Recent experimental realization of superconducting quantum dot devices with intradot attraction $U$ [Nature (London) 521, 196 (2015); Phys. Rev. X 6, 041042 (2016)] offers unique opportunities to study the charge Kondo effect in a superconducting environment. In such devices pseudospin flips are caused by two tunneling processes. One is the cotunneling of normal electrons which generates near-gap Kondo resonances in the single-electron spectral density. This negative-$U$ charge Kondo effect is more robust than the conventional spin Kondo effect against the suppression by the superconductivity. The other tunneling is the mean-field Cooper-pair tunneling which produces a zero-energy bound state in the pair spectral density. Interesting crossover physics from the strongly-correlated Kondo screening to the mean-field polarization of local pseudospin is demonstrated. Due to the interplay of these two tunnelings, the supercurrent is suppressed for intermediate couplings, but it can increase to the unitary limits both in the strong and weak coupling regimes. We obtain the magnetic field-dependent supercurrent which is consistent with the key experimental findings.

We present a general method for combining two crystals into an interface. The method finds all possible interfaces between the crystals with small coincidence cells and identifies the strain and area of the corresponding two-dimensional cells of the two crystal surfaces. We apply the method to the two semiconductor alloys ${\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ and ${\mathrm{Ga}}_x{\mathrm{In}}_{1-x}\mathrm{As}$ combined with a selection of pure metals or with NbTiN to create semiconductor/superconductor interfaces. The lattice constant of the alloy can be tuned by composition and we can extract the alloy lattice parameters corresponding to zero strain in both the metal and the alloy. The results can be used to suggest new epitaxially matched interfaces between two materials.

The energies as a function of the magnetic field $\left(H\right)$ and the pressure are studied theoretically in the tight-binding model for the two-dimensional organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$, in which massless Dirac fermions are realized. The effects of the uniaxial pressure $\left(P\right)$ are studied by using the pressure-dependent hopping parameters. The system is semimetallic with the same area of an electron pocket and a hole pocket at $P<3.0$ kbar, where the energies $({\varepsilon }_{\mathrm{D}}^0)$ at the Dirac points locate below the Fermi energy $({\varepsilon }_{\mathrm{F}}^0)$ when $H=0$. We find that at $P=2.3$ kbar the Dirac cones are critically tilted. In that case a type of band crossing occurs at “three-quarter” Dirac points; i.e., the dispersion is quadratic in one direction and linear in the other three directions. We obtain magnetic field dependencies of the Landau levels $\left({\varepsilon }_n\right)$: ${\varepsilon }_n-{\varepsilon }_{\mathrm{D}}^0\propto {\left(nH\right)}^{4/5}$ at $P=2.3$ kbar (three-quarter Dirac points) and $|{\varepsilon }_n-{\varepsilon }_{\mathrm{F}}^0{|\propto \left(nH\right)}^2$ at $P=3.0$ kbar (the critical pressure for the semimetallic state). We also study the magnetization as a function of the inverse magnetic field. We obtain two types of quantum oscillations. One is the usual de Haas–van Alphen (dHvA) oscillation, and the other is the unusual dHvA-like oscillation which is seen even in the system without the Fermi surface.

We investigate the photoresponse of intrinsic graphene in an in-plane electric field. Toward that end, we employ a microscopic approach that allows us to determine the time- and momentum-resolved charge-carrier distributions as a result of the interplay between the field-induced acceleration of optically excited carriers and Coulomb- and phonon-driven carrier scattering. Calculating the generated photocurrent that is determined by the asymmetry of the carrier distribution, we reveal the microscopic foundation of the photoconduction effect in graphene. In particular, we discuss the possibility of tuning the photocurrent via externally accessible knobs, such as electric field, temperature, and substrate. Furthermore, we study the impact of Auger-induced carrier multiplication on the photocurrent in graphene.

We show that, in certain circumstances, exact excitation energies appear as locally site-independent (or flat) modes if one records the excitation spectrum of the effective Hamiltonian while sweeping through the lattice in the variational matrix-product-state formulation of the density matrix renormalization group, a remarkable property since the effective Hamiltonian is only constructed to target the ground state. Conversely, modes that are very flat over several consecutive iterations are systematically found to correspond to faithful excitations. We suggest to use this property to extract accurate information about excited states using the standard ground-state algorithm. The results are spectacular for critical systems, for which the low-energy conformal tower of states can be obtained very accurately at essentially no additional cost, as demonstrated by confirming the predictions of boundary conformal field theory for two simple minimal models: the transverse-field Ising model and the critical three-state Potts model. This approach is also very efficient to detect the quasidegenerate low-energy excitations in topological phases and to identify localized excitations in systems with impurities. Finally, using the variance of the Hamiltonian as a criterion, we assess the accuracy of the resulting matrix-product-state representations of the excited states.

