A semiconducting nanowire with strong Rashba spin-orbit coupling and coupled to a superconductor can be tuned by an external Zeeman field into a topological phase with Majorana zero modes. Here we theoretically investigate how this exotic topological superconductor phase manifests in Josephson junctions based on such proximitized nanowires. In particular, we focus on critical currents in the short junction limit ($L_{\mathrm{N}}\ll \xi$, where $L_{\mathrm{N}}$ is the junction length and $\xi$ is the superconducting coherence length) and show that they contain important information about nontrivial topology and Majoranas. This includes signatures of the gap inversion at the topological transition and a unique oscillatory pattern that originates from Majorana interference. Interestingly, this pattern can be modified by tuning the transmission across the junction, thus providing complementary evidence of Majoranas and their energy splittings beyond standard tunnel spectroscopy experiments, while offering further tunability by virtue of the Josephson effect.

Using a scanning tunneling microscope, confined electron states are studied that exist above subsurface nanometer-sized voids at Pb(111), where potential barriers at the parallel vacuum-Pb(111) and Pb(111)-void interfaces establish a principal series of quantum well states that are further confined laterally by strong reflection at the open boundaries at the edges of the void. The influence of the size, depth, and shape of the voids on the effectiveness of the lateral confinement is discussed. Standing wave patterns observed in differential conductance maps unravel the dispersion of the relevant underlying Pb electron states.

Further to planar single-layer hexagonal structures, GaN and AlN can also form free-standing, single-layer structures constructed from squares and octagons. We performed an extensive analysis of dynamical and thermal stability of these structures in terms of ab initio finite-temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single-layer square-octagon structures of GaN and AlN display directional mechanical properties and have wide, indirect fundamental band gaps, which are smaller than their hexagonal counterparts. These density functional theory band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain, the fundamental band gaps decrease and can be closed. The electronic and magnetic properties of these single-layer structures can be modified by adsorption of various adatoms, or by creating neutral cation-anion vacancies. The single-layer structures attain magnetic moment by selected adatoms and neutral vacancies. In particular, localized gap states are strongly dependent on the type of vacancy. The energetics, binding, and resulting electronic structure of bilayer, trilayer, and three-dimensional (3D) layered structures constructed by stacking the single layers are affected by vertical chemical bonds between adjacent layers. In addition to van der Waals interaction, these weak vertical bonds induce buckling in planar geometry and enhance their binding, leading to the formation of stable 3D layered structures. In this respect, these multilayers are intermediate between van der Waals solids and wurtzite crystals, offering a wide range of tunability.

We study analytically topological properties of a noninteracting modified dimerized Kitaev chain and an exactly solvable interacting dimerized Kitaev chain under open boundary conditions by analyzing two introduced edge correlation functions. The interacting dimerized Kitaev chain at the symmetry point $\mathrm{\Delta }=t$ and the chemical potential $\mu =0$ can be exactly solved by applying two Jordan-Wigner transformations and a spin rotation, which permits us to calculate the edge correlation functions analytically. We demonstrate that the two edge correlation functions can be used to characterize the trivial, Su-Schrieffer-Heeger-like topological and topological superconductor phases of both the noninteracting and interacting systems and give their phase diagrams.

The work describes a first-principles-based computational strategy for studying structural phase transitions, and in particular, for determination of the so-called Landau-Devonshire potential—the classical zero-temperature limit of the Gibbs energy, expanded in terms of order parameters. It exploits the configuration space attached to the eigenvectors of the modes frozen in the ground state, rather than the space spanned by the unstable modes of the high-symmetry phase, as done usually. This allows us to carefully probe the part of the energy surface in the vicinity of the ground state, which is most relevant for the properties of the ordered phase. We apply this procedure to ${\mathrm{BiFeO}}_3$ and perform ab initio calculations in order to determine potential energy contributions associated with strain, polarization, and oxygen octahedra tilt degrees of freedom, compatible with its two-formula unit cell periodic boundary conditions.

