We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a path to control spin currents in superconductors.

Density-functional calculations are performed to investigate the experimentally reported field-induced phase transition in thin-film ${\mathrm{ZrO}}_2$ [J. Müller et al., Nano Lett. 12, 4318 (2012)]. We find a small energy difference of $\sim 1$ meV/f.u. between the nonpolar tetragonal and polar orthorhombic structures, characteristic of antiferroelectricity. The requisite first-order transition between the two phases, which atypically for antiferroelectrics have a group-subgroup relation, results from coupling to other zone-boundary modes, as we show with a Landau-Devonshire model. Tetragonal ${\mathrm{ZrO}}_2$ is thus established as a lead-free antiferroelectric with excellent dielectric properties and compatibility with silicon. In addition, we demonstrate that a ferroelectric phase of ${\mathrm{ZrO}}_2$ can be stabilized through epitaxial strain, and suggest an alternative stabilization mechanism through continuous substitution of Zr by Hf.

We performed an optical spectroscopy measurement on single crystals of ${\mathrm{Ba}}_2{\mathrm{Ti}}_2{\mathrm{Fe}}_2{\mathrm{As}}_4\mathrm{O}$, which is a newly discovered superconductor showing a coexistence of superconductivity and density wave orders. The study reveals spectral changes associated with both density wave and superconductivity phase transitions. The density wave phase transition at $T_{\mathrm{DW}}\approx 125$ K leads to the reconstruction of Fermi surfaces which removes about half of Drude spectral weight. The ratio of $2{\Delta }_{\mathrm{DW}}/k_B{T}_{\mathrm{DW}}\approx 11.9$ is considerably larger than the mean-field value based on the weak-coupling BCS theory. At the lowest temperature in the superconducting state, further spectral change associated with the superconducting condensate is observed. The low frequency optical conductivity could be well modeled within the Mattis-Bardeen approach with two isotropic gaps of ${\Delta }_1\left(0\right)=3.4$ meV and ${\Delta }_2\left(0\right)=7.9$ meV. The superconducting properties of the ${\mathrm{Ba}}_2{\mathrm{Ti}}_2{\mathrm{Fe}}_2{\mathrm{As}}_4\mathrm{O}$ compound are similar to those of ${\mathrm{BaFe}}_{1.85}{\mathrm{Co}}_{0.15}{\mathrm{As}}_2$.

Magnetic shape-memory Heusler alloys are multiferroics stabilized by the correlations between electronic, magnetic, and structural order. To study these correlations we use time-resolved x-ray diffraction and magneto-optical Kerr effect experiments to measure the laser induced dynamics in a Heusler alloy ${\mathrm{Ni}}_2\mathrm{MnGa}$ film and reveal a set of time scales intrinsic to the system. We observe a coherent phonon which we identify as the amplitudon of the modulated structure and an ultrafast phase transition leading to a quenching of the incommensurate modulation within 300 fs with a recovery time of a few ps. The thermally driven martensitic transition to the high temperature cubic phase proceeds via nucleation within a few ps and domain growth limited by the speed of sound. The demagnetization time is 320 fs, which is comparable to the quenching of the structural modulation.

We observe the real-time breaking of single Cooper pairs by monitoring the radio-frequency impedance of a superconducting double quantum dot. The Cooper pair breaking rate in the microscale islands of our device decreases as temperature is reduced, saturating at 2 kHz for temperatures beneath 100 mK. In addition, we measure in real time the quasiparticle recombination into Cooper pairs. Analysis of the recombination rates shows that, in contrast to bulk films, a multistage recombination pathway is followed.

Graphene sheets encapsulated between hexagonal boron nitride (hBN) slabs display superb electronic properties due to very limited scattering from extrinsic disorder sources such as Coulomb impurities and corrugations. Such samples are therefore expected to be ideal platforms for highly tunable low-loss plasmonics in a wide spectral range. In this article we present a theory of collective electron density oscillations in a graphene sheet encapsulated between two hBN semi-infinite slabs (hBN/G/hBN). Graphene plasmons hybridize with hBN optical phonons forming hybrid plasmon-phonon modes. We focus on scattering of these modes against graphene's acoustic phonons and hBN optical phonons, two sources of scattering that are expected to play a key role in hBN/G/hBN stacks. We find that at room temperature the scattering against graphene's acoustic phonons is the dominant limiting factor for hBN/G/hBN stacks, yielding theoretical inverse damping ratios of hybrid plasmon-phonon modes of the order of 50–60, with a weak dependence on carrier density and a strong dependence on illumination frequency. We confirm that the plasmon lifetime is not directly correlated with the mobility: In fact, it can be anticorrelated.

