The appearance of topological effects in systems exhibiting a nontrivial topological band structure strongly relies on the coherent wave nature of the equations of motion. Here, we reveal topological dynamics in a classical stochastic random walk version of the Su-Schrieffer-Heeger model with no relation to coherent wave dynamics. We explain that the commonly used topological invariant in the momentum space translates into an invariant in a counting-field space. This invariant gives rise to clear signatures of the topological phase in an associated escape time distribution.

Lattice deformations couple to the low-energy electronic excitations of graphene as vector fields similar to the electromagnetic potential. The observation of strain-induced pseudo Landau levels with scanning tunnel microscopy experiments has been one of the most exciting events in the history of graphene. Nevertheless, the experimental observation presents some ambiguities. Similar strain patterns show different images that are sometimes difficult to interpret. In this Rapid Communication, we show that, for some strain configurations, the deformation potential acts as a parallel electric field able to destabilize the Landau level structure via a mechanism identical to that occurring for real electromagnetic fields. This effect also alters the estimations of the value of the pseudomagnetic field, which can be significantly bigger. The mechanism applies equally if the electric field has an external origin, which opens the door to an electric control of giant pseudomagnetic fields in graphene.

The effects of Fe dopants on the electronic bands structure of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\mathrm{OH}$)FeSe are investigated by a band unfolding ($k$-projection) technique and first-principles supercell calculations. Doping 20% Fe into the LiOH layers causes electron donation to the FeSe layers, significantly changing the profile of bands around the Fermi level. Because of the weak bonding between the LiOH and FeSe layers the magnetic configuration of the dopants has only minor effects on the band structure. The electronic bands for the surface FeSe layer of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}\mathrm{OH}$)FeSe show noticeable differences compared to those of the inner layers, both in the location of the Fermi level and in details of the bands near the high symmetry points, resulting from different effective doping levels and the broken symmetry at the surface. The band structure for the surface FeSe layer with checkerboard antiferromagnetic order is reasonably consistent with angle-resolved photoemission results. The $3d$ transition metals Mn and Co have similar doping effects on the band structure of (LiOH)FeSe.

We study optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{0.92}{\mathrm{Y}}_{0.08}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) using angle-resolved two-photon photoemission spectroscopy. Three spectral features are resolved near 1.5, 2.7, and 3.6 eV above the Fermi level. By tuning the photon energy, we determine that the 2.7-eV feature arises predominantly from unoccupied states. The 1.5- and 3.6-eV features reflect unoccupied states whose spectral intensities are strongly modulated by the corresponding occupied states. These unoccupied states are consistent with the prediction from a cluster perturbation theory based on the single-band Hubbard model. Through this comparison, a Coulomb interaction strength $U$ of 2.7 eV is extracted. Our study complements equilibrium photoemission spectroscopy and provides a direct spectroscopic measurement of the unoccupied states in cuprates. The determined Coulomb $U$ indicates that the charge-transfer gap of optimally doped Bi2212 is 1.1 eV.

We have investigated the electronic structure of ${\mathrm{Na}}_x{\mathrm{Ca}}_{1-x}{\mathrm{Cr}}_2{\mathrm{O}}_4$ using x-ray absorption spectroscopy together with Anderson impurity model calculations. We show ${\mathrm{Na}}_x{\mathrm{Ca}}_{1-x}{\mathrm{Cr}}_2{\mathrm{O}}_4$ takes an unusual mixed-valence electronic state in which the positive charge-transfer (CT) and the negative CT states coexist. While ${\mathrm{CaCr}}_2{\mathrm{O}}_4$ (one end member) exhibits a typical CT nature with strong covalent character, Na substitution causes a negative CT state with an oxygen hole. In ${\mathrm{NaCr}}_2{\mathrm{O}}_4$ (the other end member), positive CT and negative CT states coexist with equal weight. This unusual electronic state is in sharp contrast to the conventional mixed-valence description in which the ground state can be described by the mixture of ${\mathrm{Cr}}^{3+}$ ($3d^3$) and ${\mathrm{Cr}}^{4+}$ ($3d^2$).

