Transport properties of graphene-superconductor junction have been studied extensively to understand the interplay of the relativistic Dirac quasiparticles and superconductivity. Though shot noise measurements in graphene have been performed to realize many theoretical predictions, both at zero magnetic field as well as quantum Hall (QH) regime, its junction with superconductor remain unexplored. Here, we have carried out the shot noise measurements in an edge contacted bilayer graphene–niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we have observed a Fano factor $\sim 1/3$ above the superconducting gap $\left(\mathrm{\Delta }\right)$ and a transition to an enhanced Fano factor $\sim 0.5$ below the superconducting gap. By changing the carrier density we have found a continuous reduction of Fano factor for both types of carriers; however, the enhancement of Fano factor within the superconducting gap by a factor of $\sim 1.5$ is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the graphene-superconductor interface.

The spin dynamics of positively ($X^{+}$) and negatively ($X^{-}$) charged excitons in $\mathrm{InP}/{\mathrm{In}}_{0.48}{\mathrm{Ga}}_{0.52}\mathrm{P}$ quantum dots subject to magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e., for $X^{-}$ trion photoexcitation. The presence of $X^{-}$ and $X^{+}$ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In the absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization is fitted by a form which describes the electron spin relaxation in “frozen” nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved.

We study the spectral function and quasiparticle scattering in Li-decorated graphene (Li@graphene) with an atomistic $T$-matrix formalism and uncover adatom-induced spectral effects which shed light on experimentally observed angle-resolved photoemission spectroscopy (ARPES) features. From transport studies, alkali adatoms are known to introduce charged-impurity scattering limiting the carrier mobility. Here, we demonstrate that Li adatoms furthermore give rise to a low-energy impurity band centered at the $\mathrm{\Gamma }$ point which originates from the hybridization between the atomic $2s$ state of the Li adatoms and graphene “surface” states. We show that the impurity band is strongly dependent on the concentration $c_{\mathrm{Li}}$ of Li adatoms, and aligns with the Li-induced Fermi level on the Dirac cone at $c_{\mathrm{Li}}\sim 8\%$ ($E_F\approx 1.1$ eV). Finally, we show that adatom-induced quasiparticle scattering increases dramatically at energies above $\sim 1\mathrm{eV}$ close to the van Hove singularity in the graphene density of states, giving rise to a large linewidth broadening on the Dirac cone with a concomitant downshift and a characteristic kink in the conduction band. Our findings are highly relevant for future studies of ARPES, transport, and superconductivity in adatom-doped graphene.

We present a comprehensive investigation of the anisotropic magnetic and magnetocaloric properties of quasi-two-dimensional weak itinerant ferromagnet trigonal ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ single crystals. Magnetic-anisotropy-induced satellite transition $T^{*}$ is observed at low fields applied parallel to the $ab$ plane below the Curie temperature ($T_c\sim 230\mathrm{K}$). $T^{*}$ is featured by an anomalous magnetization downturn, similar to that in structurally related ${\mathrm{CrI}}_3$, which possibly stems from an in-plane antiferromagnetic alignment induced by out-of-plane ferromagnetic spins canting. Magnetocrystalline anisotropy is also reflected in magnetic entropy change $\mathrm{\Delta }{S}_M(T,H)$ and relative cooling power. Given the high Curie temperature, ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ crystals are materials of interest for nanofabrication in basic science and applied technology.

