We consider the spin-1/2 antiferromagnetic Heisenberg model on a bilayer honeycomb lattice including interlayer frustration in the presence of an external magnetic field. In the vicinity of the saturation field, we map the low-energy states of this quantum system onto the spatial configurations of hard hexagons on a honeycomb lattice. As a result, we can construct effective classical models (lattice-gas as well as Ising models) on the honeycomb lattice to calculate the properties of the frustrated quantum Heisenberg spin system in the low-temperature regime. We perform classical Monte Carlo simulations for a hard-hexagon model and adopt known results for an Ising model to discuss the finite-temperature order-disorder phase transition that is driven by a magnetic field at low temperatures. We also discuss an effective-model description around the ideal frustration case and find indications for a spin-flop-like transition in the considered isotropic spin model.

We study Josephson junctions with weak links consisting of two parallel disordered arms with magnetic properties: ferromagnetic, half-metallic, or normal with magnetic impurities. In the case of long links, the Josephson effect is dominated by mesoscopic fluctuations. In this regime, the system realizes a ${\phi }_0$ junction with sample-specific ${\phi }_0$ and critical current. Cooper pair splitting between the two arms plays a major role and leads to $2{\mathrm{\Phi }}_0$ periodicity of the current as a function of flux between the arms. We calculate the current and its flux and polarization dependence for the three types of magnetic links.

We show that the interaction between a superconducting order parameter and the magnetic moment of an atomic cluster in a two-dimensional $s$-wave superconductor with Rashba spin-orbit coupling generates magnetic anisotropy that can be stronger or comparable to the magnetic anisotropy due to the crystal field and the shape of the cluster. Transport current through the superconductor produces the effective magnetic field acting on the cluster's magnetic moment. The direction of the effective field depends on the direction of the current, thus allowing one to manipulate the magnetic moment by the superconducting current. Due to the large density of the superconducting current this method of magnetization reversal can be more advantageous at low temperatures than the spin-transfer torque method that requires a large spin-polarized current through a normal metal.

Multiferroic orthorhombic $o-{\mathrm{LuMnO}}_3$ exhibits large ferroelectric polarization induced by an $E$-type magnetic order. Recently, the $E$-type magnetic phase in ${\mathrm{LuMnO}}_3$ was proposed to feature magnetic moments tilted away from the collinear ordering. We employed neutron diffraction to determine the symmetry of the magnetic order in $o-{\mathrm{LuMnO}}_3$. We observed that below $T_N=39$ K, the ${\mathrm{Mn}}^{3+}$ spins order into an incommensurate amplitude-modulated phase that obeys the Pbnm crystal symmetry and is paraelectric. The incommensurate phase locks into a commensurate phase at $T_C=35.5$ K described by a fully antiferromagnetic and noncollinear $E$-type order. This noncollinear $E$-type ordering breaks the spatial inversion symmetry and induces a spontaneous polarization at $T_C$. At $T=2$ K, an appreciably large electric polarization was observed similar to that of other orthorhombic manganites featuring $E$-type magnetic order. We also present a Pbnm symmetry-allowed Dzyaloshinskii-Moriya interaction that explains the noncollinear $E$-type order in the commensurate phase. These results are in qualitative agreement with the type of distortions from collinear $E$-type antiferromagnetic order found using Monte Carlo simulation for rare-earth manganites [M. Mochizuki et al., Phys. Rev. B 84, 144409 (2011)].

