We study theoretically the effects of interfacial Rashba and Dresselhaus spin-orbit coupling in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions—with allowing for tunneling barriers between the ferromagnetic and superconducting layers—by solving the Bogoljubov–de Gennes equation for realistic heterostructures and applying the Furusaki-Tsukada technique to calculate the electric current at a finite temperature. The presence of spin-orbit couplings leads to out-of-plane and in-plane magnetoanisotropies of the Josephson current, which are giant in comparison to current magnetoanisotropies in similar normal-state ferromagnet/normal metal (F/N) junctions. Especially huge anisotropies appear in the vicinity of $0-\pi$ transitions, caused by the exchange-split bands in the ferromagnetic metal layer. We also show that the direction of the Josephson critical current can be controlled (inducing $0-\pi$ transitions) by the strength of the spin-orbit coupling and, more crucial, by the orientation of the magnetization. Such a control can bring new functionalities into Josephson junction devices.

In this joint experimental and theoretical work magnetic properties of the ${\mathrm{Cu}}^{2+}$ mineral szenicsite ${\mathrm{Cu}}_3\left({\mathrm{MoO}}_4\right){\left(\mathrm{OH}\right)}_4$ are investigated. This compound features isolated triple chains in its crystal structure, where the central chain involves an edge-sharing geometry of the ${\mathrm{CuO}}_4$ plaquettes, while the two side chains feature a corner-sharing zigzag geometry. The magnetism of the side chains can be described in terms of antiferromagnetic dimers with a coupling larger than 200 K. The central chain was found to be a realization of the frustrated antiferromagnetic $J_1\text{−}{J}_2$ chain model with $J_1\simeq 68$ K and a sizable second-neighbor coupling $J_2$. The central and side chains are nearly decoupled owing to interchain frustration. Therefore, the low-temperature behavior of szenicsite should be entirely determined by the physics of the central frustrated $J_1\text{−}{J}_2$ chain. Our heat-capacity measurements reveal an accumulation of magnetic entropy at low temperatures and suggest a proximity of the system to the Majumdar-Ghosh point of the antiferromagnetic $J_1\text{−}{J}_2$ spin chain, $J_2/J_1=0.5$.

The possibility to engineer the coupling of spin and valley physics is explored in ferroelectric oxide heterostructures with $e_g^2$ electronic configuration. We show that the polar structural distortion induces the appearance of spin-valley coupled properties, at the same time as being responsible for a topological transition from a quantum spin-Hall insulating phase to a trivial band insulator. The coupled spin-valley physics is affected by the topological band inversion in a nontrivial way; while the valley-dependent spin polarization of both conduction and valence bands is preserved, a change of the Berry curvature and of spin-valley selection rules is predicted, leading to different circular dichroic response as well as valley and spin Hall effects.

We study the expectation values of observables and correlation functions at long times after a global quantum quench. Our focus is on metallic (“gapless”) fermionic many-body models and small quenches. The system is prepared in an eigenstate of an initial Hamiltonian, and the time evolution is performed with a final Hamiltonian which differs from the initial one in the value of one global parameter. We first derive general relations between time-averaged expectation values of observables as well as correlation functions and those obtained in an eigenstate of the final Hamiltonian. Our results are valid to linear and quadratic order in the quench parameter $g$ and generalize prior insights in several essential ways. This allows us to develop a phenomenology for the thermalization of local quantities up to a given order in $g$. Our phenomenology is put to a test in several case studies of one-dimensional models representative of four distinct classes of Hamiltonians: quadratic ones, effectively quadratic ones, those characterized by an extensive set of (quasi-) local integrals of motion, and those for which no such set is known (and believed to be nonexistent). We show that for each of these models, all observables and correlation functions thermalize to linear order in $g$. The more local a given quantity, the longer the linear behavior prevails when increasing $g$. Typical local correlation functions and observables for which the term $\mathcal{O}\left(g\right)$ vanishes thermalize even to order $g^2$. Our results show that lowest-order thermalization of local observables is an ubiquitous phenomenon even in models with extensive sets of integrals of motion.

