Nanoscale superconducting quantum interference devices (nano-SQUIDS) where the weak links are made from nanobridges, i.e., nanobridge SQUIDs (NBSs), are one of the most sensitive magnetometers for nanoscale magnetometry. Because of very strong nonlinearity in the nanobridge-electrode joints, the applied magnetic flux (${\mathrm{\Phi }}_a$)–critical current ($I_c$) characteristics of NBSs differ very significantly from conventional tunnel-junction SQUIDs, especially when the nanobridges are long and/or the screening parameter is large. However, in most of the theoretical descriptions, NBSs have been treated as the conventional tunnel-junction SQUIDs, which are based on the dc Josephson effect. Here, I present a model demonstrating that for long nanobridges and/or large screening parameters the $I_c\left({\mathrm{\Phi }}_a\right)$ of a NBS can be explained by merely considering the fluxoid quantization in the NBS loop and the energy of the NBS; it is not necessary to take the Josephson effect into consideration. I also demonstrate that using the model, one can derive useful expressions such as the modulation depth and transfer function. I discuss the role of the kinetic inductance fraction ($\kappa$) in determining $I_c\left({\mathrm{\Phi }}_a\right)$. I compare the predictions of the present model with the experimental data already published by several groups.

The exciton dynamics in transverse magnetic field is investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The observed linear polarization of the quantum well photoluminescence has two contributions, one of which arises from the crystalline structure of the quantum well. It does not depend on temperature and demonstrates a strong spectral dependence across the emission band. The other one is induced by a transverse magnetic field. It strongly decreases with increasing temperature, has no spectral dependence, and demonstrates an unexpectedly long-time dynamics. The experimental findings can be explained in the framework of the developed theoretical model which accounts for the quantum well anisotropy, the Zeeman effect of electrons and holes in the transverse magnetic field, and the redistribution of excitons over the spin sublevels. It provides quantitative agreement with the experiment and allows us to evaluate, for the studied structure, the heavy-hole in-plane $g$-factor tensor, which turns out to be extremely anisotropic with principal values of opposite signs and the same magnitude of 0.25.

To predict transport properties of disordered systems, especially ac transport properties, one has to calculate the disorder average of the correlation of the multiple Green's function at different energies. To avoid brute force calculation, diagrammatic perturbation expansion must be used along with the coherent potential approximation (CPA). In this paper, we develop a theoretical formalism based on the nonequilibrium Green's function that maps the average of the correlation of the multiple Green's function into an average over a single generalized Green's function. After the mapping, this formalism is structurally very similar to the CPA and completely eliminates the need to perform diagrammatic expansion. As a demonstration of our theory, the dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias in the presence of Anderson disorder are calculated by directly taking the disorder average of the generating function of full counting statistics (FCS) within the CPA. Our numerical results on dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias show remarkable agreement with that obtained by the brute force calculation. The phase diagram in the frequency versus disorder strength plane has been efficiently calculated using the generalized FCS-CPA method.

Partially overlapping two parallel armchair nanotubes are investigated theoretically with the $\pi$ orbital tight bonding model. Considering the interlayer Hamiltonian as perturbation, we obtain approximate analytical formulas of the interlayer transmission rates $T_{{\sigma }^{\prime },\sigma }$ from channel $\sigma$ to ${\sigma }^{\prime }$ for all four combinations $({\sigma }^{\prime },\sigma )=(\pm ,\pm )$ and $(\pm ,\mp )$, where suffixes + and $-$ represent symmetric and antisymmetric channels, respectively, with respect to the mirror plane of each tube. Landauer's formula conductance is equal to the sum of them in units of $2e^2/h$. According to the perturbation calculation, the interlayer Hamiltonian is transformed into the parameter $w_{{\sigma }^{\prime },\sigma }$ that determines the analytical formula of $T_{{\sigma }^{\prime },\sigma }$. By comparison with the exact numerical results, the effective range of the analytical formulas is discussed. In the telescoped coaxial contact, the off-diagonal part $T_{-,+}+T_{+,-}$ is very small compared to the diagonal part $T_{+,+}+T_{-,-}$. In the side contact, on the other hand, the off-diagonal part is more significant than the diagonal part in the zero energy peak of the conductance.