A powder inelastic neutron-scattering study of ${\mathrm{HoBaFe}}_4{\mathrm{O}}_7$ (HBFO) revealed characteristic magnetic excitations associated with geometrical spin frustration. Dispersionless excitations in energy ($\omega$) and wave-vector ($Q$) space are observed at several discrete energies. Some of them can be attributed to crystal-field excitations of ${\mathrm{Ho}}^{3+}$ in octahedral symmetry, but the others are explained instead by the vibronic state of the ${\mathrm{Fe}}^{2+}$ dynamical Jahn-Teller effect with orbit-spin coupling, indicating interaction among the spin, the orbital, and the lattice degree of freedom. The antiferromagnetic HBFO lattice has cubic symmetry, and both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ reside on corner-sharing tetrahedra with a number ratio of 3:1. Even though ${\mathrm{Fe}}^{2+}$ is a Jahn-Teller active ion in tetrahedral symmetry, the system does not exhibit any static lattice distortion to the lowest temperature studied (4 K). The observed excitations can be understood by considering the dynamical interaction among spin-orbital-lattice degrees of freedom, indicating that spin fluctuation due to the frustration effect induces the dynamical Jahn-Teller effect, although in most cases a Jahn-Teller active ion ${\mathrm{Fe}}^{2+}$ takes the static Jahn-Teller effect in a magnetic oxide system.

We present a low temperature scanning tunneling spectroscopy (STS) study of the superconducting properties of monolayers of $\mathrm{FeS}{\mathrm{e}}_{0.5}\mathrm{T}{\mathrm{e}}_{0.5}$ grown on the three-dimensional (3D) topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_{1.2}\mathrm{T}{\mathrm{e}}_{1.8}$. While the morphology and the overall transition temperature resembles those of similarly doped bulk crystals, we find a two-fold anisotropic $s$-wave gap function. The two-fold nature of the gap symmetry is evident from the Bogoliubov quasiparticle interference (QPI) pattern, which shows distinct $C_2$ symmetric scattering intensities. Spatially resolved spectroscopic data shows a strong inhomogeneity in the size and anisotropy strength of the energy gaps, which cannot be correlated merely to the local chemical disorder. Instead, we argue that the gap inhomogeneity emerges with a similar mechanism as in disordered superconductors. Our sample system provides an ideal platform to study unconventional superconductivity in close proximity to a topological insulator.

This paper exposes in a detailed manner the recent findings about the SU(2) scenario for the underdoped phase of the cuprate superconductors. The SU(2) symmetry is formulated as a rotation between the $d$-wave superconducting (SC) phase and a $d$-wave charge order. We define the operators responsible for the SU(2) rotations and we derive the nonlinear $\sigma$ model associated with it. In this framework, we demonstrate that SU(2) fluctuations are massless in finite portions of the Brillouin zone corresponding to the antinodal regions $\left(0,\pi \right)$ and $\left(\pi ,0\right)$. We argue that the presence of SU(2) fluctuations in the antinodal region leads to the opening of Fermi arcs around the Fermi surface and to the formation of the pseudogap. Moreover, we show that SU(2) fluctuations lead, in turn, to the emergence of a finite momentum SC order—or pair density wave (PDW)—and more importantly to a new kind of excitonic particle-hole pairs liquid, the resonant excitonic state (RES), which is made of patches of preformed particle-hole pairs with multiple momenta. When the RES liquid becomes critical, we demonstrate that electronic scattering through the critical modes leads to anomalous transport properties. This new finding can account for the strange metal (SM) phase at finite temperature, on the right-hand side of the SC dome, shedding light on another notoriously mysterious part of the phase diagram of the cuprates.

We study the surface Andreev bound states (SABSs) and quasiparticle tunneling spectroscopy of three-dimensional (3D) chiral superconductors by changing their surface (interface) misorientation angles. We obtain an analytical formula for the SABS energy dispersion of a general pair potential, for which an original $4\times 4$ BdG Hamiltonian can be reduced to two $2\times 2$ blocks. The resulting SABS for 3D chiral superconductors with a pair potential given by $k_z{(k_x+i{k}_y)}^{\nu }$ ($\nu =1,2$) has a complicated energy dispersion owing to the coexistence of both point and line nodes. We focus on the tunneling spectroscopy of this pairing in the presence of an applied magnetic field, which induces a Doppler shift in the quasiparticle spectra. In contrast to the previously known Doppler effect in unconventional superconductors, a zero-bias conductance dip can change into a zero-bias conductance peak owing to an external magnetic field. We also study SABSs and tunneling spectroscopy for possible pairing symmetries of ${\mathrm{UPt}}_3$. For this purpose, we extend a standard formula for the tunneling conductance of unconventional superconductor junctions to treat spin-triplet nonunitary pairings. Magnetotunneling spectroscopy, i.e., tunneling spectroscopy in the presence of a magnetic field, can serve as a guide to determine the pairing symmetry of this material.