At low temperature, the experimental gap of silicon decreases as $E_g\left(T\right)=E_g\left(0\right)-A{T}^4$. The main reason is electron-phonon renormalization. The physics behind the $T^4$-power law is more complex than has been realized. Renormalization at low $T$ by intraband scattering requires a nonadiabatic treatment in order to correctly include acoustic phonons and avoid divergences from piezoacoustic phonon interactions. The result is an unexpected low $T$ term $E_g\left(0\right)+A^{\prime }{T}^p$ with positive coefficient $A^{\prime }$, and power $p=4$ for nonpiezoelectric materials, and power $p=2$ for piezoelectric materials. The acoustic phonons in piezoelectric semiconductors generate a piezoelectric field, modifying the electron-phonon coupling. However, at higher $T$, thermally excited acoustic phonons of energy $\hslash v_{s}q$ and intraband excitation energies ${\epsilon }_q-{\epsilon }_0={\hslash }^2{q}^2/2m^{*}$ become comparable in size. Above this temperature, the low $q$ and higher $q$ intraband acoustic phonon contributions to $T^p$ rapidly cancel, leaving little thermal effect. Then the contribution from interband scattering by acoustic phonons is dominant. This has the power law $T^4$ for both nonpiezoelectric and piezoelectric semiconductors. The shift can then have either sign, but usually reduces the size of gaps as $T$ increases. It arises after cancellation of the $T^2$ terms that appear separately in Debye-Waller and Fan parts of the acoustic phonon interband renormalization. The cancellation occurs because of the acoustic sum rule.

Transformation optics, a geometrical recipe for metamaterial design, was originally conceived as a tool to smoothly modify the trajectory of light using continuous coordinate transformations. Here, we show how discontinuous transformations can be used as a geometric framework to understand and manipulate phenomena at the surface of nanophotonic structures. In particular, we show how the Goos-Hänchen shift—a lateral shift exhibited by totally reflected beams—can be tailored and we provide a classification and complete analytical description of this effect in existing complex media.

A weak coupling quantum master equation provides reliable steady-state results only in the van Hove limit, i.e., when the system-lead coupling approaches zero. Recently, J. Thingna et al. [Phys. Rev. E 88, 052127 (2013)] proposed an alternative approach, based on an analytic continuation of the Redfield solution, to evaluate the steady-state reduced density matrix up to second order in the system-bath coupling. The approach provides accurate results for harmonic oscillator and spin-bosonic systems. We apply this approach to study steady-state fermionic systems and the calculation on an exactly solvable double quantum dot system shows that the method is rigorously valid up to second order in system-lead coupling only near equilibrium, i.e., linear response regime. We further compare to the Redfield and the secular Redfield (Lindblad-type) master equations that are inaccurate in all parameter regimes. Lastly, we consider the nontrivial problem of strong Coulomb interaction and illustrate the interplay between system-lead coupling, interdot tunneling, and Coulomb strength that can be captured only via the analytic continuation method.

We present a detailed study of local and nonlocal correlations in the electronic structure of elemental transition metals carried out by means of the quasiparticle self-consistent GW ($\mathrm{QS}\mathit{GW}$) and dynamical mean field theory (DMFT). Recent high resolution ARPES and Haas-van Alphen data of two typical transition metal systems (Fe and Ni) are used as a case study. (i) We find that the properties of Fe are very well described by $\mathrm{QS}\mathit{GW}$. Agreement with cyclotron and very clean ARPES measurements is excellent, provided that final-state scattering is taken into account. This establishes the exceptional reliability of $\mathrm{QS}\mathit{GW}$ also in metallic systems. (ii) Nonetheless $\mathrm{QS}\mathit{GW}$ alone is not able to provide an adequate description of the Ni ARPES data due to strong local spin fluctuations. We surmount this deficiency by combining nonlocal charge fluctuations in $\mathrm{QS}\mathit{GW}$ with local spin fluctuations in DMFT. (iii) Finally we show that the dynamics of the local fluctuations are actually not crucial. The addition of an external static field can lead to similarly good results if nonlocal correlations are included through $\mathrm{QS}\mathit{GW}$.

