Recently, a family of nonmagnetic kagome metals ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K}, \mathrm{Rb}, \mathrm{and} \mathrm{Cs})$ has attracted significant attention for realizing the intertwining of quantum order and nontrivial topology. However, these compounds have been identified to host complex band structures. Therefore, it is desirable to design and synthesize novel kagome materials with a simple band topology and good transport properties. In this study, using first-principles calculations, we present the electronic properties and the intrinsic spin Hall effect of ${\mathrm{V}}_6{\mathrm{Sb}}_4$, the latest experimentally synthesized vanadium-based compounds that form a kagome bilayer. In the absence of spin-orbital coupling (SOC), this compound is a Dirac nodal line semimetal with symmetry-protected nodal rings near the Fermi level. Within the SOC, the spin-rotation symmetry breaks the gaps of the nodal rings with a small band gap. Furthermore, based on the Wannier tight-binding approach and the Kubo formula, we reveal a large spin Hall effect in ${\mathrm{V}}_6{\mathrm{Sb}}_4$, which intrinsically originates from the spin Berry curvature. Our work further expands nonmagnetic kagome compounds for applications in spintronics accompanied by exotic quantum order.

The operation of many classical and quantum systems in nonequilibrium steady state is constrained by cost-precision (dissipation-fluctuation) tradeoff relations, delineated by the thermodynamic uncertainty relation (TUR). However, coherent quantum electronic nanojunctions can escape such a constraint, showing finite charge current and nonzero entropic cost with vanishing current fluctuations. Here we analyze the absence, and restoration, of cost-precision tradeoff relations in fermionic nanojunctions under different affinities: voltage and temperature biases. With analytic work and simulations, we show that both charge and energy currents can display the absence of cost-precision tradeoffs if we engineer the transmission probability as a boxcar function—with a perfect transmission and hard energy cutoffs. Specifically for charge current under voltage bias, the standard TUR may be immediately violated as we depart from equilibrium, and it is exponentially suppressed with increased voltage. However, beyond idealized, hard-cutoff energy-filtered transmission functions, we show that realistic models with soft cutoffs or imperfect transmission functions follow cost-precision tradeoffs and eventually recover the standard TUR sufficiently far from equilibrium. The existence of cost-precision tradeoff relations is thus suggested as a generic feature of realistic nonequilibrium quantum transport junctions.

The exploration of superconductivity in low-dimensional materials has attracted intensive attention for decades. Based on first-principles electronic structure calculations, we have systematically investigated the electronic and superconducting properties of the two-dimensional electride ${\mathrm{Ba}}_2\mathrm{N}$ in the monolayer limit. Our results show that monolayer ${\mathrm{Ba}}_2\mathrm{N}$ has a low work function of 3.0 eV and a predicted superconducting transition temperature ($T_c$) of 3.4 K. The superconductivity can be further improved with the tensile strain, which results from the increase of density of states at the Fermi level as well as the enhanced coupling between inner-layer electrons and phonons. Remarkably, at the $4\%$ tensile strain, the acoustic branches have noticeable softening at the $K$ point of the Brillouin zone and the superconducting $T_c$ can reach 10.8 K. The effect of lattice strain on the electron transfer from the superficial region to the inner-layer region of monolayer ${\mathrm{Ba}}_2\mathrm{N}$ may also apply to other electride materials and influence their physical properties.

Low-temperature optical spectroscopy in applied magnetic fields provides clear evidence of magnetoelastic coupling in the spin ice material ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. In far infrared (IR) reflectometry measurements, we observe field-dependent features around 30, 61, 72, and 78 meV, energies corresponding to crystal electronic field (CEF) doublets. The calculations of the crystal field Hamiltonian model confirm that the observed features in IR spectra are consistent with magnetic-dipole-allowed excitations from the ground state to higher ${}^5\mathrm{I}_8$ CEF levels. We present the CEF parameters that best describe our field-dependent IR reflectivity measurements. Additionally, we identify a weak field-dependent shoulder near one of the CEF doublets. This indicates that this level is split even in zero field, which we associate with a vibronic bound state. Modeling of the observed splitting shows that the phonon resides at slightly lower energy compared to the CEF level that it couples to, which is in contrast with previously published inelastic neutron measurements. The magnetic field dependence of the vibronic state shows a gradual decoupling of the phonon with the CEF level as it shifts. This approach should work in pyrochlores and other systems that have magnetic dipole transitions in the IR spectroscopic range, which can elucidate the presence and the ability to tune the nature of vibronic states in a wide variety of materials.