We put forward the concept of a bulk Rashba effect emerging in a multiferroic material, such as an antiferromagnetic system with a polar crystal structure. According to symmetry considerations, while time-reversal and space-inversion symmetries are both broken, there exist specific spin flipping operations that relate opposite spin sites in the magnetic crystal structure. As a consequence, at certain high-symmetry points in the momentum space, the magnetic point group allows the spin angular momentum to be locked to the linear momentum, a typical feature of the Rashba effect. In such a case, spin-splitting effects induced by spin-orbit coupling can arise, similar to what happens in nonmagnetic Rashba systems. As a prototypical example, ab initio calculations of antiferromagnetic ${\mathrm{BiCoO}}_3$ in the polar structure reveal that a large Rashba-like band and spin splitting occurs at the conduction-band bottom, having a large weight from $\mathrm{Bi}-p$ orbital states. Moreover, we show that the spin texture of such a multiferroic can be modulated by applying a magnetic field. In particular, an external in-plane magnetic field is predicted not only to induce spin canting, but also a distortion of the energy isocontours and a shift of the spin vortex (centered on the high-symmetry point and characteristic of Rashba effect) along a direction perpendicular to the applied field.

Parity-time ($\mathcal{PT}$) symmetry plays an important role both in non-Hermitian and topological systems. In non-Hermitian systems, $\mathcal{PT}$ symmetry can lead to an entirely real-energy spectrum, while in topological systems, $\mathcal{PT}$ symmetry gives rise to stable and protected Dirac points. Here, we study a $\mathcal{PT}$-symmetric system which is both non-Hermitian and topological, namely, a $\mathcal{PT}$-symmetric Dirac semimetal with non-Hermitian perturbations in three dimensions. We find that, in general, there are only two types of symmetry-allowed non-Hermitian perturbations, namely, non-Hermitian kinetic potentials and non-Hermitian anticommuting potentials. For both of these non-Hermitian potentials, we investigate the band topology for open and periodic boundary conditions, determine the exceptional points, and compute the surface states. We find that with periodic boundary conditions, the non-Hermitian kinetic potential leads to exceptional rings, while the non-Hermitian anticommuting potential generates exceptional spheres, which separate regions with broken $\mathcal{PT}$ symmetry from regions with unbroken $\mathcal{PT}$ symmetry. With open boundary conditions, the non-Hermitian kinetic potential induces a non-Hermitian skin effect, which is localized on both sides of the sample due to symmetry, while the non-Hermitian anticommuting potential leads to Fermi ribbon surface states.

Recent studies reveal that four-phonon scattering can be important in determining thermal conductivities of solids. However, these studies have been focused on materials where the thermal conductivity $\kappa$ is dominated by acoustic phonons, and the role of four-phonon scattering, although significant, is still smaller than that of three-phonon scattering at room temperature. In this work, taking AlSb as an example for which optical phonons contribute significantly to the thermal conductivity under the three-phonon picture, we demonstrate that four-phonon scattering can be more important than three-phonon scattering as it diminishes optical phonon thermal transport and therefore significantly reduces the thermal conductivities of materials. Also, our calculations show that four-phonon scattering can diminish the isotope effect on $\kappa$. Specifically, including four-phonon scattering reduces the room-temperature $\kappa$ of isotopically pure BAs from 3322 W/mK to 1721 W/mK, a $48\%$ reduction. Four-phonon scattering reduces the room-temperature $\kappa$ of the isotopically pure and naturally occurring AlSb by $70\%$ and $50\%$, respectively, indicating that four-phonon scattering is more important than three-phonon scattering. The reductions for isotopically pure and naturally occurring cubic GaN are about $34\%$ and $27\%$, respectively. For isotopically pure wurtzite GaN, the reduction is about $13\%$ at room temperature and $25\%$ at 400 K. These results provided important guidance for experimentalists for achieving high thermal conductivities in III-V compounds for applications in the semiconductor industry.

Two-dimensional electron-hole gases in colloidal semiconductors have a wide variety of applications. Therefore, a proper physical understanding of these materials is of great importance. In this paper we present a detailed theoretical analysis of the recent experimental results by Tomar et al. [J. Phys. Chem. C 123, 9640 (2019)] that show an unexpected stability of excitons in CdSe nanoplatelets at high photoexcitation densities. Including the screening effects by free charges on the exciton properties, our analysis shows that CdSe nanoplatelets behave very differently from bulk CdSe, and in particular do not show a crossover to an electron-hole plasma in the density range studied experimentally, even though there is substantial overlap between the excitons at the highest densities achieved. From our results we also conclude that a quantum degenerate exciton gas is realized in the experiments, which opens the prospect of observing superfluidity in CdSe nanoplatelets in the near future.

