The double exchange model describing the coupling between conduction electrons and localized magnetic moments is relevant for a large family of physical systems including manganites. Here we reveal that the one-dimensional double exchange model with an incommensurate magnetic elliptical spiral is a topological insulator with a Chern number $2\mathbb{Z}$ in the two-dimensional space with one physical dimension and one ancillary dimension spanned by the Goldstone mode of the spiral. Moreover, the electronic states can be localized for a strong local exchange coupling. The topological protected edge states are responsible for the pumping of electron charge, and give rise to multiferroic response. Our work uncovers hitherto undiscovered nontrivial topology and Anderson localization in the double exchange model with possible applications to perovskite manganites.

Thermodynamic and spectral properties of electrons coupled to quantum phonons are studied in the framework of the one-dimensional spinless Holstein model. Using quantum Monte Carlo simulations and an efficient exact estimator for the specific heat based on the properties of the perturbation expansion we calculate the specific heat and compressibility for the entire range of electron-phonon couplings and phonon frequencies. Our results reveal how the Peierls gap and long-range order at zero temperature as well as the significant renormalization of electrons and phonons—revealed by the corresponding spectral functions—manifest themselves in the specific heat. We also quantitatively assess the validity for the widely used classical phonon approximation and identify the electronic contribution predicted for a Tomonaga-Luttinger liquid.

Since the first observation of weak ferromagnetism in the charge-transfer salt $\kappa -{\left(\text{BEDT-}\mathrm{TTF}\right)}_2\text{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\text{Cl}$ [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of a ferromagnetic state in this class of organic materials has been reported. Here, static and dynamic spin susceptibility measurements on $\kappa -{\left(\text{BEDT-}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Br}$ exhibit weak ferromagnetism below 20 K on the geometrically frustrated background. Our experimental results suggest that frustrated spins in the molecular dimers suppress long-range antiferromagnetic order, forming a spin-glass-type ground state of the triangular lattice in the insulating phase which locally contains ferromagnetic polarons. Moreover, specific-heat data reveal an excess peak located around 5 K, indicating the glassy nature of the electrons as well.

We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$.

We report on the observation of terahertz radiation induced photoconductivity and of terahertz analog of the microwave-induced resistance oscillations (MIRO) in HgTe-based quantum well (QW) structures of different width. The MIRO-like effect has been detected in QWs of 20 nm thickness with inverted band structure and a rather low mobility of about $3\times {10}^5{\mathrm{cm}}^2$/V s. In a number of other structures with QW widths ranging from 5 to 20 nm and lower mobility we observed an unconventional nonoscillatory photoconductivity signal which changes its sign upon magnetic field increase. This effect was observed in structures characterized by both normal and inverted band ordering, as well as in QWs with critical thickness and linear dispersion. In samples having Hall bar and Corbino geometries an increase of the magnetic field resulted in a single and double change of the sign of the photoresponse, respectively. We show that within the bolometric mechanism of the photoresponse these unusual features imply a nonmonotonic behavior of the transport scattering rate, which should decrease (increase) with temperature for magnetic fields below (above) the certain value. This behavior is found to be consistent with the results of dark transport measurements of magnetoresistivity at different sample temperatures. Our experiments demonstrate that photoconductivity is a very sensitive probe of the temperature variations of the transport characteristics, even those that are hardly detectable using standard transport measurements.

We study an analytical model of a Rashba nanowire that is partially covered by and coupled to a thin superconducting layer, where the uncovered region of the nanowire forms a quantum dot. We find that, even if there is no topological superconducting phase possible, there is a trivial Andreev bound state that becomes pinned exponentially close to zero energy as a function of magnetic field strength when the length of the quantum dot is tuned with respect to its spin-orbit length such that a resonance condition of Fabry-Perot type is satisfied. In this case, we find that the Andreev bound state remains pinned near zero energy for Zeeman energies that exceed the characteristic spacing between Andreev bound state levels but that are smaller than the spin-orbit energy of the quantum dot. Importantly, as the pinning of the Andreev bound state depends only on properties of the quantum dot, we conclude that this behavior is unrelated to topological superconductivity. To support our analytical model, we also perform a numerical simulation of a hybrid system while explicitly incorporating a thin superconducting layer, showing that all qualitative features of our analytical model are also present in the numerical results.