The classical belief about the mechanism of spin reorientation phase transition (SRPT) and ground-state magnetic structure in ${\mathrm{SmCrO}}_3$ has become intriguing because of inconsistent bulk magnetization observations. The presence of highly neutron-absorbing Sm atom has so far evaded the determination of microscopic magnetic structure. In the present report, we have utilized very high-energy “hot neutrons” to overcome the Sm absorption and to determine the thermal evolution of magnetic configurations. Unambiguously, three distinct phases are observed: the uncompensated canted antiferromagnetic structure ${\mathrm{\Gamma }}_4(G_x,A_y,F_z;F_z^R)$ occurring below the Néel temperature ($T_N=191$ K), the collinear antiferromagnetic structure ${\mathrm{\Gamma }}_1(A_x,G_y,C_z;C_z^R)$ occurring below 10 K, and a nonequilibrium configuration with cooccurring ${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_4$ phases in the neighborhood of the SRPT (10 K $\le T\le$ 40 K). In differing to the earlier predictions, we divulge the SRPT to be a discontinuous transition where chromium spins switch from the $a-b$ crystallographic plane to the $b-c$ crystallographic plane in a discrete manner with no allowed intermediate configuration. The canting angle of chromium ions in the $a-b$ plane is unusually not a thermal constant, rather it is empirically discerned to follow exponential behavior. The competition between magnetocrystalline anisotropy and free energy derived by isotropic and antisymmetric exchange interactions between different pairs of magnetic ions is observed to govern the mechanism of SRPT.

We develop a Green's function formalism for spin transport through heterostructures that contain metallic leads and insulating ferromagnets. While this formalism in principle allows for the inclusion of various magnonic interactions, we focus on Gilbert damping. As an application, we consider ballistic spin transport by exchange magnons in a metal-insulator-metal heterostructure with and without disorder. For the former case, we show that the interplay between disorder and Gilbert damping leads to spin current fluctuations. For the case without disorder, we obtain the dependence of the transmitted spin current on the thickness of the ferromagnet. Moreover, we show that the results of the Green's function formalism agree in the clean and continuum limit with those obtained from the linearized stochastic Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a natural starting point for numerical studies of magnon transport in heterostructures that contain normal metals and magnetic insulators.

We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-1 Heisenberg model on the anisotropic triangular lattice that interpolates between the regime of weakly coupled Haldane chains $\left(J^{\prime }\ll J\right)$ and the isotropic triangular lattice $\left(J^{\prime }=J\right)$. We use the density-matrix renormalization group (DMRG) and Schwinger boson theory performed at the Gaussian correction level above the saddle-point solution. Our DMRG results show an abrupt transition between decoupled spin chains and the spirally ordered regime at ${(J^{\prime }/J)}_c\sim 0.42$, signaled by the sudden closing of the spin gap. Coming from the magnetically ordered side, the computation of the spin stiffness within Schwinger boson theory predicts the instability of the spiral magnetic order toward a magnetically disordered phase with one-dimensional features at ${(J^{\prime }/J)}_c\sim 0.43$. The agreement of these complementary methods, along with the strong difference found between the intra- and the interchain DMRG short spin-spin correlations for sufficiently large values of the interchain coupling, suggests that the interplay between the quantum fluctuations and the dimensional crossover effects gives rise to the one-dimensionalization phenomenon in this frustrated spin-1 Hamiltonian.

We consider Kohn-Luttinger mechanism for superconductivity in a two-dimensional electron gas confined to a narrow well between two grounded metallic planes with two occupied subbands with Fermi momenta $k_{FL}>k_{FS}$. On the basis of a perturbative analysis, we conclude that non-$s$-wave superconductivity emerges even when the bands are parabolic. We analyze the conditions that maximize $T_c$ as a function of the distance to the metallic planes, the ratio $k_{FL}/k_{FS}$, and $r_s$, which measures the strength of Coulomb correlations. The largest attraction is in $p$-wave and $d$-wave channels, of which $p$ wave is typically the strongest. For $r_s=O\left(1\right)$ we estimate that the dimensionless coupling $\lambda \approx {10}^{-1}$, but it likely continues increasing for larger $r_s$ (where we lose theoretical control).

The effect of van der Waals dispersion correction in combination with density functional theory is investigated on a canonical amorphous phase-change material. Density functional theory (DFT), using the generalized gradient approximation, usually fails to reproduce the structure of amorphous tellurides, which manifests by an overestimation of the interatomic bond distances, and particularly the Ge-Te one involved in local geometries (tetrahedral or defect octahedral). Here, we take into account dispersion forces in a semiempirical way and apply such DFT simulations to amorphous GeTe. We obtain a substantial improvement of the simulated structure factor and pair-correlation function, which now reproduce the experimental counterparts with an unprecedented accuracy, including on a recent partial contribution from anomalous x-ray scattering and from x-ray absorption. A detailed analysis of the corresponding structures indicates that the dispersion correction reduces the Ge-Te bond length, increases the fraction of tetrahedral germanium, and reduces the presence of heteropolar so-called fourfold ABAB rings. Given that these structural features have been stressed to be central for the understanding of the phase-change mechanism, the present results challenge our current understanding of the crystal to amorphous transformation at play.