We have investigated the charge transport in the paramagnetic metallic phase of hole-doped pyrochlore iridates. The temperature dependence of thermopower shows a remarkable peak, typically $45\mu \mathrm{V}$/K at 20 K, in a wide doping range from a heavily doped region to nearby the Mott transition. Calculations based on local density approximation and dynamical mean-field theory show the presence of quadratic band touching (QBT) in these compounds. This quantitatively elucidates the thermopower characteristics and specific heat of the doping-induced metallic state, suggesting that QBT is a common and robust feature in the paramagnetic metallic phase of hole-doped pyrochlore iridates even in the proximity of the Mott transition.

${}^{115}\mathrm{In}$ nuclear magnetic resonance data are presented for a series of ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{CoIn}}_5$ crystals with different La dilutions $x$. Multiple In(1) sites associated with different numbers of nearest-neighbor cerium atoms exhibit different Knight shifts and spin lattice relaxation rates. Analysis of the temperature dependence of these sites reveals both an evolution of the heavy electron coherence as a function of dilution, as well as spatial inhomogeneity associated with a complete suppression of antiferromagnetic fluctuations in the vicinity of the La sites. Quantum critical fluctuations persist within disconnected Ce clusters with dilution levels up to 75%, despite the fact that specific heat shows Fermi liquid behavior in dilute samples.

Electron transport in clean 2D systems with weak electron-phonon (e-ph) coupling can transition from an Ohmic to a ballistic or a hydrodynamic regime. The ballistic regime occurs when electron-electron (e-e) scattering is weak whereas the hydrodynamic regime arises when this scattering is strong. Despite this difference, we find that vortices and a negative nonlocal resistance believed to be quintessentially hydrodynamic are equally characteristic of the ballistic regime. These non-Ohmic regimes cannot be distinguished in DC transport without changing experimental conditions. Further, as our kinetic calculations show, the hydrodynamic regime in DC transport is highly fragile and is wiped out by even sparse disorder and e-ph scattering. We show that microwave-frequency AC sources by contrast readily excite hydrodynamic modes with current vortices that are robust to disorder and e-ph scattering. Indeed, current reversals in the non-Ohmic regimes occur via repeated vortex generation and mergers through reconnections, as in classical 2D fluids. Crucially, AC sources give rise to strong correlations across the entire device that unambiguously distinguish all regimes. These correlations in the form of nonlocal current-voltage and voltage-voltage phases directly check for the presence of a nonlocal current-voltage relation signifying the onset of non-Ohmic behavior as well as also for the dominance of bulk interactions, needed to confirm the presence of a hydrodynamic regime. We use these probes to demarcate all regimes in an experimentally realizable graphene device and find that the ballistic regime has a much larger extent in parameter space than the hydrodynamic regime.

Solid-solid and solid-liquid transformations were examined in Ge(100), using in situ x-ray diffraction measurements during uniaxial strain compression and release. For final stresses above 15.7 GPa, the Ge transformed to a highly textured tetragonal $\beta$-Sn phase. At 31.5 GPa and above, Ge transformed to the molten phase. Full stress release (uniaxial strain) from the $\beta$-Sn phase, from the melt boundary, and from the completely molten phase, resulted in reversion to an untextured cubic diamond (cd) phase. These findings demonstrate that the cd to $\beta$-Sn phase change is reversible, and that recrystallization from the liquid phase occurs on nanosecond timescales during release.