Transport through quantum coherent conductors, such as atomic junctions, is described by conduction channels. Information about the number of channels and their transmissions can be extracted from various sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or shot noise. We complement this set of methods by introducing the superconducting excess current as a new tool to continuously extract the transport channel transmissions of an atomic scale junction in a scanning tunneling microscope. In conjunction with ab initio simulations, we employ this technique in atomic aluminum junctions to determine the influence of the structure adjacent to the contact atoms on the transport properties.

The $\alpha \text{−}{T}_3$ lattice model interpolates a honeycomb (graphene-like) lattice and a $T_3$ (also known as dice) lattice via the parameter $\alpha$. These lattices are made up of three atoms per unit cell. This gives rise to an additional dispersionless flat band touching the conduction and valence bands. Electrons in this model are analogous to Dirac fermions with an enlarged pseudospin, which provides unusual tunneling features like omnidirectional Klein tunneling, also called super-Klein tunneling (SKT). However, it is unknown how small deviations in the equivalence between the atomic sites, i.e., variations in the $\alpha$ parameter, and the number of tunnel barriers changes the transmission properties. Moreover, it is interesting to learn how tunneling occurs through regions where the energy spectrum changes from linear with a middle flat band to a hyperbolic dispersion. In this paper we investigate these properties, its dependence on the number of square barriers and the $\alpha$ parameter for either gapped and gapless cases. Furthermore, we compare these results to the case where electrons tunnel from a region with linear dispersion to a region with a bandgap. In the latter case, contrary to tunneling through a potential barrier, the SKT is no longer observed. Finally, we find specific cases where transmission is allowed due to a symmetry breaking of sublattice equivalence.

Magnetic impurities placed in the superconductor can lead to the emergence of Yu-Shiba-Rusinov bound states. Coupling between the impurity and the substrate depends on the density of states (DOS) at the Fermi level and can be tuned by DOS singularities. In this paper, we study the role of DOS singularities using the real space Bogoliubov–de Gennes equations for chosen lattice models. To uncover the role of these singularities (Dirac point, van Hove singularity, or the flat band), we study honeycomb, kagome, and Lieb lattices. We show that the properties of the Shiba state strongly depend on the type of lattice. Nevertheless some behaviors are generic, e.g., dependence of the critical magnetic coupling on the DOS at the Fermi level. However, the Shiba states realized in the Lieb lattice exhibit extraordinary properties, which can be explained by the presence of a few nonequivalent sublattices. Depending on the location of the magnetic impurity in the chosen sublattice, the value of critical magnetic coupling $J_{\text{c}}$ can be reduced or enhanced when the flat band is located at the Fermi level. In this context, we also present differences in the local DOS and coherence lengths for different sublattices in the Lieb lattice.

Moiré superlattices have recently been extensively studied in both electronic and photonic systems, e.g., magic-angle bilayer graphene showing superconductivity and twisted bilayer photonic crystals leading to magic-angle lasers. However, the moiré physics is barely studied in the field of magnonics, i.e., in using spin waves for information processing. In this work, we report magnon flat-band formation in twisted bilayer magnonic crystals at the optimal “magic angle” and interlayer exchange coupling combination using micromagnetic simulations. At the flat-band frequency, magnons undergo a strong two-dimensional confinement with a lateral scale of about 185 nm. The magic-angle magnonic nanocavity occurs at the $AB$ stacking region of a moiré unit cell, unlike its photonic counterpart which is at the $AA$ region, due to the exchange-induced magnon spin torque. The magnon flat band originates from band structure reformation induced by interlayer magnon-magnon coupling. Our results enable efficient accumulation of magnon intensity in a confined region that is key for potential applications such as magnon Bose-Einstein condensation and even magnon lasing.

We theoretically predict that the critical current $I_c$ and magnetization $M$ of a hybrid superconductor/normal-metal (SN) strip may have nonmonotonous dependence on a perpendicular magnetic field—so-called peak effect. In contrast to familiar peak effect, which is connected either with vortex entry to the superconductor or with peculiarities of vortex pinning, the found phenomenon exists at low fields, in the vortex-free (Meissner) phase. We argue that the effect appears at specific parameters of the studied hybrid structure when its in-plane current-supervelocity relation has two maxima. We expect that the same peak effect may exist in two-band superconductors (like ${\mathrm{MgB}}_2$) where similar current-supervelocity dependence was predicted at low temperatures.

