We consider a small itinerant ferromagnet exposed to an external magnetic field and strongly driven by a thermally induced spin current. For this model, we derive the quasiclassical equations of motion for the magnetization where the effects of a dynamical nonequilibrium distribution function are taken into account self-consistently. We obtain the Landau-Lifshitz-Gilbert equation supplemented by a spin-transfer torque term of Slonczewski form. We identify a regime of persistent precessions in which we find an enhancement of the thermoelectric current by the pumping current.

A fundamental pillar of quantum mechanics concerns indistinguishable quantum particles. In three dimensions they may be classified into fermions or bosons, having, respectively, antisymmetric or symmetric wave functions under particle exchange. One of the numerous manifestations of this quantum statistics is the tendency of fermions (bosons) to antibunch (bunch). In a two-particle scattering experiment with two possible outgoing channels [C. K. Hong, Z. Y. Ou, and L. Mandel, Phys. Rev. Lett. 59, 2044 (1987)], the probability of the two particles to arrive each at a different terminal is enhanced (with respect to classical particles) for fermions, and reduced for bosons. Here we show that by entangling the particles with an external degree of freedom, we can engineer quantum statistical transmutation, which may cause fermions to bunch. Our analysis may have consequences on the observed fractional statistics of anyons, including non-Abelian statistics, with serious implications for quantum computing operations in the presence of external degrees of freedom.

In our theoretical study in which we combine a nonequilibrium Green's function approach with density functional theory (DFT) we investigate compounds containing a ferrocene moiety which is connected to (i) thiol anchor groups on both sides in a para-connection and (ii) a pyridyl anchor group on one side in a meta-connection and a thiol group on the other side in a para-connection, in both cases with acetylenic spacers in between the ferrocene and the anchors. We predict possible single-molecule junction geometries within a scanning tunneling microscopy (STM) setup, where we find that the conductance trend for the set of conformations is intriguing in the sense that the conductance does not decrease while the junction length increases, which we analyze and explain in terms of the Fermi level alignment. We also find a pattern for the current-voltage ($I-V$) curves within the linear-response regime for both molecules we study, where the conductance variation with the molecular configurations is surprisingly small.

Reduced electron screening in two dimensions plays a fundamental role in determining exciton properties, which dictates optoelectronic and photonic device performances. Considering the explicit electron-hole interaction within the $GW$ plus Bethe-Salpeter equation (BSE) formalism, we first study the excitonic properties of pristine phosphorene and investigate the effects of strain and impurity coverage. The calculations reveal strongly bound excitons in these systems with anisotropic spatial delocalization. Further, we present a simplified hydrogenic model with anisotropic exciton mass and effective electron screening as parameters, and the corresponding results are in excellent agreement with the present $GW$-BSE calculations. The simplified model is then used to investigate exciton renormalization in few-layer and heterostructure phosphorene. The changes in carrier effective mass along with increasing electron screening renormalize the exciton binding in these systems. We establish that the present model, in which the parameters are calculated within computationally less expensive first-principles calculations, can predict exciton properties with excellent accuracy for larger two-dimensional systems, for which the many-body $GW$-BSE calculations are impossible.

The Casimir pressure is investigated in three-layer systems where the intervening stratum possesses magnetic properties. This subject is gaining in importance in connection with ferrofluids and their use in various microelectromechanical devices. We present general formalism of the Lifshitz theory adapted to the case of ferrofluid sandwiched between two dielectric plates (walls). The Casimir pressure is computed for the cases of kerosene- and water-based ferrofluids containing a $5\%$ fraction of magnetite nanoparticles with different diameters between silica glass walls. For this purpose, we have found the dielectric permittivities of magnetite and kerosene along the imaginary frequency axis employing the available optical data and used the familiar dielectric properties of silica glass and water, as well as the magnetic properties of magnetite. We have also computed the relative difference in the magnitudes of the Casimir pressure which arises on addition of magnetite nanoparticles to pure carrier liquids. It is shown that for nanoparticles of 20 nm diameter at 2 $\mu \mathrm{m}$ separation between the walls this relative difference exceeds $140\%$ and $25\%$ for kerosene- and water-based ferrofluids, respectively. An interesting effect is found that at a fixed separation between the walls an addition of magnetite nanoparticles with some definite diameter makes no impact on the Casimir pressure. The physical explanation for this effect is provided. Possible applications of the obtained results are discussed.