An atomistic effective Hamiltonian, along with a presently developed analytical model, are employed to investigate and analyze low-frequency polar, antipolar and antiferrodistortive phonons at finite temperature in a prototypical hybrid improper ferroelectric, that is, the $\left({\mathrm{BiFeO}}_3\right)/\left({\mathrm{NdFeO}}_3\right)$ [abbreviated as ${\left(\mathrm{BFO}\right)}_1/{\left(\mathrm{NFO}\right)}_1$] 1:1 superlattice. In the high-temperature paraelectric phase, phonons having both polar and antipolar characters are found to exist, as a result of a bilinear coupling between different cation motions, with these phonons having frequencies that are basically independent of temperature. In contrast, phonons having fluctuations of either in-phase or antiphase octahedral tiltings are soft in this high-temperature phase (with these two fluctuations being uncoupled), which results in their condensation below some critical temperature and the emergence of a low-temperature phase. In this latter low-temperature phase, trilinear energetic couplings between these two types of octahedral tiltings and Bi and Nd cations' motions lead to the appearance of a spontaneous polarization, consistent with the nature of hybrid improper ferroelectricity. These trilinear energetic couplings also yield an increase in the number of phonons possessing both polar and antipolar characters in the low-temperature phase, with most of these phonons softening when approaching the ferroelectric-to-paraelectric transition from below, as a result of the fact that they also exhibit antiferrodistortive features. The different temperature behaviors of polar modes at high versus low temperatures emphasize the uniqueness of hybrid improper ferroelectrics.

Quantum tunneling is a valuable resource exploited by quantum annealers to solve complex optimization problems. Tunneling events also occur during projective quantum Monte Carlo (PQMC) simulations, and in a class of problems characterized by a double-well energy landscape their rate was found to scale linearly with the first energy gap, i.e., even more favorably than in physical quantum annealers, where the rate scales with the gap squared. Here we investigate how a guiding wave function—which is essential to make many-body PQMC simulations computationally feasible—affects the tunneling rate. The chosen test beds are a continuous-space double-well problem, the ferromagnetic quantum Ising chain, and the recently introduced shamrock model. As guiding wave function, we consider an approximate Boltzmann-type ansatz, the numerically exact ground state of the double-well model, and a neural-network wave function based on a Boltzmann machine. Remarkably, for each ansatz we find the same asymptotic linear scaling of the tunneling rate that was previously found in the PQMC simulations performed without a guiding wave function. We also provide a semiclassical theory for the double-well with exact guiding wave function that explains the observed linear scaling. These findings suggest that PQMC simulations guided by an accurate ansatz represent a valuable benchmark for physical quantum annealers and a potentially competitive quantum-inspired optimization technique.

The structure and lattice dynamics of the hybrid improper ferroelectric compound ${\mathrm{Ca}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Ti}}_2{\mathrm{O}}_7$ (for $x=0,0.6,$ and 0.9) have been studied with a combination of diffraction, inelastic scattering experiments, Raman spectroscopy, and calorimetry measurements, as well as first-principles simulations. Using inelastic neutron scattering, we have measured the phonon density of states (DOS) for $x=0.9$, which revealed a strong broadening but little change in phonon energies on heating from 10 K to 728 K across the ferroelectric phase transition temperature, $T_{FE}$. Using inelastic x-ray scattering, the momentum-resolved phonon dispersions were measured from 80 K to 950 K on a single crystal (for $x=0.6$), and also revealed a strong phonon broadening but a small energy shift for acoustic modes on heating across $T_{FE}$. Our Raman measurements (for $x=0.6$) showed robust rotational and oxygen breathing modes but soft tilt modes, consistent with previous measurements on similar compounds. Our density functional calculations achieve good agreement with both the phonon DOS and dispersions measured. We did not observe any unusual quadratic dispersion for $c$-polarized transverse acoustic modes, at odds with a recently predicted quasi-2D character, for either undoped ($x=0$) or doped ($x=0.6$) compounds.