Noncentrosymmetric materials are a promising paradigm to explore unconventional superconductivity. In particular, several Re-containing noncentrosymmetric materials have attracted considerable attention due to a superconducting state with broken time-reversal symmetry. A comprehensive study on the superconducting ground state of NbReSi was performed using magnetization, resistivity, and muon spin rotation/relaxation measurements. Zero-field muon spectroscopy results showed the absence of spontaneous magnetic field below the superconducting transition temperature, $T_c$ = 6.29(3) K, indicating the preserved time-reversal symmetry. Transverse field muon spin rotation measurements confirms an $s$-wave nature of the sample with $\mathrm{\Delta }\left(0\right)/k_B{T}_c$ = 1.73(2).

It is well known that disorder (structural or thermal) induces electronic scattering processes which determine the resistivity of crystalline metals. In this paper, we employ a variant of the Kubo-Greenwood formula to compute the space-projected conductivity and study the atomistic details of conduction in models of crystalline aluminum within the framework of density functional theory. We consider point and extended lattice defects and show with spatial detail how the defect locally affects conduction. We simulate thermal disorder and determine how the disorder affects the conductivity in real space and reveal the spatial nature of thermal fluctuations. Furthermore, we show that well below the Debye temperature, a classical thermal molecular dynamics simulation reproduces the form of temperature-dependent conductivity predicted by the Bloch-Grüneisen formula.

Magnetochiral properties are enriched in curved magnetic nanostructures, in which dipole-dipole and exchange couplings are their physical sources. In such systems, direct implications are evidenced in the magnetization dynamics, where a noticeable frequency shift appears between two counterpropagating spin waves. In this paper, the spin-wave asymmetry induced by the curvature is theoretically studied in thick ferromagnetic nanotubes with a vortex ground state. The spin-wave spectra are obtained using semianalytical calculations and the dynamical matrix method for thin and thick nanotubes. Under the thickness increase, radial standing spin waves are observed at low frequencies, while the nonreciprocal properties are improved. Such standing waves exhibit a nonreciprocal spin-wave dispersion, but it is not as prominent as the asymmetry of the low-frequency in-phase modes. In the limit of small wave vectors, analytical expressions are reported for the spin-wave dispersion, where the resonance frequency, the frequency shift of two counterpropagating waves, and the critical field that destabilizes the vortex state are determined. The obtained frequency shift allows us to estimate the influence of thickness and curvature on the nonreciprocity of the spin waves. These results constitute a significant advance in the fundamental understanding of the spin-wave dynamics of ferromagnetic nanotubes, predicting new phenomena and providing expressions that are easy to interpret and that allow us, therefore, to promote the study of magnetization dynamics in curved structures.

The effect of symmetry on the resonance spectra of antiferromagnetically coupled oscillators has attracted new interest with the discovery of symmetry-breaking induced anticrossings. Here, we experimentally characterize the resonance spectrum of a synthetic antiferromagnet Pt/CoFeB/Ru/CoFeB/Pt, where we are able to independently tune the effective magnetization of the two coupled magnets. To model our results, we apply the mathematical methods of group theory to the solutions of the Landau-Lifshitz-Gilbert equation. This general approach, usually applied to quantum-mechanical systems, allows us to identify the main features of the resonance spectrum in terms of symmetry breaking and to make a direct comparison with crystal antiferromagnets.

The total angular momentum of a closed system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has been extinguished. Such conservation, however, is never satisfied in practice in any real-time first-principles description of the demagnetization process. Furthermore, there is growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here we critically analyze the issue and conclude that current state-of-the-art simulations violate angular momentum conservation already at the electronic level of description. This shortcoming originates from an oversimplified description of the spin-orbit coupling, which includes atomic contributions but neglects completely that of itinerant electrons. We corroborate our findings with time-dependent simulations using model tight-binding Hamiltonians, and show that indeed such conservation can be reintroduced by an appropriate choice of spin-orbit coupling. The consequences of our findings on recent experiments are also discussed.

We study theoretically the collective quantum dynamics occurring in various interacting superconducting qubit arrays (SQAs) in the presence of a spread of individual qubit frequencies. The interaction is provided by mutual inductive coupling between adjacent qubits (short-range Ising interaction) or inductive coupling to a low-dissipative resonator (long-range exchange interaction). In the absence of interaction, the Fourier transform of the temporal correlation function of the total polarization ($z$ projection of the total spin), i.e., the dynamic susceptibility $C\left(\omega \right)$, demonstrates a set of sharp small magnitude resonances corresponding to the transitions of individual superconducting qubits. We show that even a weak interaction between qubits can overcome the disorder with a simultaneous formation of the collective excited states. This collective behavior manifests itself by a single large resonance in $C\left(\omega \right)$. In the presence of a weak nonresonant microwave photon field in the low-dissipative resonator, the positions of dominant resonances depend on the number of photons, i.e., the collective ac Stark effect. Coupling of an SQA to the transmission line allows a straightforward experimental access of the collective states in microwave transmission experiments and, at the same time, to employ SQAs as sensitive single-photon detectors.