The conventional Landau-Lifshitz-Gilbert (LLG) equation is a widely used tool to describe the dynamics of local magnetic moments, viewed as classical vectors of fixed length, with their change assumed to take place simultaneously with the cause. Here we demonstrate that recently developed [M. D. Petrović et al., Phys. Rev. Appl. 10, 054038 (2018)] self-consistent coupling of the LLG equation to a time-dependent quantum-mechanical description of electrons—where nonequilibrium spin density from time-dependent nonequilibrium Green function (TDNEGF) calculations is inserted within a torque term into the LLG equation while local magnetic moments evolved by the LLG equation introduce time-dependent potential in the quantum Hamiltonian of electrons—microscopically generates time-retarded damping in the LLG equation described by a memory kernel that is also spatially dependent. For sufficiently slow dynamics of local magnetic moments on the memory time scale, the kernel can be expanded into power series to extract the Gilbert damping (proportional to the first time derivative of magnetization) and magnetic inertia (proportional to the second time derivative of magnetization) terms whose parameters, however, are time-dependent in contrast to time-independent parameters used in the conventional LLG equation. We use examples of single or multiple local magnetic moments precessing in an external magnetic field, as well as field-driven motion of a magnetic domain wall (DW), to quantify the difference in their time evolution computed from the conventional LLG equation versus the TDNEGF+LLG quantum-classical hybrid approach. The faster DW motion predicted by the TDNEGF+LLG approach reveals that important quantum effects, stemming essentially from a finite amount of time that it takes for a conduction electron spin to react to the motion of classical local magnetic moments, are missing from conventional classical micromagnetics simulations. We also demonstrate a large discrepancy between the TDNEGF+LLG-computed numerically exact and, therefore, nonperturbative result for charge current pumped by a moving DW and the same quantity computed by a perturbative spin motive force formula combined with the conventional LLG equation.

The normal-state behavior of the temperature-dependent Hall coefficient in cuprate superconductors is investigated using linear-response theory. The Hall conductivity is of paramount importance in that its sign and magnitude directly reflect the sign of the charge carriers and the size of particle-hole asymmetry effects. Here we apply a strong-pairing fluctuation theory that incorporates pseudogap effects known to be important in cuprate transport. As a result, in the vicinity of the transition temperature our theoretical approach goes beyond the conventional superconducting fluctuation formalism. In this regime, pseudogap effects are evident in both the transverse and longitudinal conductivities and the bosonic response is explicitly gauge invariant. The presence of a gap in the excitation spectrum is also apparent at higher temperatures, where the gapped fermionic quasiparticles are the dominant contribution to the Hall coefficient. The observed nonmonotonic temperature dependence of the Hall coefficient therefore results from a delicate interplay between the fermionic quasiparticles and the bosonic fluctuations. An important feature of our work is that the sign of the Hall conductivity from the Cooper-pair fluctuations is the same as that of their fermionic constituents. Thus, we find no sign change in the Hall coefficient above the transition temperature. This prediction is corroborated by experiments, away from special charge ordering stoichiometries. The theoretical results presented in this paper provide crucial signatures that can be experimentally verified, enabling validation of the present theory.

Room-temperature superconductivity has been one of the most challenging subjects in modern physics. Recent experiments reported that lanthanum hydride ${\mathrm{LaH}}_{10\pm x}$ ($x<1$) raises a superconducting transition temperature $T_{\mathrm{c}}$ up to $\sim 260$ (or 250) K at high pressures around 190 (170) GPa. Here, based on first-principles calculations, we reveal that compressed ${\mathrm{LaH}}_{10}$ has symmetry-protected Dirac-nodal-line states, which split into holelike and electronlike bands at the high-symmetry points near the Fermi energy ($E_{\mathrm{F}}$), thereby producing a van Hove singularity (vHs). The crystalline symmetry and the band topology around the high-symmetry points near $E_{\mathrm{F}}$ are thus demonstrated to be important for room-temperature superconductivity. Further, we identify that the electronic states at the vHs are composed of strongly hybridized La $f$ and H $s$ orbitals, giving rise to a peculiar characteristic of electrical charges with anionic La and both anionic and cationic H species. Consequently, a large number of electronic states at the vHs are strongly coupled to the H-derived high-frequency phonon modes that are induced via the unusual, intricate bonding network of ${\mathrm{LaH}}_{10}$, therefore yielding a high $T_{\mathrm{c}}$. Our findings elucidate the microscopic mechanism of the observed high-$T_{\mathrm{c}}$ BCS-type superconductivity in ${\mathrm{LaH}}_{10}$, which can be generic to another recently observed high-$T_{\mathrm{c}}$ hydride ${\mathrm{H}}_3\mathrm{S}$.