A quenched-in state of thermal equilibrium (at 723 K) in a single crystal of Cr-Fe-Co-Ni close to equal atomic percent was studied. Atom probe tomography revealed a single-phase state with no signs of long-range order. The presence of short-range order (SRO) was established by diffuse x-ray scattering exploiting the variation in scattering contrast close to the absorption edges of the constituents: At the incoming photon energies of 5969, 7092, and 8313 eV, SRO maxima that result from the linear superposition of the six partial SRO scattering patterns, were always found at $X$ position. Electronic structure calculations showed that this type of maximum stems from the strong Cr-Ni and Cr-Co pair correlations, that are furthermore connected with the largest scattering contrast at 5969 eV. The calculated effective pair interaction parameters revealed an order-disorder transition at approximately 500 K to a $L{1}_2$-type (Fe,Co,Ni)${}_3\mathrm{Cr}$ structure. The calculated magnetic exchange interactions were dominantly of the antiferromagnetic type between Cr and any other alloy component and ferromagnetic between Fe, Co, and Ni. They yielded a Curie temperature ($T_{\mathrm{C}}$) of 120 K, close to experimental findings. Despite the low value of $T_{\mathrm{C}}$, the global magnetic state strongly affects chemical and elastic interactions in this system. In particular, it significantly increases the ordering tendency in the ferromagnetic state compared to the paramagnetic one.

We observed surface acoustic wave (SAW) propagation on a multiferroic material ${\mathrm{CuB}}_2{\mathrm{O}}_4$ with use of two interdigital transducers (IDTs). The period of IDT fingers is as short as 1.6 $\mu \mathrm{m}$ so that the frequency of SAW is 3 GHz, which is comparable with that of magnetic resonance. In the antiferromagnetic phase, the SAW excitation intensity varied with the magnitude and direction of the magnetic field, owing to the dynamical coupling between SAWs and antiferromagnetic resonance of ${\mathrm{CuB}}_2{\mathrm{O}}_4$. The microscopic mechanism is discussed based on the symmetrically allowed magnetoelastic coupling.

Pairing of $\pi$ electronic state structures with functional or metallic atoms makes them possible to engineer physical and chemical properties. Herein, we predict the reorientation of magnetization of Co on hexagonal BN ($h\text{−}\mathrm{BN}$) and graphene multilayers. The driving mechanism is the formation of the tetrahedral bonding between $s{p}^3$ and $d$ orbitals at the interface. More specifically, the intrinsic $\pi$ bonding of $h\text{−}\mathrm{BN}$ and graphene is transformed to $s{p}^3$ as a result of strong hybridization with metallic $d_{z^2}$ orbital. The different features of these two tetrahedral bondings, $s{p}^2$ and $s{p}^3$, are well manifested in charge density and density of states in the vicinity of the interface, along with associated band structure near the $\overline{K}$ valley. Our findings provide an approach to tailoring magnetism by means of degree of the interlayer hybrid bonds in two-dimensional layered materials.

The effect of anharmonic vibrational contributions to the finite-temperature pressure-driven B1-B2 structural phase transition of LiH is studied by using the stochastic self-consistent harmonic approximation method in combination with ab initio density functional theory and the quasiharmonic approximation. Contrary to previous experimental results based on multiple-shock compression, we find that the B1-B2 transition pressure is not significantly reduced at high temperatures. Moreover, we find that the B2 phase is dynamically unstable at low temperatures within harmonic theory in a wide range of pressures where its enthalpy is lower than that of the B1 phase, and the inclusion of anharmonic effects stabilizes the B2 phase in this pressure range. Our results imply that a third, yet unknown phase must exist in the phase diagram of LiH, in addition to the B1 and B2 phases, in order to explain the shock compression result.