Two electrons in a quantum dot repel each other: their interaction can be characterized by a positive interaction energy. From the theory of superconductivity, we also know that mechanical vibrations of the crystal lattice can make the electron-electron interaction attractive. Analogously, if a quantum dot interacts with a mechanical degree of freedom, the effective interaction energy can be negative; that is, the electron-electron interaction might be attractive. In this work, we propose and theoretically study an engineered electromechanical system that exhibits electron-electron attraction: a quantum dot suspended on a nonlinear mechanical resonator, tuned by a bottom and a top gate electrode. We focus on the example of a dot embedded in a suspended graphene ribbon, for which we identify conditions for electron-electron attraction. Our results suggest the possibility of electronic transport via tunneling of packets of multiple electrons in such devices, similar to that in superconducting nanostructures, but without the use of any superconducting elements.

Nonlinear electrostatic interaction between the surface ions of electrochemical nature and ferroelectric dipoles gives rise to the coupled ferroionic states in nanoscale ferroelectrics. Here, we investigate the role of the surface ion formation energy on the polarization states and its reversal mechanisms, domain structure, and corresponding phase diagrams of ferroelectric thin films. Using 3D finite element modeling, we analyze the distribution and hysteresis loops of ferroelectric polarization and ionic charge, and the dynamics of the domain states. These calculations performed over large parameter space delineate the regions of single- and polydomain ferroelectric, ferroionic, antiferroionic, and nonferroelectric states as a function of surface ion formation energy, film thickness, applied voltage, and temperature. We further map the analytical theory for 1D systems onto an effective Landau-Ginzburg free energy and establish the correspondence between the 3D numerical and 1D analytical results. This approach allows us to perform an overview of the ferroionic system phase diagrams and explore the specifics of polarization reversal and domain evolution phenomena.

We study the effect of spin-orbit interaction on one-dimensional U(1)-invariant frustrated magnets with dominant critical nematic fluctuations. The spin-orbit coupling explicitly breaks the U(1) symmetry of arbitrary global spin rotations about the high-symmetry axis down to $Z_2$ (invariance under a $\pi$ rotation). Given that the nematic order parameter is invariant under a $\pi$ rotation, it is relevant to ask if other discrete symmetries can be spontaneously broken. Here we demonstrate that the spin-orbit coupling induces a bond density wave that spontaneously breaks the translational symmetry and opens a gap in the excitation spectrum.

If, during the preparation, an external magnetic field is applied upon cooling we say it has been field cooled. A novel mechanism for insulator-metal transition and superconductivity in field-cooled spin-$1/2$ antiferromagnets on bcc lattice is discussed. Applying a magnetic field along the sublattice B magnetization, we change the magnetic and transport properties of the material. There is a critical value $H_{\text{cr}1}$. When the magnetic field is below the critical one $H<H_{\text{cr}1}$ the prepared material is a spin-$1/2$ antiferromagnetic insulator. When $H>H_{\text{cr}1}$ the sublattice A electrons are delocalized and the material is metal. There is a second critical value $H_{\text{cr}2}>H_{\text{cr}1}$. When $H=H_{\text{cr}2}$, it is shown that the Zeeman splitting of the sublattice A electrons is zero and they do not contribute to the magnetization of the system. At this quantum partial order point (QPOP) the sublattice B transversal spin fluctuations (magnons) interact with sublattice A electrons inducing spin antiparallel $p$-wave superconductivity which coexists with magnetism. At zero temperature the magnetic moment of sublattice B electrons is maximal. Below the Néel temperature $\left(T_N\right)$ the gap is approximately constant with a small increase when the system approaches $T_N$. It abruptly falls down to zero at temperatures above $T_N$.

