We theoretically study the local density of states in superconducting proximity structures where two superconducting terminals are attached to a side surface of a normal-metal wire. Using the quasiclassical Green's function method, the energy spectrum is obtained for both spin-singlet $s$-wave and spin-triplet $p$-wave junctions. In both cases, the decay length of the proximity effect at zero temperature is limited by a depairing effect due to inelastic scattering. In addition to the depairing effect, in $p$-wave junctions the decay length depends sensitively on the transparency at the junction interfaces, which is a unique property of odd-parity superconductors where the anomalous proximity effect occurs.

We study the behavior of Cooper pair amplitudes that emerge when a two-dimensional superconductor is coupled to two parallel nanowires, focusing on the conditions for realizing odd-frequency pair amplitudes in the absence of spin-orbit coupling or magnetism. In general, any finite tunneling between the superconductor and the two nanowires induces odd-frequency spin-singlet pair amplitudes in the substrate as well as a substantial odd-frequency interwire pairing, both of which vanish locally. Interestingly, in the regime of strong superconductor-nanowire tunneling, we find that the presence of two nanowires allows for the conversion of nonlocal odd-frequency pairing to local even-frequency pairing. By studying this higher-order symmetry conversion process, we are able to identify a notable effect of the odd-frequency pairing in the superconductor on local quantities accessible by experiments. Specifically, we find that the odd-frequency pairing plays a direct role in the emergence of certain subgap features in the local density of states and, importantly, it is responsible for a reduction of the maximum Josephson current between the two nanowires, measurable using Josephson scanning tunneling microscopy. We discuss ways to control the sizes of these effects induced by odd-frequency superconductivity by tuning the parameters describing the nanowires.

We consider Dirac electrons on the honeycomb lattice Kondo coupled to spin-1/2 degrees of freedom on the kagome lattice. The interactions between the spins are chosen along the lines of the Balents-Fisher-Girvin model that is known to host a ${\mathbb{Z}}_2$ spin-liquid and a ferromagnetic phase. The model is amenable to sign free auxiliary-field quantum Monte Carlo simulations. While in the ferromagnetic phase the Dirac electrons acquire a gap, they remain massless in the ${\mathbb{Z}}_2$ spin-liquid phase. Since our model has an odd number of spins per unit cell, this phase is a non-Fermi liquid that violates the conventional Luttinger theorem which relates the Fermi surface volume to the particle density in a Fermi liquid. This non-Fermi liquid is a specific realization of the so-called fractionalized Fermi liquid proposed in the context of heavy fermions. We probe the violation of the Luttinger sum rule in this non-Fermi liquid phase via conventional observables such as the spectral function, and also by studying the mutual information between the electrons and the spins.

Scattering of matter waves, present at multiple quantum setups, is required to be maximized in several applications from sensing and imaging to quantum switching and memory. The simplest case of a homogeneous spherical scatterer is rigorously solved and optimized with respect to its size, texture, and background by considering the effective quantum properties from a long directory of materials. In this context, several alternative layouts with maximal scattering efficiency are proposed, offering additional degrees of freedom in design without requirements for careful engineering of quantum texture. Many of the reported inclusions exhibit substantial selectivity in their scattering response, admitting them to operate as sharp filters for particle energies or sensitive detectors of electron beams.

First-principles calculations of spin transmission and perpendicular magnetic anisotropy in artificially stacked Fe(110)/hBN/metal heterostructures have been employed to study the dynamical response of tunnel junctions to applied axial fields. By projecting the effective electric-field gradient densities and magnetic shielding constants across constitutive atomic layers in the scatter region of the device, an unusual site-dependent spin response is unraveled at the Fe/hBN and hBN/metal heterobilayer interfaces. Analysis of the Fermi-level topology reveals an exotic electronic phase characterized by electric-field induced spin flip relative to the ferromagnetic ground state. The calculations reveal unique signatures in the spin transport phase and dynamical effects are observed as field tunable perpendicular magnetic anisotropy. In particular, magnetization alignment preferences are observed in the spin transmission, electric-field gradient localization, and a vanishing magnetic shielding at finite electric fields. Results show that V-capped stacks have a relatively superior perpendicular magnetic anisotropy suggesting that the atomic species of the free layer plays a dominant role in spin-based information storage using Fe(110)/hBN/M stacks.