The magnetic structure of the quasi-two-dimensional antiferromagnet ${\mathrm{Rb}}_{1-x}{\mathrm{K}}_x\mathrm{Fe}{\left({\mathrm{MoO}}_4\right)}_2$ with weakly distorted triangular lattice was studied with ${}^{87}\mathrm{Rb}$ NMR. The samples with $0\le x\le 0.15$ were studied in the field region near $H_{\mathrm{sat}}/3$, where quantum and thermal fluctuations play a decisive role in the formation of the magnetic structure of undoped frustrated triangular $\mathrm{RbFe}{\left({\mathrm{MoO}}_4\right)}_2$. Line-shape analysis reveals that in samples with $x=0,0.025$, and 0.075 the magnetic structure is stabilized by the thermal fluctuations, whereas in the sample with $x=0.15$ the magnetic structure with short-range order within each individual triangular plane and interplanar disorder is realized. The short-range static correlations in this two-dimensional ordered state are in agreement with a theoretical prediction for the $XY$ triangular-lattice antiferromagnet with nonmagnetic impurities by Maryasin and Zhitomirsky [Phys. Rev. Lett. 111, 247201 (2013)].

Recent experimental reports suggested the existence of a finite-temperature insulator in the vicinity of the superconductor-insulator transition. The rapid decay of conductivity over a narrow temperature range was theoretically linked to both a finite-temperature transition to a many-body-localized state, and to a charge-Berezinskii-Kosterlitz-Thouless transition. Here we report of low-frequency noise measurements of such insulators to test for many-body localization. We observed a huge enhancement of the low-temperatures noise when exceeding a threshold voltage for nonlinear conductivity and discuss our results in light of the theoretical models.

Kitaev's model of spins interacting on a honeycomb lattice describes a quantum spin liquid, where an emergent static ${\mathbb{Z}}_2$ gauge field is coupled to Majorana fermions. In the presence of an external magnetic field and for a range of interaction strengths, the system behaves as a gapped, non-Abelian quantum spin liquid. In this phase, the vortex excitations of the emergent ${\mathbb{Z}}_2$ gauge field have Majorana zero modes bound to them. Motivated by recent experimental progress in measuring and characterizing real materials that could exhibit spin-liquid behavior, we analytically calculate the dynamical spin structure factor in the non-Abelian phase of the Kitaev's honeycomb model. In particular, we treat the case of quenched disorder in the vortex configurations. Our calculations reveal a peak in the low-energy dynamical structure factor that is a signature of the spin-liquid behavior. We map the effective Hamiltonian to that of a chiral $p$-wave superconductor by using the Jordan-Wigner transformation. Subsequently, we analytically calculate the wave functions of the Majorana zero modes, the energy splitting for finite separation of the vortices and, finally, the dynamical structure factor in presence of quenched disorder.

We report on a reoptimization of the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential dedicated to the prediction of the band gaps of three-dimensional (3D) and layered hybrid organic-inorganic perovskites (HOPs) within pseudopotential-based density functional theory methods. These materials hold promise for future photovoltaic and optoelectronic applications. We begin by determining a set of parameters for 3D HOPs optimized over a large range of materials. Then we consider the case of layered HOPs. We design an empirical relationship that facilitates the prediction of band gaps of layered HOPs with arbitrary interlayer molecular spacers with a computational cost considerably lower than that of more advanced methods like hybrid functionals or $GW$. Our study also shows that substituting interlayer molecular chains of layered HOPs with Cs atoms is an appealing and cost-effective route to band gap calculations. Finally, we discuss the pitfalls and limitations of TB-mBJ for HOPs, notably its tendency to overestimate the effective masses due to the narrowing of the band dispersions. We expect our results to extend the use of TB-mBJ for other low-dimensional materials.

The development of intrinsic vacancies in SnSe single crystals was investigated as a function of annealing temperature by means of positron annihilation spectroscopy accompanied by transport measurements. It has been demonstrated that two types of vacancies are present in single-crystalline SnSe. While Sn vacancies dominate in the low-temperature region, Se vacancies and vacancy clusters govern the high-temperature region. These findings are supported by theoretical calculations enabling direct detection and quantification of the most favorable type of vacancies. The experiments show that Sn vacancies couple with one or more Se vacancies with increasing temperature to form vacancy clusters. Interestingly, the clusters survive the α→β transition at ≈800 K and even grow in size with temperature. The concentration of both Se vacancies and vacancy clusters increases with temperature, similar to thermoelectric performance. This indicates that the extraordinary thermoelectric properties of SnSe are related to point defects. We suggest that either these defects vary the band structure in favor of high thermoelectric performance or introduce an energy-dependent scattering of free carriers realizing, in fact, energy filtering of the free carriers. Cluster defects account for the glasslike thermal conductivity of SnSe at elevated temperatures.

