singular. In particular, $Σ_{mi}\sim n^{3/2}$ as $ n\Rightarrow 0$ and $Σ_{mi}\sim (T-T_c (\vec h))^{3/2} .$ $T_c (\vec h)$ is the transition temperature in the presence of external magnetic field at MI phase transition. \par The singular behavior of $Σ_{mi}$ leads to an emphasized hysteresis at MI transition.

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arXiv2013

Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi

Symmetry or topology protected Dirac fermion states in two and three dimensions constitute novel quantum systems that exhibit exotic physical phenomena. However, none of the studied spin-orbit materials are suitable for realizing bulk multiplet Dirac states for the exploration of interacting Dirac physics. Here we present experimental evidence, for the first time, that the compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or overlap leads to a Lifshitz transition in momentum space - a condition for realizing interactions involving Dirac states. By carefully preparing the samples at a non-natural-cleavage (100) crystalline surface, we uncover many novel electronic and spin properties in Na3Bi by utilizing high resolution angle- and spin-resolved photoemission spectroscopy measurements. We observe two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone center point $Γ$, which exhibit a Fermi surface Lifshitz transition and a saddle point singularity. Furthermore, our data shows evidence for the possible existence of theoretically predicted weak 2D nontrivial spin-orbit surface state with helical spin polarization that are nestled between the two bulk Dirac cones, consistent with the theoretically calculated (100) surface-arc-modes. Our main experimental observation of a rich multiplet of Dirac structure and the Lifshitz transition opens the door for inducing electronic instabilities and correlated physical phenomena in Na3Bi, and paves the way for the engineering of novel topological states using Na3Bi predicted in recent theory.

cond-mat.mes-hall↗

arXiv2014

Surprise(s) in magnets without net moments

We are all familiar with ferromagnetic and antiferromagnetic materials, in which the localized ionic moments (in case of ionic insulators) or the electronic spins (in case of metals) go into a long-range ordered state with a net macroscopic moment (in case of ferromagnets) or a net macroscopic sublattice magnetization (in case of antiferromagnets). However this behaviour is far from ubiquitous even in ionic insulators with well-developed local moments. Indeed, there are many ionic insulators in which the dominant interactions between the local moments compete with each other, leading to a cooperative paramagnetic state with no ordering of the moments down to the lowest temperatures accessible to experiments. The physics of such magnets without net moments has some interesting and surprising aspects, which are touched upon in this brief review.

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arXiv2014

A Memory of Majorana Fermions through Quantum Quench

We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. The long-time evolution of the system and its topological characterization depend on a pseudomagnetic field ${\bs R}_{\mbox{\text eff}}(k)$, which connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the robustness of the Majorana zero-mode associated with the entanglement cut in the topologically nontrivial phase and identify the parameter space in which the mode can survive in the infinite-time limit.

cond-mat.mes-hall↗

arXiv2014

Attachment of Surface "Fermi Arcs" to the Bulk Fermi Surface: "Fermi-Level Plumbing" in Topological Metals

The role of "Fermi arc" surface-quasiparticle states in "topological metals" (where some Fermi surface sheets have non-zero Chern number) is examined. They act as "Fermi-level plumbing" conduits that transfer quasiparticles among groups of apparently-disconnected Fermi sheets with non-zero Chern numbers to maintain equality of their chemical potentials, which is required by gauge invariance. Fermi arcs have a chiral tangential attachment to the surface projections of sheets of the bulk Fermi Surface: the total Chern number of each projection equals the net chirality of arc-attachments to it. Information from the Fermi arcs is needed to unambiguously determine the quantized part of the anomalous Hall effect that is not determined at the bulk Fermi surface.