Recent investigations have found that the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation exchange-correlation functional significantly over-magnetizes elemental Fe, Co, and Ni solids. For the paradigmatic case, bcc Fe, the error relative to experiment is $\gtrsim 20\%$. A comparative analysis of magnetization results from SCAN and its deorbitalized counterpart, SCAN-L, leads to identification of the source of the discrepancy. It is not from the difference between Kohn-Sham (SCAN-L) and generalized Kohn-Sham (SCAN) procedures. The key is the iso-orbital indicator $\alpha$ (the ratio of the local Pauli and Thomas-Fermi kinetic energy densities). Its deorbitalized counterpart ${\alpha }_L$ has more dispersion in both spin channels with respect to magnetization in an approximate region between 0.6 and 1.2 bohrs around an Fe nucleus. The overall effect is that the SCAN switching function evaluated with ${\alpha }_L$ reduces the energetic disadvantage of the down channel with respect to up compared to the original $\alpha$, which in turn reduces the magnetization. This identifies the cause of the SCAN magnetization error as insensitivity of the SCAN switching function to $\alpha$ values in the approximate range $0.5\lesssim \alpha \lesssim 0.8$ and oversensitivity for $\alpha \gtrsim 0.8$.

We report tests of various density functionals for ferromagnetic Fe, Co, and Ni with a focus on characterizing the behavior of the so-called strongly constrained and appropriately normed (SCAN) functional. It is found that SCAN is closer in behavior to functionals that yield localized behavior, such as hybrid functionals, than other semilocal functionals that are tested. The results are understood in terms of a tendency to differentiate orbitals, favoring integer occupation, which is necessary for a correct description of atomic systems, but inappropriate for the open shell metallic ferromagnetic metals studied here. Not only is the exchange splitting for open shells enhanced with SCAN, but also as seen in Ni, there is much more band dependence with significantly more overestimation for bands corresponding to the partially filled orbitals.

The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100–300 μs lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of ∼10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.

The Altshuler-Aronov (AA) effect is one of the most basic quantum many-body effects in the mesoscopic regime. It originates from the coexistence of disorder and electron-electron interaction. In this paper, we reformulate the Feynman diagrammatic theory of the AA effect in a real-space nonequilibrium Green's function framework, in which an effective medium technique (via coherent potential) is employed to evaluate disorder-induced vertices in the diagrams. As such the developed real-space formalism is compatible with the prevailing nanodevice simulation paradigm, leading to an effective numerical approach to calculating the AA effects on electronic structures and quantum transport properties of nanostructures. As an application, we analyze the $I\text{−}V$ characteristics and the full local density of states (DOS) profile of an Anderson-Hubbard lattice sandwiched between biased electrodes. We show how the DOS anomaly due to the AA mechanism is reshaped by the geometrical confinement and the nonequilibrium effects in a nanosystem. Our numerical findings are well understood by the analytical results we provide in this paper.

We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Since the laser electric field can be considered as a time-periodic perturbation, we apply the Floquet theory and perform the inverse frequency expansion by focusing on the high frequency region. Using the obtained effective Floquet Hamiltonian, we study nonequilibrium magnon dynamics away from the adiabatic limit and its effect on topological phenomena. We show that a linearly polarized laser can generate helical edge magnon states and induce the magnonic spin Nernst effect, whereas a circularly polarized laser can generate chiral edge magnon states and induce the magnonic thermal Hall effect. In particular, in the latter, we find that the direction of the magnon chiral edge modes and the resulting thermal Hall effect can be controlled by the chirality of the circularly polarized laser through the change from the left-circular to the right-circular polarization. Our results thus provide a handle to control and design magnon topological properties in the insulating AF.