Orbital ordering below 30 K was previously observed in the ferromagnetic ${\text{YTiO}}_3$ compound both by polarized neutron diffraction (PND) and x-ray magnetic diffraction (XMD). In this paper we report a procedure for the joint refinement of a unique spin-density model based on both PND and XMD data. The distribution of the unpaired $3d$ electron of titanium is clearly seen on the magnetization density reconstructed by the maximum entropy method from the PND data collection at 5 K. The ${\text{Ti}}^{3+}$ $3d$ orbital populations obtained by joint model refinement are discussed in terms of the orbital ordering scheme. Small but significant magnetic moments on apical oxygen ${\text{O}}_1$ and yttrium atoms are found. The agreement between experimental and theoretical spin densities obtained using density functional theory is discussed.

Unpaired electrons in ${\mathrm{YTiO}}_3$ ferromagnetic crystal (below 30 K) were studied by polarized neutron diffraction (PND) and incoherent x-ray magnetic Compton scattering (MCS). These experiments provide both position and momentum representations of the electrons at the origin of the magnetic behavior, mostly those in the $t_{2g}$ state of Ti atoms. A two-dimensional reconstruction was conducted from experimental and theoretical directional magnetic Compton profiles to obtain the two-dimensional magnetic electron momentum density. A “superposition” method is proposed to examine the coherence between results for position and momentum spaces, respectively. This model-free approach allows a straightforward cross-checking of PND and MCS experiments. An “isolated Ti model” is proposed to emphasize the role played by ${\mathrm{O}}_1$ in the ferromagnetic coupling between Ti and its neighboring atoms.

We investigate bias voltage effects on the spin-dependent transport properties of Fe/$\mathrm{MgAl}{}_2\mathrm{O}{}_4$/Fe(001) magnetic tunneling junctions (MTJs) by comparing them with those of Fe/MgO/Fe(001) MTJs. By means of the nonequilibrium Green's function method and the density functional theory, we calculate bias voltage dependencies of magnetoresistance (MR) ratios in both the MTJs. We find that in both the MTJs, the MR ratio decreases as the bias voltage increases and finally vanishes at a critical bias voltage $V_{\mathrm{c}}$. We also find that the critical bias voltage $V_{\mathrm{c}}$ of the $\mathrm{MgAl}{}_2\mathrm{O}{}_4$-based MTJ is clearly larger than that of the MgO-based MTJ. Since the in-plane lattice constant of the Fe/$\mathrm{MgAl}{}_2\mathrm{O}{}_4$/Fe(001) supercell is twice that of the Fe/MgO/Fe(001) one, the Fe electrodes in the $\mathrm{MgAl}{}_2\mathrm{O}{}_4$-based MTJs have an identical band structure to that obtained by folding the Fe band structure of the MgO-based MTJs in the Brillouin zone of the in-plane wave vector. We show that such a difference in the Fe band structure is the origin of the difference in the critical bias voltage $V_{\mathrm{c}}$ between the $\mathrm{MgAl}{}_2\mathrm{O}{}_4$- and MgO-based MTJs.

We demonstrate that in a weak-coupled two-band superconducting slab the interaction between vortices penetrating the sample and its boundaries leads to the phenomenon of vortex splitting, which divides composite vortices and creates fractional ones. The interaction between vortices, attractive for different bands and repulsive for the same band, which is controlled by the electric current density flowing through the system, leads to an ordered alternating arrangement of the vortices. This arrangement creates nonequilibrium interband phase textures or domains with different signs of the Josephson energy of the interaction between the band condensates. Such phase textures have a significant effect on the dissipation caused by the vortex motion. In particular, in the phase-texture regime the onset of the dissipation is shifted to higher current densities.