We introduce a first-principles study to clarify the carrier-scattering property at the $\mathrm{SiC}/{\mathrm{SiO}}_2$. Interestingly, the electron transport at the conduction-band edge is significantly affected by the introduction of oxygen, even though there are no electrically active defects. The origin of the large scattering is explained by the behavior of the internal-space states (ISSs). Moreover, the effect of the ISSs is larger than that of the electrically active carbon-related defects. This result indicates that an additional scattering not considered in a conventional $\mathrm{Si}/{\mathrm{SiO}}_2$ occurs at the $\mathrm{SiC}/{\mathrm{SiO}}_2$.

We present the results of transport measurements in a hybrid system consisting of an arch-shaped quantum point contact (QPC) and a reflector; together, they form an electronic cavity in between them. On tuning the arch-QPC and the reflector, an asymmetric resonance peak in resistance is observed at the one-dimension to two-dimension transition. Moreover, a dip in resistance near the pinch-off of the QPC is found to be strongly dependent on the reflector voltage. These two structures fit very well with the Fano line shape. The Fano resonance was found to get weakened on applying a transverse magnetic field, and smeared out at 100 mT. In addition, the Fano-like shape exhibited a strong temperature dependence and gradually smeared out when the temperature was increased from 1.5 to 20 K. The results might be useful in realizing devices for quantum information processing.

In the present work we developed an orbital coupling model for cubic full Heusler compounds that provides a unified set of rules that account for the chemical ordering, magnetic moment, and composition of the most promising candidates for half-metallicity. The origin and limitations of the rules are clearly described. To the best of our knowledge all of the several dozen half-metallic Heusler compounds known in the literature that follow the $M_t=N_t-24$ or $M_t=N_t-28$ generalized Slater-Pauling behavior satisfy the derived half-metallicity rule. Calculations performed by using density functional theory—performed for 259 compounds—confirm the validity of our model and derived rules for broad classes of Heusler compounds.

Predicting the compositional phase stability of strongly correlated electron materials is an outstanding challenge in condensed matter physics. In this work, we employ the density functional theory plus $U$ $\left(\mathrm{DFT}+U\right)$ formalism to address the effects of local correlations due to transition metal $d$ electrons on compositional phase stability in the prototype phase stable and separating materials ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ and olivine ${\mathrm{Li}}_x{\mathrm{FePO}}_4$, respectively. We introduce a spectral decomposition of the $\mathrm{DFT}+U$ total energy, revealing the distinct roles of the filling and ordering of the $d$ orbital correlated subspace. The on-site interaction $U$ drives both of these very different materials systems towards phase separation, stemming from enhanced ordering of the $d$ orbital occupancies in the $x=0$ and $x=1$ species, whereas changes in the overall filling of the $d$ shell contribute negligibly. We show that $\mathrm{DFT}+U$ formation energies are qualitatively consistent with experiments for phase stable ${\mathrm{Li}}_x{\mathrm{CoO}}_2$, phase separating ${\mathrm{Li}}_x{\mathrm{FePO}}_4$, and phase stable ${\mathrm{Li}}_x{\mathrm{CoPO}}_4$. However, we find that charge ordering plays a critical role in the energetics at intermediate $x$, strongly dampening the tendency for the Hubbard $U$ to drive phase separation. Most relevantly, the phase stability of ${\mathrm{Li}}_{1/2}{\mathrm{CoO}}_2$ within $\mathrm{DFT}+U$ is qualitatively incorrect without allowing charge ordering, which is problematic given that neither charge ordering nor the band gap that it induces are observed in experiment. We demonstrate that charge ordering arises from the correlated subspace interaction energy as opposed to the double counting. Additionally, we predict the Li order-disorder transition temperature for ${\mathrm{Li}}_{1/2}{\mathrm{CoO}}_2$, demonstrating that the unphysical charge ordering within $\mathrm{DFT}+U$ renders the method problematic, often producing unrealistically large results. Our findings motivate the need for other advanced techniques, such as DFT plus dynamical mean-field theory, for total energies in strongly correlated materials.