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arXiv2014

Experimental Discovery of Topological Surface States - A New Type of 2D Electron Systems (Review Article)

Topological Surface States (TSS) represent new types of two dimensional electron systems with novel and unprecedented properties distinct from any quantum Hall-like or spin-Hall effects. Their Z$_2$ topological order can be realized at room temperatures without magnetic fields and they can be turned into magnets, exotic superconductors or Kondo insulators leading to worldwide interest and activity in the topic. We review the basic concepts defining such topological matter and the experimental discovery via the key experimental probe that revealed the Z$_2$ topological order in the bulk of these spin-orbit interaction dominated insulators for the first time. This review focuses on the key results that demonstrated the fundamental Z$_2$ topological properties such as spin-momentum locking, non-trivial Berry's phases, mirror Chern number, absence of backscattering, bulk-boundary correspondence (topology), protection by time-reversal and other discrete (mirror) symmetries and their remarkable persistence up to the room temperature elaborating on results first briefly discussed in an early review by M.Z. Hasan and C.L. Kane [Rev. of Mod. Phys., 82, 3045 (2010)]. Additionally, key results on broken symmetry phases such as quantum magnetism and superconductivity induced in topological materials are briefly discussed. Topological insulators beyond the Z$_2$ classification such as Topological Crystalline Insulators (TCI) are discussed based on their spin properties (mirror Chern invariants). The experimental methodologies detailed here have been used in most of the subsequent studies of Z$_2$ topological physics in almost all bulk topological insulator materials to this date.

cond-mat.mes-hall↗

arXiv2014

Large enhancement of emergent magnetic fields in MnSi with impurities and pressure

We report a study of the topological Hall effect (THE) in Fe-doped MnSi and compare with results from pure MnSi under pressure. We find that Fe doping increases the THE, indicating an enhancement of the magnitude of the emergent gauge field. This is consistent with the concurrent reduction in the length scale of the skyrmion lattice. For both pressurized and doped samples, we calculate the emergent magnetic field based on the size of the measured THE, and compare it with a theoretical upper-bound. We find that the ratio of these two remains more or less constant with pressure or Fe doping, but differs greatly from that of pure MnSi at ambient pressure. We discuss the implications of this ratio with respect to trends in the saturated magnetic moment and helical pitch length as T_C \rightarrow 0 via doping and pressure, respectively.

cond-mat.str-el↗

arXiv2014

Conductance spectroscopy in ferromanget/superconductor hybrids

We present theoretical model for the proximity effect in F-SFF-F structures (F is ferromagnet, S is superconductor) with non-collinear magnetizations vectors in the F-layers and with arbitrary magnitudes of exchange fields. Electrical conductance of these structures is analyzed within the Keldysh-Usadel formalism in the diffusive regime as a function of a misorientation angle between magnetizations of the F-layers and transparencies of SF and FF interfaces. We show that long-range triplet superconducting correlations manifest themselves either as a zero-bias peak in the case of perfect transparency of FF interface, or as a two-peak structure in the case of finite transparency. The predicted features may serve as a diagnostic tool for characterization of interfaces in superconducting hybrid structures

cond-mat.supr-con↗

arXiv2014

Experimental Proposal to Detect Topological Ground State Degeneracy

One of the most profound features of topologically ordered states of matter, such as the fractional quantum Hall (FQH) states, is that they possess topology-dependent ground state degeneracies that are robust to all local perturbations. Here we propose to directly detect these topological degeneracies in an experimentally accessible setup. The detection scheme uses electrical conductance measurements in a double layer FQH system with appropriately patterned top and bottom gates. We discuss two experimental platforms; in the first, the detection of topologically degenerate states coincides with the detection of $Z_N$ parafermion zero modes. We map the relevant physics to a single-channel $Z_N$ quantum impurity model, providing a novel generalization of the Kondo model. Our proposal can also be adapted to detect the $Z_N$ parafermion zero modes recently discovered in FQH line junctions proximitized with superconductivity.

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arXiv2014

Long-lived nonequilibrium states in the Hubbard model with an electric field

We study the single-band Hubbard model in the presence of a large spatially uniform electric field out of equilibrium. Using the Keldysh nonequilibrium formalism, we solve the problem using perturbation theory in the Coulomb interaction U. We present numerical results for the charge current, the total energy of the system and the double occupancy on an infinite-dimensional hypercubic lattice with nearest-neighbor hopping. The system is isolated from an external bath and is in the paramagnetic state. We show that an electric field pulse can drive the system to a steady nonequilibrium state, which does not evolve into a thermal state. We compare results obtained within second-order perturbation theory (SOPT), self-consistent second-order perturbation theory (SCSOPT) and iterated perturbation theory (IPT). We also discuss the importance of initial conditions for a system which is not coupled to an external bath.