Interlayer resistance is reported in a layered organic superconductor ${\beta }^{″}\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ with $T_{\mathrm{c}}\approx 5$ K. At 0.7 K in parallel magnetic fields, the interlayer resistance has nonzero values in a wide field region from 2 T to the upper critical field $\left(\sim 13\mathrm{T}\right)$, which can be ascribed to the Josephson vortex (JV) dynamics. Significant hysteresis between 2 T and 8 T and a steplike structure between 4 T and 8 T are found, both of which become less evident with increasing temperature. In the second field-derivative curves, successive small dips are observed between 10 T and 12 T, which are ascribed to the commensurability (CM) effect between the JV lattice and the wavelength ${\lambda }_{\mathrm{FFLO}}$ of the order parameter oscillation in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The CM effect is observed only in nearly parallel fields, showing that the FFLO phase is stabilized only in a limited field angle region. The temperature-field phase diagram and ${\lambda }_{\mathrm{FFLO}}$ values are obtained.

The prototypical antiferroelectric ${\mathrm{PbZrO}}_3$ has several unsettled questions, such as the nature of the antiferroelectric transition, a possible intermediate phase, and the microscopic origin of the $Pbam$ ground state. Using first-principles calculations, we show that no phonon becomes truly soft at the cubic-to-$Pbam$ transition temperature, and the order-disorder character of this transition is clearly demonstrated based on molecular dynamics simulations and potential energy surfaces. The out-of-phase octahedral tilting is an important degree of freedom, which can collaborate with other phonon distortions and form a complex energy landscape with multiple minima. Candidates of the possible intermediate phase are suggested based on the calculated kinetic barriers between energy minima, and the development of a first-principles-based effective Hamiltonian. The use of this latter scheme further reveals that specific bilinear interactions between local dipoles and octahedral tiltings play a major role in the formation of the $Pbam$ ground state, which contrasts with most of the previous explanations.

Superconducting circuits consisting of a few low-anharmonic transmons coupled to readout and bus resonators can perform basic quantum computations. Since the number of qubits in such circuits is limited to not more than a few tens, the qubits can be designed to operate within the dispersive regime where frequency detunings are much stronger than coupling strengths. However, scaling up the number of qubits will bring the circuit out of the regime, and this invalidates current theories. We develop a formalism that allows to consistently diagonalize the superconducting circuit Hamiltonian beyond the dispersive regime. This will allow to study qubit-qubit interaction unperturbatively, therefore, our formalism remains valid and accurate at small or even negligible frequency detuning; thus, our formalism serves as a theoretical ground for designing qubit characteristics for scaling up the number of qubits in superconducting circuits. We study the most important circuits with single- and two-qubit gates, i.e., a single transmon coupled to a resonator and two transmons sharing a bus resonator. Surprisingly, our formalism allows to determine the circuit characteristics, such as dressed frequencies and Kerr couplings, in closed-form formulas that not only reproduce perturbative results, but also extrapolate beyond the dispersive regime and can ultimately reproduce (and even modify) the Jaynes-Cumming results at resonant frequencies.

We derive an expression for the effective Josephson coupling from the microscopic Hubbard model. It serves as a starting point for the description of phase fluctuations of local Cooper pairs in $d_{x^2-y^2}$-wave superconductors in the framework of an effective $XY$ model of plaquettes, the Josephson lattice. The expression for the effective interaction is derived by means of the local-force theorem, and it depends on local symmetry-broken correlation functions that we obtain using the cluster dynamical mean-field theory. Moreover, we apply the continuum limit to the Josephson lattice to obtain an expression for the gradient term in the Ginzburg-Landau theory and compare predicted London penetration depths and Kosterlitz-Thouless transition temperatures with experimental data for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$.

Twisted bilayer graphene has been argued theoretically to host exceptionally flat bands when the angle between the two layers falls within a magic range near $1.1^{\circ }$. This is now strongly supported by experiment, which furthermore reveals dramatic correlation effects in the magic range due to the relative dominance of interactions when the bandwidth is suppressed. Experimentally, quantum oscillations exhibit different Landau level degeneracies when the angles fall in or outside the magic range; these observations can contain crucial information about the low-energy physics. In this paper, we report a thorough theoretical study of the Landau level structure of the noninteracting continuum model for twisted bilayer graphene as the magnetic field and the twist angle are tuned. We first show that a discernible difference exists in the butterfly spectra when twist angle falls in and outside the magic range. Next, we carry out semiclassical analysis in detail, which quantitatively determines the origin of the low-energy Landau levels from the zero field band structure. We find that the Landau level degeneracy predicted in the above analyses is capable of partially explaining features of the quantum oscillation experiments in a natural way. Finally, topological aspects, validity, and other subtle points of the model are discussed.

