We analyze a Josephson junction between two superconductors interconnected through a normal-state nanostructure made of two parallel nanowires with embedded quantum dots. We study the influence of interference effects due to the Aharonov-Bohm (AB) and Aharonov-Casher (AC) phases for local and nonlocal (split) Cooper pairs. In the AB effect the phase of electron is affected by magnetic flux, while in the AC effect the phase of the electron in solid state can be modified due to the Rashba spin-orbit coupling. In the low-transmission regime the AB and AC effects can be related to only local or nonlocal Cooper pair transport, respectively. We demonstrate that by the addition of the quantum dots the Cooper pair splitting can be made perfectly efficient and that the AC phase is different for non-spin-flip and spin-flip transport processes.

Spin pumping from the full Heusler alloy $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ film into the transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{S}}_2$ is reported. Plasma-assisted sulfurization of ion-beam sputtered Mo films of different nominal thicknesses is employed to first fabricate large area high-quality $\mathrm{Mo}{\mathrm{S}}_2$ sheets [thicknesses: 1, 2, 3, and 4 monolayers (MLs)] on $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates, followed by deposition of $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ films with a fixed thickness of 8 nm. The spin pumping is investigated by measuring the changes in the damping constant in the Al(5 nm) capped $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}/\mathrm{Mo}{\mathrm{S}}_2$ bilayers using ferromagnetic resonance spectroscopy. The study demonstrates that even 1 ML of $\mathrm{Mo}{\mathrm{S}}_2$ possesses high enough spin-orbit coupling strength to enhance damping from $5.5(\pm 0.2)\times {10}^{-3}$ in $\mathrm{Al}\left(5\mathrm{nm}\right)/\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}\left(8\mathrm{nm}\right)$ to a nearly saturated value of $8.3(\pm 0.2)\times {10}^{-3}$ in $\mathrm{Al}\left(5\mathrm{nm}\right)/\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}\left(8\mathrm{nm}\right)/\mathrm{Mo}{\mathrm{S}}_2\left(1\mathrm{ML}\right)$, which is suppressed by inserting a thin Al layer at the $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}/\mathrm{Mo}{\mathrm{S}}_2$ interface. The observed enhancement in damping is in agreement with the results from first-principles calculations based on density functional theory. These results open up a paradigm for designing spintronic devices based on heterostructures comprising a full Heusler alloy and the inherently stable $\mathrm{Mo}{\mathrm{S}}_2$.

It is known that orthorhombic $R\mathrm{Mn}{\mathrm{O}}_3$ multiferroics $\left(R=\mathrm{magnetic}\mathrm{rare}\mathrm{earth}\right)$ with low symmetry exhibit a large rotating magnetocaloric effect because of their strong magnetocrystalline anisotropy. In this paper, we demonstrate that the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ single crystals also unveil a giant rotating magnetocaloric effect that can be obtained by spinning them in constant magnetic fields around their $a$ or $b$ axes. When the $\mathrm{ErMn}{\mathrm{O}}_3$ crystal is rotated with the magnetic field initially parallel to the $c$ axis, the resulting entropy change reaches maximum values of 7, 17, and 20 J/kg K under 2, 5, and 7 T, respectively. These values are comparable to or even larger than those shown by some of the best orthorhombic phases. More interestingly, the generated anisotropic thermal effect is about three times larger than that exhibited by the hexagonal $\mathrm{HoMn}{\mathrm{O}}_3$ single crystal. The enhancement of the rotating magnetocaloric effect in the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ compound arises from the unique features of the $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic sublattice. In fact, the $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic moments located at $2a$ sites experience a first-order metamagnetic transition close to 3 K along the $c$ axis resulting in a peaked magnetocaloric effect over a narrower temperature range. In contrast, the “paramagnetic” behavior of $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic moments within the $ab$ plane produces a larger magnetocaloric effect over a wider temperature range. Therefore, the magnetocaloric effect anisotropy is maximized between the $c$ and the $ab$ directions, leading to a giant rotating magnetocaloric effect.

We investigate nonradiative decay and absorption rates of two-level quantum emitters embedded in a metal at low temperatures. We obtain the expressions for both nonradiative transition rates and identify a unique, experimentally accessible way to obtain both nonradiative transition rates via electronic transport in the host metallic system. Our findings not only provide a microscopic description of the nonradiative channels in metals, but they also allow one to identify, determine, and differentiate them from other decay channels, which is crucial to the understanding and controlling of the light-matter interactions at the nanoscale.

Charge neutral objects usually attract each other in the nanoscale and it is actually not favorable for nanoscale manipulation as attractive forces tend to make objects stick together. It would be highly desirable if the sign of the force between charge neutral objects could be controlled by surface chemical modification. In this work, we show that the static electric field generated by a submonolayer of chemisorbed adatoms can be used to control the sign of the forces between neutral surfaces. The local density functional method combined with an electrostatic stress tensor approach is used to study the forces between tungsten surfaces with stripes of noble metal atoms adsorbed on top. When the metal substrate is partially covered by the adatom stripes, the electric field generated by the local variation of the work function can extend into the vacuum, which in turn can attract or repel other surfaces in close vicinity. Chemisorption may hence offer a good strategy to manipulate nanoscale objects.