We investigate a large variation in a diagonal component of the linear magnetoelectric (ME) susceptibility in the polar ferrimagnet $\mathrm{M}{\mathrm{n}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$ while changing the magnetic-ion site from orbital-quenched $\mathrm{M}{\mathrm{n}}^{2+}$ to $\mathrm{F}{\mathrm{e}}^{2+}$ with strong uniaxial anisotropy. In $\mathrm{M}{\mathrm{n}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$, the linear ME susceptibility is dominated by the nonrelativistic exchange striction mechanism, showing a positive value at low temperature and a critical behavior toward negative divergence around the transition temperature. This negative peak value becomes four times larger when $50\%$ of $\mathrm{M}{\mathrm{n}}^{2+}$ ions are replaced with the $\mathrm{F}{\mathrm{e}}^{2+}$ ion, highlighting the beneficial perspective of the compositionally tunable ME effect. As the doped Fe increases in fraction, gradual negative shift of the ME coefficient is observed around the lowest temperature, which manifests the relativistic single-site origin of the ME susceptibility. Further doping with $x\gtrsim 75\%$ in $(\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{F}{\mathrm{e}}_x{)}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$, the ME coefficient becomes negative in the entire temperature region. Such a composition dependence can be explained in terms of the chemical preference for the two types of magnetic sites of Mn/Fe ions. The present paper demonstrates the large tunability of ME effect by substituting the magnetic ion with the primary structural/magnetic characters kept intact.

Strained fold-like deformations on armchair graphene nanoribbons (AGNRs) can be properly engineered in experimental setups, and could lead to a controlling tool for gaps and transport properties. Here, we analyze the electronic properties of folded AGNRs relating to the electronic responses and the mechanical deformation. An important and universal parameter for the gap engineering is the ribbon percent-width variation, i.e., the difference between the deformed and undeformed ribbon widths. AGNRs band gap can be tuned mechanically in a well-defined bounded range of energy values, eventually leading to a metallic system. This characteristic provides a controllable degree of freedom that allows manipulation of electronic currents. We show that the numerical results are analytically predicted by solving the Dirac equation for the strained system.

We discuss a magnetically ordered (“superfluid”) phase near quantum transition to the Bose-glass phase in a simple modeling system, a Heisenberg antiferromagnet with spatial dimension $d>2$ in an external magnetic field with disorder in exchange coupling constants. Our analytical consideration is based on hydrodynamic description of long-wavelength excitations. Results obtained are valid in the entire critical region near the quantum critical point (QCP), allowing us to describe a possible crossover from one critical behavior to another. We demonstrate that the system behaves in full agreement with predictions by M. P. Fisher et al. [Phys. Rev. B 40, 546 (1989)] in close vicinity to the QCP. We find as an extension to that analysis that the anomalous dimension $\eta =2-d$ and $\beta =\nu d/2$, where $\beta$ and $\nu$ are critical exponents of the order parameter and the correlation length, respectively. The density of states per spin of low-energy localized excitations is found to be independent of $d$ (“superuniversal”). We show that many recent experimental and numerical results obtained in various three-dimensional (3D) systems can be described by our formulas using percolation critical exponents. Then, it is a possibility that a percolation critical regime arises in the ordered phase in some 3D systems not very close to the QCP.

Antiferromagnets (AFMs) are presently considered as promising materials for applications in spintronics and random access memories due to the robustness of information stored in the AFM state against perturbing magnetic fields. In this respect, AFM multiferroics may be attractive alternatives for conventional AFMs as the coupling of magnetism with ferroelectricity (magnetoelectric effect) offers an elegant possibility of electric-field control and switching of AFM domains. Here we report the results of comprehensive experimental and theoretical investigations of the quadratic magnetoelectric (ME) effect in single crystals and highly resistive ceramics of $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_3$ (PFN) and $(1-x)\mathrm{Pb}\left(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3-x\mathrm{PbTi}{\mathrm{O}}_3\left(\mathrm{PFN}\text{−}x\mathrm{PT}\right)$. We are interested primarily in the temperature range of the multiferroic phase, $T<150\mathrm{K}$, where the ME coupling coefficient is extremely large (as compared to the well-known multiferroic $\mathrm{BiFe}{\mathrm{O}}_3$) and shows sign reversal at the paramagnetic-to-antiferromagnetic phase transition. Moreover, we observe strong ME response nonlinearity in the AFM phase in the magnetic fields of only a few kOe. To describe the temperature and magnetic field dependencies of the above unusual features of the ME effect in PFN and PFN-$x\mathrm{PT}$, we use a simple phenomenological Landau approach which explains experimental data surprisingly well. Our ME measurements demonstrate that the electric field of only 20–25 kV/cm is able to switch the AFM domains and align them with ferroelectric ones even in PFN ceramic samples.