Our development of the self-consistent mean-field (SCMF) kinetic theory for nonuniform alloys leads to the statement that kinetic correlations induced by the vacancy diffusion mechanism have a dramatic effect on nanoscale diffusion phenomena, leading to nonlinear features of the interdiffusion coefficients. Lattice rate equations of alloys including nonuniform gradients of chemical potential are derived within the Bragg-Williams statistical approximation and the third shell kinetic approximation of the SCMF theory. General driving forces including deviations of the free energy from a local equilibrium thermodynamic formulation are introduced. These deviations are related to the variation of vacancy motion due to the spatial variation of the alloy composition. During the characteristic time of atomic diffusion, multiple exchanges of the vacancy with the same atoms may happen, inducing atomic kinetic correlations that depend as well on the spatial variation of the alloy composition. As long as the diffusion driving forces are uniform, the rate equations are shown to obey in this form the Onsager formalism of thermodynamics of irreversible processes (TIP) and the TIP-based Cahn-Hilliard diffusion equation. If now the chemical potential gradients are not uniform, the continuous limit of the present SCMF kinetic equations does not coincide with the Cahn-Hilliard (CH) equation. In particular, the composition gradient and higher derivative terms depending on kinetic parameters add to the CH thermodynamic-based composition gradient term. Indeed, a diffusion equation written as a mobility multiplied by a thermodynamic formulation of the driving forces is shown to be inadequate. In the reciprocal space, the thermodynamic driving force has to be multiplied by a nonlinear function of the wave vector accounting for the variation of kinetic correlations with composition inhomogeneities. Analytical expressions of the effective interdiffusion coefficient are given for two limit behaviors of the vacancy, the latter treated as either a conservative species (fixed concentration) or a nonconservative species (time-dependent equilibrium concentration). Relying on the same vacancy diffusion model, we perform kinetic Monte Carlo simulations starting from a sinusoidal composition modulation in binary model alloys, with no interaction or nearest-neighbor interactions leading to clustering or ordering tendencies, along the $\left[100\right]$ crystallographic direction of a body centered cubic (bcc) lattice. The resulting temporal variation of the modulation amplitude is compared to the corresponding SCMF equations. Qualitative and satisfying quantitative agreements systematically strengthen our theoretical conclusions. The model alloys are shown to be representative enough of some real alloys, so that one may expect these new heterogeneous correlation effects to be non-negligible in these alloys.

One-dimensional topological phases can host localized zero-energy modes that enable high-fidelity storage and manipulation of quantum information. Majorana fermion chains support a classic example of such a phase, having zero modes that guarantee twofold degeneracy in all eigenstates up to exponentially small finite-size corrections. Chains of “parafermions”—generalized Majorana fermions—also support localized zero modes, but, curiously, only under much more restricted circumstances. We shed light on the enigmatic zero-mode stability in parafermion chains by analytically and numerically studying the spectrum and developing an intuitive physical picture in terms of domain-wall dynamics. Specifically, we show that even if the system resides in a gapped topological phase with an exponentially accurate ground-state degeneracy, higher-energy states can exhibit a splitting that scales as a power law with system size, categorically ruling out exact localized zero modes. The transition to power-law behavior is described by critical behavior appearing exclusively within excited states.

Chiral magnets support topological skyrmion textures due to the Dzyaloshinskii-Moriya spin-orbit interaction. In the presence of a sufficiently large applied magnetic field, such skyrmions are large-amplitude excitations of the field-polarized magnetic state. We investigate analytically the interaction between such a skyrmion excitation and its small-amplitude fluctuations, i.e., the magnons in a clean two-dimensional chiral magnet. The magnon spectrum is found to include two magnon-skyrmion bound states corresponding to a breathing mode and, for intermediate fields, a quadrupolar mode, which will give rise to subgap magnetic and electric resonances. Due to the skyrmion topology, the magnons scatter from an Aharonov-Bohm flux density that leads to skew and rainbow scattering, characterized by an asymmetric differential cross section with, in general, multiple peaks. As a consequence of the skew scattering, a finite density of skyrmions will generate a topological magnon Hall effect. Using the conservation law for the energy-momentum tensor, we demonstrate that the magnons also transfer momentum to the skyrmion. As a consequence, a magnon current leads to magnon pressure reflected in a momentum-transfer force in the Thiele equation of motion for the skyrmion. This force is reactive and governed by the scattering cross sections of the skyrmion; it causes not only a finite skyrmion velocity but also a large skyrmion Hall effect. Our results provide, in particular, the basis for a theory of skyrmion caloritronics for a dilute skyrmion gas in clean insulating chiral magnets.