Resonant inelastic x-ray scattering at the N $K$ edge reveals clearly resolved harmonics of the anion plane vibrations in the $\kappa$-(BEDT-${\mathrm{TTF})}_2{\mathrm{Cu}}_2$(${\mathrm{CN})}_3$ spin-liquid insulator. Tuning the incoming light energy at the $K$ edge of two distinct N sites permits us to excite different sets of phonon modes. The cyanide (CN) stretching mode is selected at the edge of the ordered N sites which are more strongly connected to the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules, while positionally disordered N sites show multimode excitation. Combining measurements with calculations on an anion plane cluster permits us to estimate the site-dependent electron-phonon coupling of the modes related to nitrogen excitation.

We show that narrow superconducting strips in superconducting (S) and normal (N) states are universally described by the model presenting them as lateral NSN proximity systems in which the superconducting central band is sandwiched between damaged edge bands with suppressed superconductivity. The width of the superconducting band was experimentally determined from the value of magnetic field at which the band transits from the Meissner state to the static vortex state. Systematic experimental study of 4.9-nm-thick NbN strips with widths in the interval from 50 nm to 20 μm, which are all smaller than the Pearl's length, demonstrates gradual evolution of the temperature dependence of the critical current with the change of the strip width.

We present a set of experimentally feasible pulse sequences that implement any single-qubit gate on a singlet-triplet spin qubit and demonstrate that these new sequences are up to three times faster than existing sequences in the literature. We show that these sequences can be extended to incorporate built-in dynamical error correction, yielding gates that are robust to both charge and magnetic field noise and up to twice as fast as previous dynamically corrected gate schemes. We present a thorough comparison of the performance of our new sequences with that of several existing ones using randomized benchmarking, considering both quasistatic and $1/f^{\alpha }$ noise models. We provide our results both as a function of evolution time and as a function of the number of gates, which respectively yield both an effective coherence time and an estimate of the number of gates that can be performed within this coherence time. We determine which set of pulse sequences gives the best results for a wide range of noise strengths and power spectra. Overall, we find that the traditional, slower sequences perform best when there is no field noise or when the noise contains significant high-frequency components; otherwise, our new, fast sequences exhibit the best performance.

Nanocomposites are promising candidates for the next generation of thermoelectric materials since they exhibit extremely low thermal conductivities as a result of phonon scattering on the boundaries of the various material phases. The nanoinclusions, however, should not degrade the thermoelectric power factor, and ideally should increase it, so that benefits to the ZT figure of merit can be achieved. In this work we employ the nonequilibrium Green's function quantum transport method to calculate the electronic and thermoelectric coefficients of materials embedded with nanoinclusions. For computational effectiveness we consider two-dimensional nanoribbon geometries, however, the method includes the details of geometry, electron-phonon interactions, quantization, tunneling, and the ballistic to diffusive nature of transport, all combined in a unified approach. This makes it a convenient and accurate way to understand electronic and thermoelectric transport in nanomaterials, beyond semiclassical approximations, and beyond approximations that deal with the complexities of the geometry. We show that the presence of nanoinclusions within a matrix material offers opportunities for only weak energy filtering, significantly lower in comparison to superlattices, and thus only moderate power factor improvements. However, we describe how such nanocomposites can be optimized to limit degradation in the thermoelectric power factor and elaborate on the conditions that achieve the aforementioned mild improvements. Importantly, we show that under certain conditions, the power factor is independent of the density of nanoinclusions, meaning that materials with large nanoinclusion densities which provide very low thermal conductivities can also retain large power factors and result in large ZT figures of merit.

One of the amazing properties of graphene is the ultrarelativistic behavior of its loosely bound electrons, mimicking massless fermions that move with a constant velocity, inversely proportional to a fine-structure constant ${\alpha }_g$ of the order of unity. The effective interaction between these quasiparticles is, however, better controlled by the coupling parameter ${\alpha }_g^{*}={\alpha }_g/\epsilon$, which accounts for the dynamic screening due to the complex permittivity $\epsilon$ of the many-valence electron system. This concept was introduced in a couple of previous studies [Reed et al., Science 330, 805 (2010) and Gan et al., Phys. Rev. B 93, 195150 (2016)], where inelastic x-ray scattering measurements on crystal graphite were converted into an experimentally derived form of ${\alpha }_g^{*}$ for graphene, over an energy-momentum region on the $\mathrm{eV}\AA {}^{-1}$ scale. Here, an accurate theoretical framework is provided for ${\alpha }_g^{*}$, using time-dependent density-functional theory in the random-phase approximation, with a cutoff in the interaction between excited electrons in graphene, which translates to an effective interlayer interaction in graphite. The predictions of the approach are in excellent agreement with the above-mentioned measurements, suggesting a calibration method to substantially improve the experimental derivation of ${\alpha }_g^{*}$, which tends to a static limiting value of $\sim 0.14$. Thus, the ab initio calibration procedure outlined demonstrates the accuracy of perturbation expansion treatments for the two-dimensional gas of massless Dirac fermions in graphene, in parallel with quantum electrodynamics.