The effects of the unique density of states (DOS) of a topological type-II nodal-line semimetal (NLS) on its thermoelectric (TE) transport properties are investigated through a combination of semianalytical and first-principles methods with “spinless ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$” as the artificial material. The DOS in such a type-II NLS possesses two van Hove singularities near the energy of the nodal line that leads to a large $S$ value compared to the normal metals. Combined with the linear band at the nodal line that gives high electrical conductivity $\sigma$, the type-II NLS can exhibit a relatively high TE power factor ($\mathrm{PF}=S^2\sigma$) at the nodal line. In particular, we find $\text{PF}\sim 60$ $\text{μ}\mathrm{W}\text{/}{\mathrm{cm}\mathrm{K}}^2$ at 300 K for the $n$-type ${\mathrm{Mg}}_3{\mathrm{Bi}}_2$ by considering the electron-phonon scattering, in which the relaxation time $\tau$ of carriers can be expressed as $\tau \propto {\text{DOS}}^{-1}$ for the type-II NLS. Furthermore, we optimize parameters for the TE power factor of type-II NLSs in general by adopting the two-band model with the DOS-dependent relaxation-time approximation. Our results suggest the type-II NLSs as a potential class of high-performance TE materials among metals and semimetals, which are traditionally considered inadequate TE materials compared to semiconductors.

We report the signature of a strain-controlled topological phase transition in the electronic structure of a quasi-one-dimensional superconductor ${\mathrm{TaSe}}_3$. Using angle-resolved photoemission spectroscopy and first-principles calculation, ${\mathrm{TaSe}}_3$ is identified to be in a weak topological insulator phase which has topologically nontrivial surface states only at the allowed planes. Under uniaxial tensile strain, a Dirac point and the topological surface state emerge on the originally forbidden $\left(10\overline{1}\right)$ plane, which demonstrates the transition to a strong topological insulator phase. Our results accomplish the experimental realization of possible topological insulating phases in ${\mathrm{TaSe}}_3$ and highlight the possibility of coupling the superconductivity with two distinct topological insulating phases in a controllable manner.

Coulomb diamonds are the archetypal signatures of Coulomb blockade, a well-known charging effect mainly observed in nanometer-sized electronic islands tunnel-coupled with charge reservoirs. Here, we identify apparent Coulomb diamond features in the scanning gate spectroscopy of a quantum point contact carved out of a semiconductor heterostructure in the quantum Hall regime. Varying the scanning gate parameters and the magnetic field, the diamonds are found to smoothly evolve to checkerboard patterns. To explain this surprising behavior, we put forward a model which relies on the presence of a nanometer-sized Fabry-Pérot quantum Hall interferometer at the center of the constriction with tunable tunneling paths coupling the central part of the interferometer to the quantum Hall channels running along the device edges. Both types of signatures, diamonds and checkerboards, and the observed transition, are reproduced by simply varying the interferometer size and the transmission probabilities at the tunneling paths. The proposed interpretation of diamond phenomenology will likely lead to revisiting previous data, and opens the way toward engineering more complex interferometric devices with nanoscale dimensions.

We study the ground state properties of the one-dimensional extended Hubbard model at half filling from the perspective of its particle reduced density matrix. We focus on the reduced density matrix of two fermions and perform an analysis of its quantum correlations and coherence along the different phases of the model. Specifically, we study its (i) entanglement entropy, (ii) ${\ell }_1$ norm of coherence, (iii) irreducible two-body cumulant matrix, and (iv) entanglement spectrum. Our results show that these different properties are complementary to each other depending on the phase of the system, exhibiting peculiar behaviors such as discontinuities and maximum or minimum values at the quantum phase transitions, thus providing a qualitative view of the phase diagram of the model. In particular, in the superconducting region, we obtain that the entanglement spectrum signals a transition from a dominant singlet (SS) to triplet (TS) pairing ordering in the system. Moreover, from the analysis of the dominant eigenvector in the reduced state, we can relate the SS-TS transition to the spatial separation between the fermion pairs in the two different pairing orderings. The entanglement gap is also able to highlight a transition—at a few-body level—in the ground state wave function, not discussed previously in the literature. While other quantifiers are less sensitive to few-body defects in the wave function, the entanglement gap can work as a magnifying glass for these, capturing such small fluctuations.