We demonstrate the potential of a periodic laser-pulse protocol for dynamic decoupling of hole spins in (In,Ga)As quantum dots from surrounding baths. When doubling the repetition rate of inversion laser pulses between two reference pulses for orienting the spins, we find that the spin coherence time is increased by a factor of 2.

An important goal in nanoelectromechanics is to cool the vibrational motion, ideally to its quantum ground state. Cooling by an applied charge current is a particularly simple and hence attractive strategy to this effect. Here we explore this phenomenon in the context of the general theory of thermoelectrics. In linear response, this theory describes thermoelectric refrigerators in terms of their cooling efficiency $\eta$ and figure of merit $ZT$. We show that both concepts carry over to phonon cooling in nanoelectromechanical systems. As an important consequence, this allows us to discuss the efficiency of phonon refrigerators in relation to the fundamental Carnot efficiency. We illustrate these general concepts by thoroughly investigating a simple double-quantum-dot model with the dual advantage of being quite realistic experimentally and amenable to a largely analytical analysis theoretically. Specifically, we obtain results for the efficiency, the figure of merit, and the effective temperature of the vibrational motion in two regimes. In the quantum regime in which the vibrational motion is fast compared to the electronic degrees of freedom, we can describe the electronic and phononic dynamics of the model in terms of master equations. In the complementary classical regime of slow vibrational motion, the dynamics is described in terms of an appropriate Langevin equation. Remarkably, we find that the efficiency can approach the maximal Carnot value in the quantum regime, with large associated figures of merit. In contrast, the efficiencies are typically far from the Carnot limit in the classical regime. Our theoretical results should provide guidance to implementing efficient vibrational cooling of nanoelectromechanical systems in the laboratory.

The paraelectric (PE) to ferroelectric (FE) first-order phase transition of ($1-x$)Pb(${\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}$)${\mathrm{O}}_3\text{−}x{\mathrm{PbTiO}}_3$ ($x=0.09$) ($T_c^c=455$ K on cooling) has been studied by the complementary use of x-ray diffuse scattering (XDS) and coherent x-ray scattering (CXS). XDS was mainly used to investigate the FE regions, while CXS was mainly used to investigate the PE regions above $T_c^c$ on cooling. The diffuse scattering intensity due to the appearance of FE regions shows a maximum at $T_{max}=460$ K. The diffuse scattering is dynamic in nature and the softening trend changes to a hardening trend at $T_{max}$. This means that the FE instability is maximum at $T_{max}$ and therefore the FE regions are well stabilized below $T_{max}$. The spatial autocorrelation function obtained by CXS, corresponding to the texture of PE regions, starts to rapidly change at about $T_{max}$ and is most unstable at $T_c^c$. We conclude that a heterophase fluctuation occurs between $T_c^c$ and $T_{max}$ near the phase transition. The heterophase fluctuation can be expected to correlate to the low-frequency dielectric dispersion and contribute to the phase transition as a precursor phenomenon of the first-order phase transition.

We present a symmetry analysis of electronic band structure including spin-orbit interaction close to the insulating gap edge in monolayer black phosphorus (“phosphorene”). Expressions for energy dispersion relation and spin-dependent eigenstates for electrons and holes are found via simplification of a perturbative expansion in wave vector $k$ away from the zone center using elementary group theory. Importantly, we expose the underlying symmetries giving rise to substantial anisotropy in optical absorption, charge, and spin transport properties, and reveal the mechanism responsible for valence band distortion and possible lack of a true direct gap.

We prove an instance of the reciprocity theorem that demonstrates that Kerr rotation, also known as the magneto-optical Kerr effect, may only arise in materials that break microscopic time-reversal symmetry. This argument applies in the linear-response regime and only fails for nonlinear effects. Recent measurements with a modified Sagnac interferometer have found finite Kerr rotation in a variety of superconductors. The Sagnac interferometer is a probe for nonreciprocity, so it must be that time-reversal symmetry is broken in these materials.