The effects of temperature and pressure on the phonons of GaN were calculated for both the wurtzite and zinc-blende structures. The quasiharmonic approximation (QHA) gave reasonable results for the temperature dependence of the phonon DOS at zero pressure but unreliably predicted the combined effects of temperature and pressure. Pressure was found to change the explicit anharmonicity, altering the thermal shifts of phonons and more notably qualitatively changing the evolution of phonon lifetimes with increasing temperature. These effects were largest for the optical modes, and phonon frequencies below approximately 5 THz were adequately predicted with the QHA. The elastic anisotropies of GaN in both wurtzite and zinc-blende structures were calculated from the elastic constants as a function of pressure at 0 K. The elastic anisotropy increased with pressure until reaching elastic instabilities at 40 GPa (zinc blende) and 65 GPa (wurtzite). The calculated instabilities are consistent with proposed transformation pathways to rocksalt GaN and place upper bounds on the pressures at which wurtzite and zinc-blende GaN can be metastable.

We report an in-depth investigation of the Anderson localization transition for classical waves in three dimensions (3D). Experimentally, we observe clear signatures of Anderson localization by measuring the transverse confinement of transmitted ultrasound through slab-shaped mesoglass samples. We compare our experimental data with predictions of the self-consistent theory of Anderson localization for an open medium with the same geometry as our samples. This model describes the transverse confinement of classical waves as a function of the localization (correlation) length, $\xi \left(\zeta \right)$, and is fitted to our experimental data to quantify the transverse spreading/confinement of ultrasound all of the way through the transition between diffusion and localization. Hence we are able to precisely identify the location of the mobility edges at which the Anderson transitions occur.

A nodeless $d$-wave state is likely in superconducting monolayer FeSe on ${\mathrm{SrTiO}}_3$. The lack of nodes is surprising but has been shown to be a natural consequence of the observed small interband spin-orbit coupling. Here we examine the evolution from a nodeless state to the nodal state as this spin-orbit coupling is increased from a topological perspective. We show that this evolution depends strongly on the orbital content of the superconducting degrees of freedom. In particular, there are two $d$-wave solutions, which we call orbitally trivial and orbitally nontrivial. In both cases, the nodes carry a $\pm 2$ topological winding number that originates from a chiral symmetry. However, the momentum space distribution of the positive and negative charges is different for the two cases, resulting in a different evolution of these nodes as they annihilate to form a nodeless superconductor. We further show that the orbitally trivial and orbitally nontrivial nodal states exhibit different Andreev flat band spectra at the edge.

We use an effective model to calculate properties of the supercurrent carried by chiral edge states of a quantum Hall weak link. This “chiral” supercurrent is qualitatively distinct from the usual Josephson supercurrent in that it cannot be mediated by a single edge alone, i.e., both right- and left-going edges are needed. Moreover, the chiral supercurrent was previously shown to obey an unusual current-phase relation with period $2{\varphi }_0=h/e$, which is twice the period of conventional Josephson junctions. We show that the “chiral” nature of this supercurrent is sharply defined, and is robust to interactions to infinite order in perturbation theory. We compare our results with recent experimental findings [Amet et al., Science 352 966 (2016)] and find that quantitative agreement in the magnitude of the supercurrent can be attained by making reasonable but critical assumptions about the superconductor quantum Hall interface.

The lattice thermal conductivity of the candidate thermoelectric material ${\mathrm{Mg}}_3{\mathrm{Sb}}_2$ is studied from first principles, with the inclusion of anharmonic, isotope, and boundary scattering processes, and via an accurate solution of the Boltzmann equation. We find that the anomalously low observed conductivity is due to grain-boundary scattering of phonons, whereas the purely anharmonic conductivity is an order of magnitude larger. Mass disorder due to alloying and off-stoichiometry is also found to contribute significantly to its decrease. Combining ab initio values vs sample size with measured grain-size distributions, we obtain an estimate of $\kappa$ vs $T$ in nanopolycrystalline material in good agreement with typical experiments, and compute the $ZT$ figure of merit in the various cases.