We continue the study of the inverse participation ratios (IPRs) of the XXZ Heisenberg spin chain initiated by Stéphan, Furukawa, Misguich, and Pasquier (2009) and continued by Misguich, Pasquier, and Luck (2016) by focusing on the case of the XX Heisenberg spin chain. For the ground state, Stéphan et al. note that calculating the IPR is equivalent to Dyson's constant term ex-conjecture. We express the IPRs of excited states as an apparently new “discrete” Hall inner product. We analyze this inner product using the theory of symmetric functions (Jack polynomials, Schur polynomials, the standard Hall inner product, and ${\omega }_{q,t}$) to determine some exact expressions and asymptotics for IPRs. We show that IPRs can be indexed by partitions, and asymptotically the IPR of a partition is equal to that of the conjugate partition. We relate the IPRs to two other models from physics, namely, the circular symplectic ensemble of Dyson and the Dyson-Gaudin two-dimensional Coulomb lattice gas. Finally, we provide a description of the IPRs in terms of a signed count of diagonals of permutohedra.

We discuss the entanglement spectrum of the ground state of a (1+1)-dimensional system in a gapped phase near a quantum phase transition. In particular, in proximity to a quantum phase transition described by a conformal field theory (CFT), the system is represented by a gapped Lorentz invariant field theory in the “scaling limit” (correlation length $\xi$ much larger than microscopic “lattice” scale “$a$”), and can be thought of as a CFT perturbed by a relevant perturbation. We show that for such (1+1) gapped Lorentz invariant field theories in infinite space, the low-lying entanglement spectrum obtained by tracing out, say, left half-infinite space, is precisely equal to the physical spectrum of the unperturbed gapless, i.e., conformal field theory defined on a finite interval of length $L_{\xi }=ln(\xi /a)$ with certain boundary conditions. In particular, the low-lying entanglement spectrum of the gapped theory is the finite-size spectrum of a boundary conformal field theory, and is always discrete and universal. Each relevant perturbation, and thus each gapped phase in proximity to the quantum phase transition, maps into a particular boundary condition. A similar property has been known to hold for Baxter's corner transfer matrices in a very special class of fine-tuned, namely, integrable off-critical lattice models, for the entire entanglement spectrum and independent of the scaling limit. In contrast, our result applies to completely general gapped Lorentz invariant theories in the scaling limit, without the requirement of integrability, for the low-lying entanglement spectrum. While the entanglement spectrum of the ground state of a gapped theory on a finite interval of length $2R$ with suitable boundary conditions, bipartitioned into two equal pieces, turns out to exhibit a crossover between the finite-size spectra of the same CFT with in general different boundary conditions as the system size $R$ crosses the correlation length from the “critical regime'' $R\ll \xi$ to the “gapped regime'' $R\gg \xi$, the physical spectrum on a finite interval of length $R$ with the same boundary conditions, on the other hand, is known to undergo a dramatic reorganization during the same crossover from being discrete to being continuous.

Based on first-principles calculations, the electronic structure of ${\mathrm{CuTeO}}_4$ is discussed in the context of superconducting cuprates. Despite some significant crystallographic differences, we find that ${\mathrm{CuTeO}}_4$ is similar to these cuprates, exhibiting a quasi-two-dimensional electronic structure that involves hybridized Cu-$d$ and O-$p$ states in the vicinity of the Fermi level, along with an antiferromagnetic insulating ground state. Hole-doping this material by substituting ${\mathrm{Te}}^{6+}$ with ${\mathrm{Sb}}^{5+}$ would be of significant interest.