We study a spinor (two-component) Bose gas confined in a one-dimensional double-valley optical lattice which has a double-well structure in momentum space. Based on field theory analysis, it is found that spinor bosons in the double-valley band may form a spin-charge mixed chiral spin quasicondensate under certain conditions. Our numerical calculations in a concrete $\pi$-flux triangular ladder system confirm the robustness of the chiral spin order against interactions and quantum fluctuations. This exotic atomic Bose-Einstein condensate exhibits spatially staggered spin loop currents without any charge dynamics despite the complete absence of spin-orbit coupling in the system, creating an interesting approach to atom spintronics. The entanglement entropy scaling allows us to extract conformal-field-theory central charge and establish the low-energy effective field theory for the chiral spin condensate as a two-component Luttinger liquid. Our predictions should be detectable in atomic experiments through spin-resolved time-of-flight techniques.

Magnon bound states emerging in one-dimensional (1D) spin chains still lack a rigorous understanding. In this Rapid Communication we show that the length-1 spin strings significantly dominate the critical properties of spinons, magnons, and free fermions in the 1D antiferromagnetic spin-1/2 chain. Using the Bethe ansatz solution, we analytically calculate the scaling functions of the thermal and magnetic properties of the model, providing a rigorous understanding of the quantum criticality of spinons. It turns out that the double maxima in specific heat elegantly mark two crossover temperatures fanning out from the critical point, indicating three quantum phases: the Tomonaga-Luttinger liquid (TLL), the quantum critical, and fully polarized ferromagnetic phases. For the TLL phase, the Wilson ratio $R_W=4K_s$ remains almost temperature independent, where $K_s$ is the Luttinger parameter. Furthermore, by applying our results, we precisely determine the quantum scalings and critical exponents of all magnetic properties in the ideal 1D spin-1/2 antiferromagnet $\mathrm{Cu}\left(\mathrm{C}{}_4\mathrm{H}{}_4\mathrm{N}{}_2\right)\left(\mathrm{NO}{}_3\right){}_2$, recently studied by Kono et al. [Phys. Rev. Lett. 114, 037202 (2015)]. We further find that the magnetization maximum used in experiments is not a good quantity to map out the finite-temperature TLL phase boundary.

Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the time scale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order-of-magnitude reduction in quasiparticle loss for a given injected quasiparticle power.

For most glass-forming liquids, the temperature dependence of viscosity is non-Arrhenius. Despite the technological and geological importance, the origin of this non-Arrhenius temperature dependence of viscosity remains elusive to date and constitutes an important but unsolved problem in condensed-matter physics. It has become increasingly clear in recent years that high-temperature elasticity and viscosity of glass-forming liquids are strongly correlated. This work proposes a modified elastic model to predict equilibrium viscosity of glass-forming liquids. The modified elastic model considers the configurational entropy as a factor controlling the activation energy for viscous flow in addition to the high-frequency shear modulus as in the Dyre shoving model. It works much better than the shoving model in fitting equilibrium viscosity for both strong and fragile systems. The modified model also has the capability to estimate the nonequilibrium isostructural viscosity of glass from the equilibrium viscosity and the temperature-dependent elasticity of the glassy state.

We studied the effect of electric field (EF) on the coercivity of two Pt/Co structures with different Co layer thicknesses. Opposite signs of the coercivity change by the gate voltage application are observed in these samples, whereas the sign of the magnetic anisotropy change is the same. We performed direct observations of the domain structures during magnetization reversal under the gate voltage and found that two samples showed significant differences in the reversal process and EF effect. This result indicates that the sign reversal of the electric field effect on coercivity observed in the present structures originates from the difference in the macroscopic magnetization reversal process.