Rashba spin-orbit coupling appears in 2D systems lacking inversion symmetry, and causes the spin-splitting of otherwise degenerate energy bands into an upper and lower helicity band. In this paper, we explore how impurity scattering affects transport in the ultralow density regime where electrons are confined to the lower helicity band. A previous study has investigated the conductivity in this regime using a treatment in the first Born approximation. In this work, we use the full $T$ matrix to uncover features of the conductivity. We first compute the conductivity within a semiclassical Boltzmann framework and show that it exhibits an unconventional density dependence due to the unusual features of the group velocity in the single-particle dispersion, as well as quantized plateaus as a function of the logarithm of the electron density. We support this with a calculation using the Kubo formula and find that these plateaus persist in the full quantum theory. We suggest that this quantization may be seen in a pump-probe experiment.

We employ ab initio calculations to investigate the charge density waves in single-layer ${\mathrm{NbSe}}_2$, and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer ${\mathrm{NbSe}}_2$ in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavored, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favor the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from a symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favor one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favor the reduction of the symmetry of the charge density distribution.

We study the transversal magnetoconductivity and magnetoresistance of a massive Dirac fermion gas. This can be used as a simple model for gapped Dirac materials. In the zero-mass limit, the case of gapless Dirac semimetals is also studied. In the case of Weyl semimetals, to reproduce the nonsaturating linear magnetoresistance seen in experiments, the use of screened charged impurities is inevitable. In this paper, these are included using the first Born approximation for the self-energy. The screening wave number is calculated using the random phase approximation with the polarization function taking into account the electron-electron interaction. The Hall conductivity is calculated analytically in the case of no impurities and is shown to be perfectly inversely proportional to the magnetic field. Thus the magnetic field dependence of the magnetoresistance is mainly determined by ${\sigma }_{xx}$. We show that in the extreme quantum limit at very high magnetic fields the gapped Dirac materials are expected to have ${\sigma }_{xx}\propto B^{-3}$ leading to ${\varrho }_{xx}\propto B^{-1}$, in contrast with the gapless case where ${\sigma }_{xx}\propto B^{-1}$ and ${\varrho }_{xx}\propto B$. At lower fields, we find that the effect of the mass term is negligible and in the region of the Shubnikov-de Haas oscillations the two systems behave almost identically. We suggest a phenomenological scattering rate that is able to reproduce the linear behavior at the oscillating region. We show that in the case of the scattering rate calculated using the Born approximation, the strength of the relative permittivity and the density of impurities affects the magnetic field dependence of the conductivity significantly.

The discovery of Weyl fermions in transition-metal monoarsenides/phosphides without inversion symmetry represents an exceptional breakthrough in modern condensed matter physics. However, exploring the inherent nature of these quasiparticles is experimentally elusive because most of the experimental probes rely on analyzing Fermi arc topology or controversial signatures such as the appearance of the chiral anomaly and the giant magnetoresistance. Here, we show that the prototypical type-I Weyl semimetals TaP and TaAs possess a giant anomalous Nernst signal with a characteristic saturation plateau beyond a critical field which can be understood as a direct consequence of the finite Berry curvature originating from the Weyl points. Our results thus promote the Nernst coefficient as an ideal bulk probe for detecting and exploring the fingerprints of emergent Weyl physics.

We show that edge-state transport in semiconductor-based quantum spin Hall systems is unexpectedly robust to magnetic fields. The origin for this robustness lies in an intrinsic suppression of the edge-state $g$-factor and the fact that the edge-state Dirac point is typically hidden in the valence band. A detailed $\mathbf{k}·\mathbf{p}$ band-structure analysis reveals that both $\mathrm{InAs}/\mathrm{GaSb}$ and $\mathrm{HgTe}/\mathrm{CdTe}$ quantum wells exhibit such buried Dirac points for a wide range of well thicknesses. By simulating transport in a disordered system described within an effective model, we demonstrate that edge-state transport remains nearly quantized up to large magnetic fields, consistent with recent experiments.

We study the electronic screening of the long-range Coulomb interaction in graphene nanoribbons (GNRs) with armchair and zigzag edges as a function of the ribbon width by employing ab initio calculations in conjunction with the random-phase approximation. We find that in GNRs with armchair edges quantum confinement effects lead to oscillatory behavior of the on-site screened Coulomb interaction with the ribbon width. Furthermore, the reduced dimensionality and the existence of a band gap result in a nonconventional screening of the Coulomb interaction; that is, it is screened at short distances and antiscreened at intermediate distances, and finally, it approaches the bare (unscreened) interaction at large distances. In the case of GNRs with zigzag edges the presence of edge states strongly affects the screening, which leads to a strong reduction of the effective on-site Coulomb interaction (Hubbard $U)$ parameters at the edge. We find that the interactions turn out to be local; the nonlocal part is strongly screened due to edge states, making GNRs with zigzag edges correlated materials. On the basis of the calculated effective Coulomb interaction parameter $U$, we discuss the appearance of ferromagnetism at zigzag edges of GNRs within the Stoner model.