Our theoretical analysis reveals that a family of full-Heusler materials exhibit unique topological surface states with type-I and type-II Dirac quasiparticles. The type-I Dirac surface state is characterized by an enclosed Fermi surface, while the type-II Dirac surfaces occur at the touching of the electron and hole pockets. In addition, due to the layered nature of the full-Heusler crystals structured with a wide range of various elements, such structures induce multiple Dirac surface states with different Lifshitz transitions protected by more than one mirror plane.

We used femtosecond optical pump-probe spectroscopy to study the photoinduced change in reflectivity of thin films of the electron-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (LCCO) with dopings of $x=0.08$ (underdoped) and $x=0.11$ (optimally doped). Above $T_c$, we observe fluence-dependent relaxation rates that begin at a temperature similar to the one where transport measurements first show signatures of antiferromagnetic correlations. Upon suppressing superconductivity with a magnetic field, it is found that the fluence and temperature dependence of relaxation rates are consistent with bimolecular recombination of electrons and holes across a gap ($2{\mathrm{\Delta }}_{\mathrm{AF}}$) originating from antiferromagnetic correlations which comprise the pseudogap in electron-doped cuprates. This can be used to learn about coupling between electrons and high-energy ($\omega >2{\mathrm{\Delta }}_{\mathrm{AF}}$) excitations in these compounds and set limits on the time scales on which antiferromagnetic correlations are static.

We report on the electronic transport properties of black phosphorus and analyze them using a two-carrier model in a wide range of pressure up to 2.5 GPa. In a semiconducting state at 0.29 GPa, the remarkable nonlinear behavior in the Hall resistance is reasonably reproduced by assuming the coexistence of two kinds of holes with different densities and mobilities. On the other hand, two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest the coexistence of high-mobility electron and hole carriers that have almost the same densities, i.e., nearly compensated semimetallic nature of black phosphorus. In the semimetallic state, analyses of both the two-carrier model and quantum oscillations indicate a systematic increase in the carrier densities as pressure increases. An observed sign inversion of Hall resistivity at low magnetic fields suggests the existence of high-mobility electrons $\left(\sim {10}^5{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}\right)$, which is roughly ten times larger than that of holes, in the semimetallic black phosphorus. We conclude that the extremely large positive magnetoresistance that has been observed in the semimetallic state cannot be reproduced by a conventional two-carrier model.

Variations in exciton absorption resulting from charge accumulation in various semiconducting donor–acceptor (DA) copolymer thin films were systematically investigated by gate modulation (GM) spectroscopy by using the field-effect transistor device structure. The GM spectra obtained for high-mobility DA copolymer thin films exhibited second-derivative like line shapes due to an effect of spectral broadening of ordinary exciton absorption spectra by accumulated charges. In contrast, the GM spectra obtained for relatively low-mobility DA copolymer thin films exhibited simple bleaching of exciton absorption spectra, as well as observed for non-DA-type polymers like poly(3-hexylthiophene-2,5-diyl) (P3HT). From a systematic comparison of the GM spectra with temperature-dependent absorption spectra for the polymers in solution, we found that the spectral broadening observed in the GM spectra can be attributed to a reduced effect on the exchange narrowing where excitonic transitions of individual polymer chains are coherently coupled within highly ordered crystalline domains in the polymer thin films. We discuss that the gate-induced charge accumulation in the polymer films effects to suppress the exciton coherence length, which contributes to the reduced exchange narrowing. We also discuss that the whole feature of the GM spectra can be understood in terms of a decomposition into ordered and disordered polymers and that the GM spectra can be used as fine probes for a degree of structural ordering in semiconductor channels of polymer field-effect transistors.