The interplay between electronic orders and superconductivity is central to the physics of unconventional superconductors, and is particularly pronounced in the iron-based superconductors. Motivated by recent experiments on FeSe, we study the superconducting pairing in its nematic phase in a multiorbital model with frustrated spin-exchange interactions. Electron correlations in the presence of nematic order give rise to an enhanced orbital selectivity in the superconducting pairing amplitudes. This orbital-selective pairing produces a large gap anisotropy on the Fermi surface. Our results naturally explain the striking experimental observations, and shed light on the unconventional superconductivity of correlated electron systems in general.

We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic field. Using time-resolved magneto-optical Kerr effect microscopy, we access to the statistics of the intermittent thermally activated domain-wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatiotemporally correlated thermally activated elementary events. Moreover, we evidence that the large-scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents.

We study the collective magnonic modes of a one-dimensional bipartite chain of magnetic spheres coupled by dipolar interactions. The magnonic modes of the infinite chain, due to the dipolar interaction, give rise to a gapped magnonic dispersion. The external magnetic fields can be invoked to quantize the Zak phase of the magnonic bands, which is used as an invariant to identify two topologically distinct phases of the system. In the case of a finite chain, the magnetic fields determine whether topologically protected localized states or Tamm-like states form at the ends of the chain. We provide an analytical expression along with numerical results which reveal parameters resulting in topologically protected end states. This bosonic system is almost similar to the Su–Schrieffer–Heeger system and is promising as a dynamically reconfigurable material for future use in magnonics.

The antiferromagnetic mixed valence ternary oxide $\alpha \text{−}{\mathrm{CoV}}_3{\mathrm{O}}_8$ displays disorder on the ${\mathrm{Co}}^{2+}$ site that is inherent to the $Ibam$ space group resulting in a local selection rule requiring that one ${\mathrm{Co}}^{2+}$ and one ${\mathrm{V}}^{4+}$ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for external influences such as chemical dopants or porous media. The zero-field structural and dynamic properties of $\alpha \text{−}{\mathrm{CoV}}_3{\mathrm{O}}_8$ have been investigated using a combination of neutron and x-ray diffraction, dc susceptibility, and neutron spectroscopy. The low-temperature magnetic and structural properties are consistent with a random macroscopic distribution of ${\mathrm{Co}}^{2+}$ over the $16k$ metal sites. However, by applying the sum rules of neutron scattering we observe that the collective magnetic excitations are parametrized with an ordered ${\mathrm{Co}}^{2+}$ arrangement and critical scattering consistent with a three-dimensional Ising universality class. The low-energy spectrum is well described by ${\mathrm{Co}}^{2+}$ cations coupled via a three-dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long-range antiferromagnetic order at $T{}_{\mathrm{N}}\sim 19\mathrm{K}$. A crystal field analysis finds two bands of excitations separated in energy at $\hslash \omega \sim 5\mathrm{meV}$ and 25 meV, consistent with a $j{}_{\mathrm{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split levels. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j{}_{\mathrm{eff}}$ manifolds, may play a key role in making the dynamics largely insensitive to disorder.