We have investigated the terahertz (THz)-pulse-induced dynamics of tetrathiafulvalene-$p$-chloranil near the boundary between the ionic and neutral phases with the use of exact numerical calculations of an extended Hubbard model coupled with lattice motion. For the ionic phase, when the applied electric field of the THz pulse opposes the electronic contribution to the electric polarization (electronic polarization) ${\overline{P}}_{\mathrm{el}}$ of the ground state and the maximum amplitude of electric field is greater than a threshold value, the THz-pulse excited state changes as ${\mathrm{I}}_{\mathrm{A}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{B}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{A}}\to \mathrm{N}\to \cdots ({\mathrm{I}}_{\mathrm{A}}$ ground-state case) or ${\mathrm{I}}_{\mathrm{B}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{A}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{B}}\to \mathrm{N}\to \cdots ({\mathrm{I}}_{\mathrm{B}}$ ground-state case), where N shows the neutral state, ${\mathrm{I}}_{\mathrm{A}}\left({\mathrm{I}}_{\mathrm{B}}\right)$ shows the ionic state with ${\overline{P}}_{\mathrm{el}}<0\left({\overline{P}}_{\mathrm{el}}>0\right)$, and the phase of the bond-length alternation of ${\mathrm{I}}_{\mathrm{A}}$ is opposite to that of ${\mathrm{I}}_{\mathrm{B}}$. For the neutral phase, when the maximum amplitude of the electric field is greater than a threshold value, the THz-pulse excited state changes as $\mathrm{N}\to {\mathrm{I}}_{\mathrm{B}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{A}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{B}}\to \cdots$ (positive electric field case) or $\mathrm{N}\to {\mathrm{I}}_{\mathrm{A}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{B}}\to \mathrm{N}\to {\mathrm{I}}_{\mathrm{A}}\to \cdots$ (negative electric-field case). The phase transitions and electronic polarization reversal are driven by time variation of the lattice order parameter, which indicates the magnitude and phase of the bond-length alternation, and the lattice motion is induced by THz-pulse excitation through the electron–lattice coupling. Transitions between the ionic and neutral phases occur and electronic polarization reverses on a picosecond timescale together with the realizable magnitude of the THz pulse both in the ionic and neutral phases near the phase boundary.

A central key to understanding heavy-fermion systems involves revealing how itinerant low-energy excitations emerge from local $f$ moments. An effective way to understand the $f$-electron behavior is to compare the electronic structure of $f$-electron compounds with a reference isostructural compound with no $f$ electrons. Here we present a systematic electronic structure study of ${\mathrm{LaCoIn}}_5$, which is a non-$f$ reference compound of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. Our angle-resolved photoemission spectroscopy study of the three-dimensional Fermi surface and band structure of ${\mathrm{LaCoIn}}_5$ highlights the rather three-dimensional electronic character of this compound. The conduction bands of ${\mathrm{LaCoIn}}_5$ are almost identical to that of ${\mathrm{CeCoIn}}_5$, though obvious differences can be found in the low-energy electronic structure. Finally, we give a quantitative analysis of the Fermi pocket change in ${\mathrm{LaCoIn}}_5$ and ${\mathrm{CeCoIn}}_5$, which may be helpful for understanding the important “large” and “small” Fermi surface issue in heavy-fermion compounds.

Recent work on layered structures of superconductors (S) or normal metals (N) in contact with ferromagnetic insulators (FI) has shown how the properties of the previous can be strongly affected by the magnetic proximity effect due to the static FI magnetization. Here we show that such structures can also exhibit a new electron thermalization mechanism due to the coupling of electrons with the dynamic magnetization, i.e., magnons in FI. We here study the heat flow between the two systems and find that in thin films the heat conductance due to the interfacial electron-magnon collisions can dominate over the well-known electron-phonon coupling below a certain characteristic temperature that can be straightforwardly reached with present-day experiments. We also study the role of the magnon band gap and the induced spin-splitting field induced in S on the resulting heat conductance and show that heat balance experiments can reveal information about such quantities in a way quite different from typical magnon spectroscopy experiments.