We have studied the interfacial Dzyaloshinskii-Moriya interaction (DMI) in $\mathrm{Pt}/\mathrm{Co}/M{\mathrm{O}}_x$ ($M=\text{Al}$, Gd) trilayers in which the degree of oxidation of the top Co interface is varied. To access reliable values of the DMI strength, we have used a method based on the measurement of the saturation velocity of field driven chiral Néel domain walls. We show that the effective DMI strength in the $\mathrm{Pt}/\mathrm{Co}/M{\mathrm{O}}_x$ trilayers varies with the oxidation degree of the $\mathrm{Co}/M{\mathrm{O}}_x$ interface. This strongly suggests that the $\mathrm{Co}/M{\mathrm{O}}_x$ interface gives a distinct contribution to the total DMI, adding to that of the Pt/Co interface. The DMI presents a maximum for the oxygen coverage maximizing also the interface magnetic anisotropy energy $K_s$. This calls for common microscopic origins for the contributions of the $\mathrm{Co}/M{\mathrm{O}}_x$ interface to DMI and $K_s$.

Mastering and understanding the magnetic couplings between magnetic electrodes separated by organic layers are crucial for developing new hybrid spintronic devices. We study the magnetic exchange interactions in organic-inorganic heterojunctions and unveil the possibility of controlling the strength of the magnetic exchange coupling between two ferromagnetic electrodes across $\pi$-conjugated molecules’ ($\alpha$-sexithiophene or para-sexiphenyl) ultrathin film. In $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4/\pi$-conjugated molecules/Co magnetic tunnel junctions, an antiferromagnetic interlayer exchange coupling with variable strength is observed according to the nature of the aromatic rings (thiophene or phenyl groups). The underlying physical mechanism is revealed by ab initio calculations relating the strength of magnetic coupling to the spin moment penetration into a molecular layer at the molecule/Co interface. The prospect that magnetic coupling between two ferromagnetic electrodes can be mediated and tuned by organic molecules opens different perspectives in the way magnetization of organic tunnel junctions or spin valves can be driven.

We report Abrikosov vortex nucleation in $\mathrm{Pt}/\mathrm{Nb}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Nb}/\mathrm{Pt}$ proximity-coupled structures under oblique ferromagnetic resonance that turns out to be detrimental to superconducting spin pumping. By measuring an out-of-plane field-angle ${\theta }_H$ dependence and comparing with Pt-absent control samples, we show that as ${\theta }_H$ increases, the degree of enhancement (suppression) of spin pumping efficiency in the superconducting state for the Pt-present (Pt-absent) sample diminishes and it reverts to the normal state value at ${\theta }_H={90}^{\circ }$. This can be explained in terms of a substantial out-of-plane component of the resonance field for the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ layer (with in-plane magnetization anisotropy and high aspect ratio) that approaches the upper critical field of the Nb, turning a large fraction of the singlet superconductor volume into the normal state.

We work out a unified theory describing both nonlocal electron transport and cross-correlated shot noise in a three-terminal normal-superconducting-normal (NSN) hybrid nanostructure. We describe noise cross correlations both for subgap and overgap bias voltages and for arbitrary distribution of channel transmissions in NS contacts. We specifically address a physically important situation of diffusive contacts and demonstrate nontrivial behavior of nonlocal shot noise exhibiting both positive and negative cross correlations depending on the bias voltages. For this case, we derive a relatively simple analytical expression for cross-correlated noise power which contains only experimentally accessible parameters.

The correlated polar semimetal ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ exhibits a rich phase diagram including two magnetic transitions ($T_N=56$ K and $T_C=48$ K) with the appearance of an insulating-like pseudogap (at $T_C$). In addition, there is a crossover back to metallic behavior at $T^{*}=30$ K, the origin of which is still under debate. We utilized ultrafast optical-pump optical-probe spectroscopy to investigate quasiparticle dynamics as a function of temperature in this enigmatic quantum material. We identify two dynamical processes, both of which are influenced by the onset of the pseudogap. This includes electron-phonon relaxation and, below $T_C$, the onset of a phonon bottleneck hindering the relaxation of quasiparticles across the pseudogap. We introduce a gap-modified two-temperature model to describe the temperature dependence of electron-phonon thermalization, and use the Rothwarf-Taylor to model the phonon bottleneck. In conjunction with density functional theory, our experimental results synergistically reveal the origin of the $T$-dependent pseudogap. Further, our data and analysis indicate that $T^{*}$ emerges as a natural consequence of $T$-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low-fluence ultrafast optics as a sensitive probe of low-energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.