We report the observation of a nonlinear elastoresistivity response for the prototypical underdoped iron pnictide $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.975}{\mathrm{Co}}_{0.025}\right)}_2{\mathrm{As}}_2$. Our measurements reveal a large quadratic term in the isotropic ($A_{1g}$) electronic response that was produced by a purely shear ($B_{2g}$) strain. The divergence of this quantity upon cooling towards the structural phase transition reflects the temperature dependence of the nematic susceptibility. This observation shows that nematic fluctuations play a significant role in determining even the isotropic properties of this family of compounds.

The detailed atomistic modeling of electrochemically deposited metal monolayers is challenging due to the complex structure of the metal-solution interface and the critical effects of surface electrification during electrode polarization. Accurate models of interfacial electrochemical equilibria are further challenged by the need to include entropic effects to obtain accurate surface chemical potentials. We present an embedded quantum-continuum model of the interfacial environment that addresses each of these challenges and study the underpotential deposition of silver on the gold (100) surface. We leverage these results to parametrize a cluster expansion of the electrified interface and show through grand canonical Monte Carlo calculations the crucial need to account for variations in the interfacial dipole when modeling electrodeposited metals under finite-temperature electrochemical conditions.

We propose a spin filter device based on the interplay of a modulated spin-orbit interaction and a uniform external magnetic field acting on a quantum wire. Half-metal phases, where electrons with only a selected spin polarization exhibit ballistic conductance, can be tuned by varying the magnetic field. These half-metal phases are proven to be robust against electron-electron repulsive interactions. Our results arise from a combination of explicit band diagonalization, bosonization techniques, and extensive density matrix renormalization group computations.

Motivated by the very recent experimental realization of electronic Lieb lattices and research interest on topological states of matter, we study the topological phase transitions driven by Anderson-type disorder on spin-orbit coupled Lieb lattices in the presence of spin-independent and -dependent staggered potentials. By combining the recursive Green's-function and self-consistent Born approximation methods, we found that both time-reversal-invariant and time-reversal-symmetry-broken spin-orbit coupled Lieb lattice systems can host the disorder-induced gapful topological phases, including the quantum spin Hall insulator (QSHI) and quantum anomalous Hall insulator (QAHI) phases. For the time-reversal-invariant case, the disorder induces a topological phase transition directly from a normal insulator (NI) to the QSHI, while for the time-reversal-symmetry-broken case, the disorder can induce either a QAHI-QSHI phase transition or a NI-QAHI-QSHI phase transition, depending on the initial state of the system. Remarkably, the time-reversal-symmetry-broken QSHI phase can be induced by Anderson-type disorder on the spin-orbit coupled Lieb lattices without time-reversal symmetry.

We present a force-biased Monte Carlo (FMC) method for structural modeling of the transition-metal clusters of Fe, Ni, and Cu with sizes of 13, 30, and 55 atoms. By employing the Finnis-Sinclair potential for Fe and the Sutton-Chen potential for Ni and Cu, the total energy of the clusters is minimized using the local gradient of the potentials in Monte Carlo simulations. The structural configurations of the clusters, obtained from the biased Monte Carlo approach, are analyzed and compared with the same configurations from the Cambridge Cluster Database (CCD) upon relaxation of the clusters using the first-principles density-functional code nwchem. The results show that the total-energy value and the structure of the FMC clusters are essentially identical to the corresponding value and the structure of the CCD clusters. A comparison of the nwchem-relax FMC and CCD structures is presented by computing the pair-correlation function, the bond-angle distribution, the coordination number of the first-coordination shell, and the Steinhardt bond-orientational order parameter, which provide information about the two- and three-body correlation functions, the local bonding environment of the atoms, and the geometry of the clusters. An atom-by-atom comparison of the FMC and CCD clusters is also provided by superposing one set of clusters onto another, and the electronic properties of the clusters are addressed by computing the density of electronic states.