There exists a significant challenge in developing efficient magnetic tunnel junctions with low write currents for nonvolatile memory devices. With the aim of analyzing potential materials for efficient current-operated magnetic junctions, we have developed a multi-scale methodology combining ab initio calculations of spin-transfer torque with large-scale time-dependent simulations using atomistic spin dynamics. In this work we introduce our multiscale approach, including a discussion on a number of possible schemes for mapping the ab initio spin torques into the spin dynamics. We demonstrate this methodology on a prototype Co/MgO/Co/Cu tunnel junction showing that the spin torques are primarily acting at the interface between the Co free layer and MgO. Using spin dynamics we then calculate the reversal switching times for the free layer and the critical voltages and currents required for such switching. Our work provides an efficient, accurate, and versatile framework for designing novel current-operated magnetic devices, where all the materials details are taken into account.

We calculate the full counting statistics (FCS) of a subsystem energy in free fermionic systems by means of the Grassmann variables. We demonstrate that the generating function of these systems can be written as a determinant formula with respect to the Hamiltonian couplings and by using the Bell's polynomials, we derive exact formulas for the subsystem energy moments. In addition, we discuss the same quantities in the quantum $XY$ spin chain, and we demonstrate that at the critical regimes the fluctuations of the energy moments decay like a power law as we expect from the conformal field theory arguments, while in noncritical regimes, the decay is exponential. Furthermore, we discuss the full counting statistics of subsystem energy in the quantum $XX$ chain.

We derive functional flow equations for the two-particle vertex and the self-energy in interacting fermion systems which capture the full frequency dependence of both quantities. The equations are applied to the hole-doped two-dimensional Hubbard model as a prototype system with entangled magnetic, charge, and pairing fluctuations. Each fluctuation channel acquires substantial dependencies on all three Matsubara frequencies, such that the frequency dependence of the vertex cannot be accurately represented by a channel sum with only one frequency variable in each term. At the temperatures we are able to access, the leading instabilities are mostly antiferromagnetic, with an incommensurate wave vector. However, at large doping, a divergence in the charge channel occurs at a finite frequency transfer, if the vertex flow is computed without self-energy feedback. This enigmatic instability was already observed in a calculation by Husemann et al. [C. Husemann, K.-U. Giering, and M. Salmhofer, Phys. Rev. B 85, 075121 (2012)], who used an approximate separable ansatz for the frequency dependence of the vertex. We identify a simple mechanism for this instability in terms of a random-phase approximation for the charge channel with a frequency dependent effective magnetic interaction as input. In spite of the strong momentum and frequency dependence of the vertex, the self-energy has a Fermi-liquid form. At the moderate interaction strength where our approach is applicable, we obtain a moderate reduction of the quasiparticle weight and a sizable decay rate with a pronounced momentum dependence. Nevertheless, the self-energy feedback into the vertex flow turns out to be crucial, as it suppresses the unphysical finite frequency charge instability.

Electric injection and detection of spin-polarized electrons in InSb semiconductors have been realized in nonlocal experimental geometry using an InSb-based “lateral spin valve.” The valve of the $\mathrm{InSb}/\mathrm{MgO}/\mathrm{C}{\mathrm{o}}_{0.9}\mathrm{F}{\mathrm{e}}_{0.1}$ composition has semiconductor/insulator/ferromagnet nanoheterojunctions in which the thickness of the InSb layer considerably exceeded the spin diffusion length of conduction electrons. The spin direction in spin diffusion current has been manipulated by a magnetic field under the Hanle effect conditions. The spin polarization of the electron gas has been registered using ferromagnetic $\mathrm{C}{\mathrm{o}}_{0.9}\mathrm{F}{\mathrm{e}}_{0.1}$ probes by measuring electrical potentials arising in the probes in accordance with the Johnson-Silsbee concept of the spin-charge coupling. The developed theory is valid at any degree of degeneracy of electron gas in a semiconductor. The spin relaxation time and spin diffusion length of conduction electrons in InSb have been determined, and the electron-spin polarization in InSb has been evaluated for electrons injected from $\mathrm{C}{\mathrm{o}}_{0.9}\mathrm{F}{\mathrm{e}}_{0.1}$ through an MgO tunnel barrier.