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arXiv2014

Layer Anti-Ferromagnetism on Bilayer Honeycomb Lattice

Bilayer honeycomb lattice, with inter-layer tunneling energy, has a parabolic dispersion relation, which causes the charge imbalance between two sublattices. Here, we investigate the metal-insulator and magnetic phase transitions on the strongly correlated bilayer honeycomb lattice by cellular dynamical mean-field theory combined with continuous time quantum Monte Carlo method. We find that different kinds of magnetic spontaneous symmetry breaking on dimer and non-dimer sites, cause a novel phase transition between normal anti-ferromagnet and layer anti-ferromagnet. We sketch the phase diagrams as the function of temperature, interaction and inter-layer hopping. Finally, we set up an experimental protocol for cold atoms in optical lattice to observe these phenomena in future experiments.

cond-mat.str-el↗

arXiv2014

Non-equilibrium noise in the (non-)Abelian fractional quantum Hall effect

We analyse the noise of the edge current of a generic fractional quantum Hall state in a tunnelling point contact system. We show that the non-symmetrized noise in the edge current for the system out-of-equilibrium is completely determined by the noise in the tunnelling current and the Nyquist-Johnson (equilibrium) noise of the edge current. Simply put, the noise in the tunnelling current does not simply add up the equilibrium noise of the edge current. A correction term arises associated with the correlation between the tunnelling current and the edge current. We show, using a non-equilibrium Ward identity, that this correction term is determined by the anti-symmetric part of the noise in the tunnelling current. This leads to a non-equilibrium fluctuation-dissipation theorem and related expressions for the excess and shot noise of the noise in the edge current. Our approach makes use of simple properties of the edge, such as charge conservation and chirality, and applies to generic constructions of the edge theory which includes edges of non-Abelian states and edges with multiple charged channels. Two important tools we make use of are the non-equilibrium Kubo formula and the non-equilibrium Ward identity. We discuss these identities in the appendix.

cond-mat.mes-hall↗

arXiv2014

Q-dependence of the spin fluctuations in the intermediate valence compound CePd3

We report inelastic neutron scattering experiments on a single crystal of the intermediate valence compound CePd3. At 300 K the magnetic scattering is quasielastic, with halfwidth of 23 meV, and is independent of momentum transfer Q. At low temperature, the Q-averaged magnetic spectrum is inelastic, exhibiting a broad peak centered near Emax = 55 meV. These results, together with the temperature dependence of the susceptibility, 4f occupation number, and specific heat, can be fit by the Kondo/Anderson impurity model. The low temperature scattering near Emax, however, shows significant variations with Q, reflecting the coherence of the 4f lattice. The intensity is maximal at (1/2, 1/2,0), intermediate at (1/2,0,0) and (0,0,0), and weak at (1/2,1/2,1/2). We discuss this Q-dependence in terms of current ideas about coherence in heavy fermion systems.

cond-mat.str-el↗

arXiv2014

Transport evidence of robust topological surface state in BiTeCl single crystals, the first strong inversion asymmetric topological insulator

Three-dimensional (3D) topological insulators (TIs) are new forms of quantum matter that are characterized by their insulating bulk state and exotic metallic surface state, which hosts helical Dirac fermions1-2. Very recently, BiTeCl, one of the polar semiconductors, has been discovered by angle-resolved photoemission spectroscopy to be the first strong inversion asymmetric topological insulator (SIATI). In contrast to the previously discovered 3D TIs with inversion symmetry, the SIATI are expected to exhibit novel topological phenomena, including crystalline-surface-dependent topological surface states, intrinsic topological p-n junctions, and pyroelectric and topological magneto-electric effects3. Here, we report the first transport evidence for the robust topological surface state in the SIATI BiTeCl via observation of Shubnikov-de Haas (SdH) oscillations, which exhibit the 2D nature of the Fermi surface and pi Berry phase. The n = 1 Landau quantization of the topological surface state is observed at B . 12 T without gating, and the Fermi level is only 58.8 meV above the Dirac point, which gives rise to small effective mass, 0.055me, and quite large mobility, 4490 cm2s-1. Our findings will pave the way for future transport exploration of other new topological phenomena and potential applications for strong inversion asymmetric topological insulators.

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