Iron is one of the archetypical ferromagnets to study the critical fluctuations at a continuous phase transition thus serving as a model system for the application of scaling theory. We report a comprehensive study of the critical dynamics at the transition from the ferro- to the paramagnetic phase in Fe, employing the high-resolution neutron spin-echo technique, modulated intensity of zero effort (MIEZE). The results show that the dipolar interactions lead to an additional damping of the critical spin fluctuations at small momentum transfers $\mathbf{q}$. The results agree essentially with scaling theory if the dipolar interactions are taken into account by means of the mode-coupling equations. However, in contrast to expectations, the dipolar wave number $q_D$ that plays a central role in the scaling function $f(\kappa /q,q_D/\kappa )$ becomes temperature dependent. In the limit of small $\mathbf{q}$ the critical exponent $z$ crosses over from 2.5 to 2.0.

Using density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the stability and response functions of ${\mathrm{CsH}}_2{\mathrm{PO}}_4$, a ferroelectric material at low temperature. This material cannot be described properly by the usual (semi)local approximations within DFT. The long-range $e^{-}-e^{-}$ correlation needs to be properly taken into account, using, for instance, Grimme's DFT-D methods, as investigated in this work. We find that DFT-D3(BJ) performs the best for the members of the dihydrogenated alkali phosphate family (${\mathrm{KH}}_2{\mathrm{PO}}_4$, ${\mathrm{RbH}}_2{\mathrm{PO}}_4$, ${\mathrm{CsH}}_2{\mathrm{PO}}_4$), leading to experimental lattice parameters reproduced with an average deviation of 0.5%. With these DFT-D methods, the structural, dielectric, vibrational, and mechanical properties of ${\mathrm{CsH}}_2{\mathrm{PO}}_4$ are globally in excellent agreement with the available experiments ($<2\%$ MAPE for Raman-active phonons). Our study suggests the possible existence of a new low-temperature phase of ${\mathrm{CsH}}_2{\mathrm{PO}}_4$, not yet reported experimentally. Finally, we report the implementation of DFT-D contributions to elastic constants within DFPT.

Strong electronic correlations related to a repulsive local interaction suppress the electronic compressibility in a single-band model, and the capacitance of a corresponding metallic film is directly related to its electronic compressibility. Both statements may be altered significantly when two extensions to the system are implemented which we investigate here: (i) we introduce an attractive nearest-neighbor interaction $V$ as antagonist to the repulsive onsite repulsion $U$, and (ii) we consider nanostructured multilayers (heterostructures) assembled from two-dimensional layers of these systems. We determine the respective total compressibility $\kappa$ and capacitance $C$ of the heterostructures within a strong coupling evaluation, which builds on a Kotliar-Ruckenstein slave-boson technique. Whereas the capacitance $C\left(n\right)$ for electronic densities $n$ close to half-filling is suppressed, illustrated by a correlation induced dip in $C\left(n\right)$, it may be appreciably enhanced close to a van Hove singularity. Moreover, we show that the capacitance may be a nonmonotonic function of $U$ close to half-filling for both attractive and repulsive $V$. The compressibility $\kappa$ can differ from $C$ substantially, as $\kappa$ is very sensitive to internal electrostatic energies which in turn depend on the specific setup of the heterostructure. In particular, we show that a capacitor with a polar dielectric has a smaller electronic compressibility and is more stable against phase separation than a standard nonpolar capacitor with the same capacitance.