${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ was recently proposed as a promising candidate for time-reversal-invariant topological superconductors. In this work, we argue that the unusual anisotropy of the Knight shift observed by Zheng and co-workers (unpublished), taken together with specific heat measurements, provides strong support for an unconventional odd-parity pairing in the two-dimensional $E_u$ representation of the $D_{3d}$ crystal point group, which spontaneously breaks the threefold rotational symmetry of the crystal, leading to a subsidiary nematic order. We predict that the spin-orbit interaction associated with hexagonal warping plays a crucial role in pinning the two-component order parameter and makes the superconducting state generically fully gapped, leading to a topological superconductor. Experimental signatures of the $E_u$ pairing related to the nematic order are discussed.

We report the observation of a bulk charge modulation in ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ (LSCO) with a characteristic in-plane wave vector of $(0.236,\pm \delta )$, with $\delta =0.011$ r.l.u. The transverse shift of the ordering wave vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal “214” cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.

The optical properties of a ${\mathrm{V}}_4{\mathrm{O}}_7$ single crystal have been investigated from the high-temperature metallic phase down to the low-temperature antiferromagnetic insulating phase. The temperature-dependent behavior of the optical conductivity across the metal-to-insulator (MIT) transition can be explained in a polaronic scenario. Charge carriers form strongly localized polarons in the insulating phase, as suggested by a far-infrared charge gap abruptly opening at $T_{\mathrm{MIT}}$ $\approx$ 237 K. In the metallic phase, instead, the presence of a Drude term is indicative of fairly delocalized charges with a moderately renormalized mass ${\mathit{m}}^{*}\approx 5{\mathit{m}}_e$. The electronic spectral weight is almost recovered on an energy scale of 1 eV, which is much narrower than in the ${\mathrm{VO}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ cases. Those findings suggest that electron-lattice interaction rather than electronic correlation is the driving force for the ${\mathrm{V}}_4{\mathrm{O}}_7$ metal-insulator transition.

The electronic structure and band alignment at metal/oxide interfaces for nonvolatile memory applications are investigated by hard x-ray photoelectron spectroscopy (HAXPES) and DC transport measurements, using acceptor doped ${\mathrm{SrTiO}}_3$ as a model memristive oxide. Metal-insulator-metal (MIM) structures with a noble metal (Pt) top electrode form a Schottky barrier and exhibit rectifying properties, while a reactive metal (Ti) as top electrode shows symmetric I(V) characteristics and a flat band situation at the interface. The transition from rectifying to ohmic I(V) relations with increasing Ti thickness is discussed with respect to the electrochemical reaction at the interface, the band alignment at the electrode/oxide interface, and the slope of the energy bands across the MIM structure.

When electron doped, the layered transition metal nitrides $\mathcal{T}\mathrm{NCl}$ ($\mathcal{T}=\mathrm{group}$ IVB transition metal ion) become impressive superconductors with critical temperature $T_c=15–26$ K. Here we take the most studied member ZrNCl as a representative and calculate the dielectric response $\epsilon \left(\omega \right)$ versus frequency and concentration of doped electronic carriers. The static dielectric constant ${\epsilon }_{\infty }=5$ is reproduced extremely well. We establish that the differences between rigid band modeling and virtual crystal treatment are small, and compare also with actual lithium doping using supercells. We obtain the variations upon changing the doping level of the reflectivity and energy loss function as well, many of which are found not to be correlated with the observed (non)variation of $T_c\left(x\right)$. The main plasmon peaks appear where the electron gas model suggests, in the range 1.2–2.0 eV for $x$ varying from 0.16 to 0.50.

We present a diagrammatic approach to construct self-energy approximations within many-body perturbation theory with positive spectral properties. The method cures the problem of negative spectral functions which arises from a straightforward inclusion of vertex diagrams beyond the $GW$ approximation. Our approach consists of a two-step procedure: We first express the approximate many-body self-energy as a product of half-diagrams and then identify the minimal number of half-diagrams to add in order to form a perfect square. The resulting self-energy is an unconventional sum of self-energy diagrams in which the internal lines of half a diagram are time-ordered Green's functions, whereas those of the other half are anti-time-ordered Green's functions, and the lines joining the two halves are either lesser or greater Green's functions. The theory is developed using noninteracting Green's functions and subsequently extended to self-consistent Green's functions. Issues related to the conserving properties of diagrammatic approximations with positive spectral functions are also addressed. As a major application of the formalism we derive the minimal set of additional diagrams to make positive the spectral function of the $GW$ approximation with lowest-order vertex corrections and screened interactions. The method is then applied to vertex corrections in the three-dimensional homogeneous electron gas by using a combination of analytical frequency integrations and numerical Monte Carlo momentum integrations to evaluate the diagrams.