We report the observation of a giant enhancement in the interconversion efficiency between spin current and charge current by the spin Hall effect made possible with the use of nanoscopic structures of Ag in a Pt layer. We fabricate heterostructures of yttrium iron garnet (YIG)/Pt, in which nanoparticles of Ag were grown in the midplane of the Pt layer. These nanostructures serve as nanoscopic molds that enhance the local spin orbit coupling by means of the Rashba effect at the Pt-Ag interfaces. The effective spin Hall angle in YIG/[Pt (3 nm)-Ag (9 nm)]/Pt (3 nm) is determined to be in the range of 27–40%, which is about one order of magnitude larger than the spin Hall angle of pure Pt, typically in the range of 2–8%. The results were confirmed by means of three different experimental techniques, namely spin pumping effect, spin Seebeck effect, and spin Hall magnetoresistance. We believe that this important discovery will contribute to advance the development of spin-based technology.

We present simulations of Bragg coherent x-ray diffractive imaging (CXDI) data from finite crystals in the frame of the dynamical theory of x-ray diffraction. The developed approach is based on a numerical solution of modified Takagi-Taupin equations and can be applied for modeling of a broad range of x-ray diffraction experiments with finite three-dimensional crystals of arbitrary shape also in the presence of strain. We performed simulations for nanocrystals of a cubic and hemispherical shape of different sizes and provided a detailed analysis of artifacts in the Bragg CXDI reconstructions introduced by the dynamical diffraction. Based on our theoretical analysis we developed an analytical procedure to treat effects of refraction and absorption in the reconstruction. Our results elucidate limitations for the kinematical approach in the Bragg CXDI and suggest a natural criterion to distinguish between kinematical and dynamical cases in coherent x-ray diffraction on a finite crystal.

We report a systematic study of the structure, electric, and magnetic properties of ${\mathrm{Ca}}_3{\mathrm{Co}}_{2-x}{\mathrm{Mn}}_x{\mathrm{O}}_6$ single crystals with $x=0.72$ and 0.26. The dc and ac magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 and 35 K ($T_{\mathrm{FE}}$) for $x=0.72$ and 0.26, respectively, with a large value of $1400\mu \mathrm{C}/{\mathrm{m}}^2$ at 8 K for the electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and has the value of 450 and $500\mu \mathrm{C}/{\mathrm{m}}^2$ at 8 K for the $x=0.72$ and 0.26 samples, respectively. The specific heat and magnetic susceptibility show no anomaly around $T_{\mathrm{FE}}$, which means that the electric polarization of these samples has no direct relationship with the magnetism. The x-ray diffraction and the Raman spectroscopy indicate that these samples may undergo Jahn-Teller distortions that could be the reason of electric polarization.

We study the quantum Ising model in $D$ dimensions with the equation-of-motion technique and the Majorana representation for spins. The decoupling scheme used for the Green's functions is based on the hierarchy of correlations in position space. To lowest order, this method reproduces the well-known mean field phase diagram and critical exponents. When correlations between spins are included, we show how the appearance of thermal fluctuations and magnons strongly affects the physical properties. In one dimension and for $B=0$ we demonstrate that, to first order in correlations, thermal fluctuations completely destroy the ordered phase. For nonvanishing transverse field we show that the model exhibits different behavior than its classical counterpart, especially near the quantum critical point. We discuss the connection with the Dyson's equation formalism and the explicit form of the self-energies.

We predict the thermal counterpart of the anomalous Josephson effect in superconductor/ferromagnet/superconductor junctions with noncoplanar magnetic texture. The heat current through the junction is shown to have the phase-sensitive interference component proportional to $cos(\theta -{\theta }_0)$, where $\theta$ is the Josephson phase difference and ${\theta }_0$ is the texture-dependent phase shift. In the generic trilayer magnetic structure with the spin-filtering tunnel barrier ${\theta }_0$ is determined by the spin chirality of magnetic configuration and can be considered as the direct manifestation of the energy transport with participation of spin-triplet Cooper pairs. In case of the ideal spin filter the phase shift is shown to be robust against spin relaxation caused by the spin-orbital scattering. Possible applications of the coupling between heat flow and magnetic precession are discussed. For the nonideal spin filters with practically relevant parameters we show that ${\theta }_0$ is much larger than the phase shift of the equilibrium Josephson current.