We derive the diagram of the topological phases accessible within a generic Hamiltonian describing quantum anomalous Hall effect for photons and electrons in honeycomb lattices in the presence of a Zeeman field and spin-orbit coupling (SOC). The two cases differ crucially by the winding number of their SOC, which is 1 for the Rashba SOC of electrons, and 2 for the photon SOC induced by the energy splitting between the TE and TM modes. As a consequence, the two models exhibit opposite Chern numbers $\pm 2$ at low field. Moreover, the photonic system shows a topological transition absent in the electronic case. If the photonic states are mixed with excitonic resonances to form interacting exciton-polaritons, the effective Zeeman field can be induced and controlled by a circularly polarized pump. This new feature allows an all-optical control of the topological phase transitions.

The interplay of spin-polarized electronic edge states with the dynamics of the host nuclei in quantum Hall systems presents rich and nontrivial transport physics. Here, we develop a Landauer-Büttiker approach to understand various experimental features observed in the integer quantum Hall setups featuring quantum point contacts. The approach developed here entails a phenomenological description of spin-resolved interedge scattering induced via hyperfine assisted electron-nuclear spin flip-flop processes. A self-consistent simulation framework between the nuclear spin dynamics and edge state electronic transport is presented in order to gain crucial insights into the dynamic nuclear polarization effects on electronic transport and in turn the electron-spin polarization effects on the nuclear spin dynamics. In particular, we show that the hysteresis noted experimentally in the conductance-voltage trace as well as in the resistively detected NMR line-shape results from a lack of quasiequilibrium between electronic transport and nuclear polarization evolution. In addition, we present circuit models to emulate such hyperfine mediated transport effects to further facilitate a clear understanding of the electronic transport processes occurring around the quantum point contact. Finally, we extend our model to account for the effects of quadrupolar splitting of nuclear levels and also depict the electronic transport signatures that arise from single and multiphoton processes.

The phenomenon of spin transfer torque (STT) has attracted a great deal of interest due to its promising prospects in practical spintronic devices. In this paper, we report a theoretical investigation of STT in a noncollinear magnetic tunnel junction under ac modulation based on the nonequilibrium Green's-function formalism, and we derive a closed formulation for predicting the time-averaged STT. Using this formulation, the ac STT of a carbon-nanotube-based magnetic tunnel junction is analyzed. Under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque on bias still holds, and the $sin\theta$ dependence on the noncollinear angle is maintained. By photon-assisted tunneling, the bias-induced components of the in-plane and out-of-plane torques can be enhanced significantly, about 12 and 75 times, respectively. Our analysis reveals the condition for achieving optimized STT enhancement and suggests that ac modulation is a very effective way for electrical manipulation of STT.

The computation of transport coefficients, even in linear response, is a major challenge for theoretical methods that rely on analytic continuation of correlation functions obtained numerically in Matsubara space. While maximum entropy methods can be used for certain correlation functions, this is not possible in general, important examples being the Seebeck, Hall, Nernst, and Reggi-Leduc coefficients. Indeed, positivity of the spectral weight on the positive real-frequency axis is not guaranteed in these cases. The spectral weight can even be complex in the presence of broken time-reversal symmetry. Various workarounds, such as the neglect of vertex corrections or the study of the infinite frequency or Kelvin limits, have been proposed. Here, we show that one can define auxiliary response functions that allow one to extract the desired real-frequency susceptibilities from maximum entropy methods in the most general multiorbital cases with no particular symmetry. As a benchmark case, we study the longitudinal thermoelectric response and corresponding Onsager coefficient in the single-band two-dimensional Hubbard model treated with dynamical mean-field theory and continuous-time quantum Monte Carlo. We thereby extend the maximum entropy analytic continuation with auxiliary functions (MaxEntAux method), developed for the study of the superconducting pairing dynamics of correlated materials, to transport coefficients.