We study the ac Hall response induced by passage of dc transport current in two- and three-dimensional metals with gyrotropic point groups—the gyrotropic Hall effect—and consider the phenomenon of current-induced optical activity in noncentrosymmetric metals as a physical application of our theory. While the effect is expected to be present in single crystals of any noncentrosymmetric metal, we expect it to be strongest in enantiomorphic Weyl semimetals. Using the semiclassical kinetic equation approach, we present several mechanisms underlying the gyrotropic Hall effect. Amongst them, the intrinsic mechanism is determined by the Berry curvature dipole, while extrinsic impurity-induced processes are related to skew scattering and side-jump phenomena. In general, the intrinsic and extrinsic contributions can be of similar magnitude. We discuss the gyrotropic Hall effect for all frequencies of practical interest, from the dc transport limit, to optical frequencies. We show that for frequencies that are small compared to relevant band splittings, the trace of the gyrotropic Hall tensor in three-dimensional materials is proportional to a topological, quantized Berry charge, and therefore is robust in gyrotropic Weyl systems. This implies that polycrystals of strongly gyrotropic Weyl semimetals will demonstrate strong current-induced optical activity, whereas the response vanishes for polycrystalline ordinary metals. Therefore the current-induced optical activity can be considered a valuable tool in identifying the topological nature of a material.

In field emission plasmas, electrons that initiate plasma formation come from the surface of a metallic electrode, or wall, with emission controlled by the electron-work function of the wall, and can be computed via the Fowler-Nordheim formula. Impinging ions modify the rate at which electrons leave the surface, and are accounted via the coefficient of secondary electron emission. However, in the case of dielectric surfaces, the microscopic mechanism by which electrons are emitted is not as well understood. While simulations of dielectric barrier discharge plasmas assume an initial density of electrons in a time-dependent simulation, whether the presence of electrons is a necessary ambient condition or whether it is a result of emission from a surface is not clear. This is particularly relevant in the context of micro and nanoscale plasma generators when surface-related effects become more important. Here we consider electron emission from dielectric surfaces in the context of dielectric barrier discharges. The configuration of interest consists of two parallel-plate metallic electrodes, each covered by a dielectric layer. Assuming that the initial electrons for plasma formation arise from the surface, we compute the rate of charge transfer from surfaces, which is a necessary, but not sufficient, condition for plasma formation. This paper presents the application of the theory of nonadiabatic transitions (dynamical level crossing) to the problem of electron emission from dielectric surfaces in dielectric barrier discharges. The microscopic model of electron transfer described here has potential applications in the design of micro and nanoscale plasma generators.

We utilize a combination of vector magnetic field and scanning tunneling microscopy to elucidate the three-dimensional field based electronic phase diagram of a correlated iron-based superconductor, LiFeAs. We observe, under a zero-field-cooled method, an ordered hexagonal vortex lattice ground state in contrast to the disordered lattice observed under a field-cooled method. It transforms to a fourfold-symmetric state by increasing the c-axis field and distorts elliptically upon tilting the field in-plane. The vortex lattice transformations correlate with the field-dependent superconducting gap that characterizes the Cooper pairing strength. The anisotropy of the vortex lattice agrees with the field-enhanced Bogoliubov quasiparticle scattering channel that is determined by the pairing symmetry in respect to its Fermi surface structure. Our systematic tuning of the vortex lattice symmetry and study of its correlation with Cooper pairing demonstrates the many-body interplay between the superconducting order parameter and emergent vortex matter.

The fractional quantum anomalous Hall (FQAH) states or fractional Chern insulator (FCI) states have been studied on two-dimensional (2D) flat lattices with different boundary conditions. Here, we propose the geometry-dependent FCI/FQAH states in which interacting particles are bounded on 2D singular lattices with arbitrary $n$-fold rotational symmetry. Based on the generalized Pauli principle, we construct trial wave functions for the singular-lattice FCI/FQAH states with the aid of an effective projection approach, and we compare them with the exact diagonalization results. High wave-function overlaps show that the singular-lattice FCI/FQAH states are certainly related to the inherent geometric factor. More interestingly, we observe some exotic degeneracy sequences of edge excitations in these singular-lattice FCI/FQAH states, and we provide an explanation that two branches of edge excitations mix together.