The Becke-Johnson model potential [A. D. Becke and E. R. Johnson, J. Chem. Phys. 124, 221101 (2006)] and the potential of the AK13 functional [R. Armiento and S. Kümmel, Phys. Rev. Lett. 111, 036402 (2013)] have been shown to mimic features of the exact Kohn-Sham exchange potential, such as step structures that are associated with shell closings and particle-number changes. A key element in the construction of these functionals is that the potential has a limiting value far outside a finite system that is a system-dependent constant rather than zero. We discuss a set of anomalous features in these functionals that are closely connected to the nonvanishing asymptotic potential. The findings constitute a formidable challenge for the future development of semilocal functionals based on the concept of a nonvanishing asymptotic constant.

$\alpha -(\text{BEDT-TTF}{)}_2{\mathrm{I}}_3$ is a prominent example of charge ordering among organic conductors. In this work, we explore the details of transport within the charge-ordered as well as semimetallic phase at ambient pressure. In the high-temperature semimetallic phase, the mobilities and concentrations of both electrons and holes conspire in such a way to create an almost temperature-independent conductivity as well as a low Hall effect. We explain these phenomena as a consequence of a predominantly interpocket scattering which equalizes mobilities of the two types of charge carriers. At low temperatures, within the insulating charge-ordered phase two channels of conduction can be discerned: a temperature-dependent activation, which follows the mean-field behavior, and a nearest-neighbor-hopping contribution. Together with negative magnetoresistance, the latter relies on the presence of disorder. The charge-ordered phase also features a prominent dielectric peak which bears a similarity to relaxor ferroelectrics. Its dispersion is determined by free-electron screening and pushed by disorder well below the transition temperature. The source of this disorder can be found in the anion layers which randomly perturb BEDT-TTF molecules through hydrogen bonds.

Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in organic solar cells (OSC). In this paper, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dynamics. In the noninteracting model, the SF yield is absent. The individual molecules are treated within Hubbard and Pariser-Parr-Pople (PPP) models and the interaction between them involves transfer terms, intersite electron repulsions, and site-charge–bond-charge repulsion terms. Initial wave packet is constructed from excited singlet state of one molecule and ground state of the other. Time development of this wave packet under the influence of intermolecular interactions is followed within the Schrödinger picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, $2{}^{1}A$ excited singlet state leads to significant SF yield while the $1{}^{1}B$ state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient $2{}^{1}A$ character and hence results in significant SF yield. Because of rapid internal conversion, the nature of the lowest excited singlet will determine the SF contribution to OSC efficiency. Furthermore, we find the fission yield depends considerably on the stacking arrangement of the polyene molecules.

A valley degree of freedom (DOF) in transition-metal dichalcogenides with broken inversion symmetry can be controlled through spin and orbital DOFs owing to their valley-contrasting characters. Another important aspect of the spin and orbital DOFs is that they affect quasiparticle scattering processes that govern the valley lifetime. Here we combine quasiparticle-interference (QPI) imaging experiments and theoretical simulations to study the roles of the spin and orbital DOFs in $3R-{\mathrm{NbS}}_2$. We find that the QPI signal arising from an intervalley scattering is noticeably weaker than that caused by an intravalley scattering. We show that this behavior is predominantly associated with the orbital DOF, signifying the different spin and orbital structures of spin-split bands at each valley. These findings provide important insights into understanding the valley-related transport properties.

By using the combination of the ab initio density functional theory and data mining of the Inorganic Crystal Structure Database, a series of monolayer materials are found to be suitable substrates for germanene or stanene, including some of the $\mathrm{Cd}{\mathrm{I}}_2$-type materials, CuI, and GaGeTe. All of the found materials, when they are used as the substrate, can almost preserve the quasifreestanding geometry, stability, and band structure of germanene or stanene. Among them, $\mathrm{Cd}{\mathrm{I}}_2$ and $\mathrm{Zn}{\mathrm{I}}_2$ can open a relatively large band gap of 0.16–0.18 (0.13–0.16) eV [without (with) spin-orbit coupling] in germanene, while preserving Dirac-cone-like band structures. Moreover, the $Z_2$ invariants of germanene on CuI and stanene on $\mathrm{Ca}{\mathrm{I}}_2$ are found to be nontrivial. The interaction between germanene and substrates can be well modeled by the low-energy tight-binding Hamiltonian of germanene under external fields. Our analysis based on the tight-binding Hamiltonian shows that suitable substrates mainly act like a “pseudoelectric” field on germanene, and that strong linear correlations are seen among the “pseudoelectric” field, extrinsic Rashba coefficient, and charge transfer.