Hydrogen plays an essential role in the growth process of artificial diamond and can easily form complexes with lattice vacancies. Despite substantial efforts to resolve the electronic structure and the ground-state properties of the hydrogen-vacancy (HV) center, the final remarks are ambiguous, while the complexes of vacancy with two and more hydrogen atoms remain unexplored. In this paper, we used spin-polarized, hybrid density-functional theory method to investigate electronic structure and magneto-optical properties of various hydrogen-vacancy clusters in diamond. Our theoretical results indicate a very strong tendency toward the formation of ${\mathrm{H}}_n\mathrm{V}$ complexes up to four hydrogen atoms that are mostly electrically and optically active centers. One of the investigated defects introduce highly correlated electronic states that pose a challenge for density-functional theory and, therefore, require special treatment when charge- and spin-density-related properties are determined. We introduced an extended Hubbard model Hamiltonian with fully ab initio provided parameters to analyze the complex electronic structure of highly correlated ${\mathrm{H}}_2{\mathrm{V}}^0$ defects. The role of quantum tunneling of hydrogen in HV center and its impact on the hyperfine structure was discussed. We demonstrate that experimentally observed ${\mathrm{HV}}^{1-}$ center is similar to well-known ${\mathrm{NV}}^{1-}$, i.e., I) it possesses triplet ${}^3\mathrm{A}$ ground state and ${}^3\mathrm{E}$ excited state in $C_{3v}$ symmetry; II) the calculated zero-phonon line is 1.71 eV (1.945 eV for ${\mathrm{NV}}^{1-}$). A detailed experimental reinvestigation based on optically detected electron paramagnetic resonance spectroscopy is suggested to verify whether the ${\mathrm{HV}}^{1-}$ center has metastable singlet shelving states between the ground and excited state triplets and, as a result, whether it may exhibit a spin-selective decay to the ground state.

We study Floquet topological transition in irradiated graphene when the polarization of incident light changes randomly with time. We numerically confirm that the noise-averaged time-evolution operator approaches a steady value in the limit of exact Trotter decomposition of the whole period during which incident light has a different polarization at each interval of the decomposition. This steady limit is found to coincide with the time-evolution operator calculated from the noise-averaged Hamiltonian. We observe that at the six corners (Dirac $K$ point) of the hexagonal Brillouin zone of graphene random Gaussian noise strongly modifies the phase band structure induced by circularly polarized light, whereas in the zone center ($\mathrm{\Gamma }$ point) even a strong noise is not able to do the same. This can be understood by analyzing the deterministic noise-averaged Hamiltonian, which has a different Fourier structure as well as a lower number of symmetries compared to the noise-free one. In one-dimensional systems noise is found to renormalize only the drive amplitude.

We use ab initio electronic structure calculations in combination with Monte Carlo simulations to investigate the magnetic and ferroelectric properties of bulk orthorhombic ${\mathrm{HoMnO}}_3$ and ${\mathrm{ErMnO}}_3$. Our goals are to explain the inconsistencies in the measured magnetic properties of the orthorhombic perovskite manganites (o-$R{\mathrm{MnO}}_3$) with small rare-earth ($R$) cations or Y, as well as the contradictions between the directions and amplitudes of the electric polarizations reported by different experimental groups. Our computations stabilize several exotic magnetic orders (so-called w-spiral, H-AFM, and I-AFM), whose presence resolve the contradictions in the measured magnetic and ferroelectric properties of o-$R{\mathrm{MnO}}_3$. We show that these orders emerge due to strong four-spin ring exchange interactions.

We study the equilibrium Josephson current between an $s$-wave superconductor and a topological superconducting nanowire with Majorana bound states (MBS) at its ends. Within a low-energy model, we show analytically that the nonlocality of the MBS allows for a finite supercurrent to flow that otherwise would vanish. In particular, we find the critical current to be a function of the difference in the spin-canting angles of the Majorana wave functions at the location of the tunnel contact. We complement our analytical calculations by numerically solving the full tight-binding model and show how to extract the main features of the low-energy model from the critical current using available experimental techniques.

Linearly polarized exciton emission from a quantum dot is very appealing for polarized light emitting diodes and quantum information science and technologies. However, it has been very difficult to access it, because conventional wurtzite quantum dots (QDs) usually favor circularly polarized exciton emission originating from a single electron bound to a hole with dominant heavy hole ($A$ band) character. Here we explore the exciton fine structure of wurtzite ZnO QDs by means of plane-wave million-atom atomistic pseudopotential calculations and a configuration interaction approach. We demonstrate that linearly polarized emission could be realized in these QDs by (i) enhancing the quantum confinement effects and (ii) applying hydrostatic pressure. Such an exciton has a polarization exclusively along the $c$-axis direction of the QDs, which derives from a single electron bounded to a hole state with a split-off hole ($C$-band) character. This work paves the way for the exploration of the fundamental properties of wurtzite ZnO QDs and their potential applications in optoelectronic devices and quantum technologies.