We study how quantum states are scrambled via braiding in systems of non-Abelian anyons through the lens of entanglement spectrum statistics. In particular, we focus on the degree of scrambling, defined as the randomness produced by braiding, at the same amount of entanglement entropy. To quantify the degree of randomness, we define a distance between the entanglement spectrum level spacing distribution of a state evolved under random braids and that of a Haar-random state, using the Kullback-Leibler divergence $D_{\mathrm{KL}}$. We study $D_{\mathrm{KL}}$ numerically for random braids of Majorana fermions (supplemented with random local four-body interactions) and Fibonacci anyons. For comparison, we also obtain $D_{\mathrm{KL}}$ for the Sachdev-Ye-Kitaev model of Majorana fermions with all-to-all interactions, random unitary circuits built out of (a) Hadamard (H), $\pi /8$ (T), and CNOT gates, and (b) random unitary circuits built out of two-qubit Haar random unitaries. To compare the degree of randomness that different systems produce beyond entanglement entropy, we look at $D_{\mathrm{KL}}$ as a function of the Page limit-normalized entanglement entropy $S/S_{\max }$. Our results reveal a hierarchy of scrambling among various models—even for the same amount of entanglement entropy—at intermediate times, whereas all models exhibit the same late-time behavior. In particular, we find that braiding of Fibonacci anyons randomizes initial product states more efficiently than the universal H+T+CNOT set.

Optical experiments on nanostructures such as molecules, one- or two-dimensional materials, are often performed with the nanostructures in close proximity of a substrate or some other polarizable media. In this case, the Bethe-Salpeter equation (BSE) can be used to calculate the optical excitations of the nanostructure by including the effect of the substrate via the screened electron-hole interaction. Here we show, that in such an approach, where the states of the substrate are not explicitly included in the BSE Hamiltonian but only enter through the screened Coulomb interaction, it is important also to screen the electron-hole exchange interaction. For the case of molecules like benzene physisorbed on the metallic Au(111) surface, the screening of the exchange interaction by the substrate redshifts the lowest optical transition by up to 300 meV. Furthermore, the screening of the exchange is essential in order to obtain the correct ordering of the size of quasiparticle and optical energy gap.

We analyze heat and charge transport through a single-level quantum dot coupled to two BCS superconductors at different temperatures to first order in the tunnel coupling. In order to describe the system theoretically, we extend a real-time diagrammatic technique that allows us to capture the interplay between superconducting correlations, strong Coulomb interactions, and nonequilibrium physics. We find that a thermoelectric effect can arise due to the superconducting proximity effect on the dot. In the nonlinear regime, the thermoelectric current can also flow at the particle-hole symmetric point due to a level renormalization caused by virtual tunneling between the dot and the leads. The heat current through the quantum dot is sensitive to the superconducting phase difference. In the nonlinear regime, the system can act as a thermal diode.

We study Anderson localization of massless two-dimensional Dirac electrons in random one-dimensional scalar and vector potentials theoretically for two different cases, in which the scalar and vector potentials are either uncorrelated or correlated. From the Dirac equation, we deduce the effective wave impedance, in which we derive the condition for total transmission and those for delocalization in our random models analytically. Based on the invariant imbedding theory, we also develop a numerical method to calculate the localization length exactly for arbitrary strengths of disorder. In addition, we derive analytical expressions for the localization length, which are extremely accurate in the weak and strong disorder limits. In the presence of both scalar and vector potentials, the conditions for total transmission and complete delocalization are generalized from the usual Klein tunneling case. We find that the incident angles at which electron waves are either completely transmitted or delocalized can be tuned to arbitrary values. When the strength of scalar potential disorder increases to infinity, the localization length also increases to infinity, both in uncorrelated and correlated cases. The detailed dependencies of the localization length on incident angle, disorder strength, and energy are elucidated and the discrepancies with previous studies and some new results are discussed. All the results are explained intuitively using the concept of wave impedance.