The out-of-time-ordered correlators (OTOCs) have been proposed and widely used recently as a tool to define and describe many-body quantum chaos. Here, we develop the Keldysh nonlinear sigma model technique to calculate these correlators in interacting disordered metals. In particular, we focus on the regularized and unregularized OTOCs, defined as $f^{\left(r\right)}\left(t\right)=\mathrm{Tr}\left[\sqrt{\widehat{\rho }}\widehat{A}\left(t\right)\sqrt{\widehat{\rho }}{\widehat{A}}^{†}\left(t\right)\right]$ and $f^{\left(u\right)}\left(t\right)=\mathrm{Tr}\left[\widehat{\rho }\widehat{A}\left(t\right){\widehat{A}}^{†}\left(t\right)\right]$, respectively (where $\widehat{A}\left(t\right)=\{\widehat{\psi }(\mathrm{r},t),{\widehat{\psi }}^{†}(0,0)\}$ is the anticommutator of fermion field operators and $\widehat{\rho }$ is the thermal density matrix). The calculation of the rate of OTOCs' exponential growth is reminiscent to that of the Altshuler-Aronov-Khmelnitskii dephasing rate in interacting metals, but here it involves two replicas of the system (two “worlds”). The intraworld contributions reproduce the Altshuler-Aronov-Khmelnitskii dephasing (that would correspond to a decay of the correlator), while the interworld terms provide a term of the opposite sign that exceeds dephasing. Consequently, both regularized and unregularized OTOCs grow exponentially in time, but surprisingly we find that the corresponding many-body Lyapunov exponents are different. For the regularized correlator, we reproduce an earlier perturbation theory result for the Lyapunov exponent that satisfies the Maldacena-Shenker-Stanford bound, ${\lambda }^{\left(r\right)}\le 2\pi k_{\mathsf{B}}T/\hslash$. However, the Lyapunov exponent of the unregularized correlator parametrically exceeds the bound, ${\lambda }^{\left(u\right)}\gg 2\pi k_{\mathsf{B}}T/\hslash$. We argue that ${\lambda }^{\left(u\right)}$ is not a reliable indicator of many-body quantum chaos as it contains additional contributions from elastic scattering events due to virtual processes that should not contribute to many-body chaos. These results bring up an important general question of the physical meaning of the OTOCs often used in calculations and proofs. We briefly discuss possible connections of the OTOCs to observables in quantum interference effects and level statistics via a generalization of the Bohigas-Giannoni-Schmit conjecture to many-body chaotic systems.

We report the interfacial shear strength of graphene on pure and oxidized Ti and Al metal surfaces using density functional theory calculations. Our results show significant changes to the graphene-metal bonding properties in the presence of an oxide phase. In particular, the strongly chemisorbed interface between graphene and pure Ti is drastically weakened by the formation of a metal-oxide phase, while the weakly physisorbed interface between graphene and pure Al is significantly strengthened through the metal oxide formation. These oxidation effects can be modulated to some extent by the presence of vacancy or Stone-Wales defects which increases the binding interactions of weaker graphene-metal interfaces. These dramatic changes to the interfacial properties by surface oxidation explain the results of recent carbon nanotube pull-out experiments from Al and Ti metal-matrix nanocomposites.

We present a generalized multiferroic phase diagram for orthorhombic $R{\mathrm{MnO}}_3\left(R=\mathrm{Gd}–\mathrm{Lu}\right)$ based on coherently grown thin films. The magnetic order was identified by neutron-diffraction and resonant soft x-ray scattering experiments. For large $R$-ions $\left(R=\mathrm{Gd}–\mathrm{Dy}\right)$, the transition temperature to a long-range ordered antiferromagnetic phase is only weakly dependent on the $R$-ion radius, but decreases monotonically with decreasing $R$-ion radius for films with $R=\mathrm{Ho}–\mathrm{Lu}$. The antiferromagnetic phase is characterized by an incommensurate order of the ${\mathrm{Mn}}^{3+}$ spins, which successively locks into a commensurate $E$-type state. These findings confirm a uniform multiferroic ground state independent of the $R$ ion and are in excellent agreement with predicted properties of strain-induced multiferroicity in these materials. In particular, strong variation of multiferroic properties in these epitaxial films compared to bulk highlights the tuning ability of strain.

Since Peierls's pioneering work, it is generally accepted that phonon-phonon scattering processes consist of momentum-conserving normal scatterings and momentum-destroying Umklapp scatterings, and that the latter always induce thermal resistance. We show in this work that Umklapp scatterings are not necessarily resistive—no thermal resistance is induced if the projected momentum is conserved in the direction of heat flow. This distinction is especially important in anisotropic materials such as graphite and black phosphorous. By introducing a direction-dependent definition of normal and Umklapp scattering, we can model thermal transport in anisotropic materials using the Callaway model accurately. This accuracy is physically rooted in the improved description of mode-specific phonon dynamics. With the new definition, we predict that second sound might persist over much longer distances than previously expected.