We study a chain of magnetic impurities on a conventional superconductor with spin-orbit coupling, treating the superconducting order fully self-consistently. We find and quantify strong hybridization between the topological edge Majorana bound states (MBSs) and in-gap Yu-Shiba-Rusinov (YSR) states, which causes increasing energy oscillations as a function of magnetic impurity strength, even when the direct MBS overlap is negligible. By treating the MBS as a topological boundary state, dependent only on the effective mass gap, we arrive at a fully parameter-free functional form of its localization that decreases with magnetic impurity strength, opposite to the behavior of the superconducting coherence length.

The structure of liquid ammonia (${\mathrm{NH}}_3$) is investigated from 1 to 6.3 GPa and up to 800 K by means of synchrotron x-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations. The XRD data are used to extract the molecular structure factor $S_{\mathrm{mol}}\left(Q\right)$, pair distribution function (PDF) $g_{\mathrm{mol}}\left(r\right)$, and the density of ${\mathrm{NH}}_3$. There is an excellent agreement between present $S_{\mathrm{mol}}\left(Q\right)$ and $g_{\mathrm{mol}}\left(r\right)$ at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state (EoS) of Tillner-Roth et al. [DKV-Tagungsbericht (Hannover, New York, 1993), Vol. 20, p. 167] than with more recent EoS models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the Becke-Lee-Yang-Parr or the Perdew-Burke-Ernzerhof exchange-correlation functional. The shapes of $S_{\mathrm{mol}}\left(Q\right)$ and $g_{\mathrm{mol}}\left(r\right)$ vary little over the investigated pressure range and suggest a compact liquid with weak orientational correlations between molecules, which is corroborated by the coordination number varying from 12.7 to $\sim 14$. The simulations are used to study the evolution of the site-site pair distribution functions, which reveals that the number of H bonds per molecule (between 1.5 and 2) do not evolve with density and that the distribution of H atoms around N atoms becomes more and more anisotropic with pressure.

The interplay of disorder and interactions is a challenging topic of condensed matter physics, where correlations are crucial and exotic phases develop. In one spatial dimension, a particularly successful method to analyze such problems is the strong-disorder renormalization group (SDRG). This method, which is asymptotically exact in the limit of large disorder, has been successfully employed in the study of several phases of random magnetic chains. Here we develop an SDRG scheme capable of providing in-depth information on a large class of strongly disordered one-dimensional magnetic chains with a global invariance under a generic continuous group. Our methodology can be applied to any Lie-algebra valued spin Hamiltonian, in any representation. As examples, we focus on the physically relevant cases of SO($N$) and Sp($N$) magnetism, showing the existence of different randomness-dominated phases. These phases display emergent SU($N$) symmetry at low energies and fall into two distinct classes, with meson-like or baryon-like characteristics. Our methodology is here explained in detail and helps to shed light on a general mechanism for symmetry emergence in disordered systems.

It was recently understood that centrosymmetric multiband superconductors that break time-reversal symmetry generically show Fermi surfaces of Bogoliubov quasiparticles. We investigate the thermodynamic stability of these Bogoliubov Fermi surfaces in a paradigmatic model. To that end, we construct the mean-field phase diagram as a function of spin-orbit coupling and temperature. It confirms the prediction that a pairing state with Bogoliubov Fermi surfaces can be stabilized at moderate spin-orbit coupling strengths. The multiband nature of the model also gives rise to a first-order phase transition, which can be explained by the competition of intra- and interband pairing and is strongly affected by cubic anisotropy. For the state with Bogoliubov Fermi surfaces, we also discuss experimental signatures in terms of the residual density of states and the induced magnetic order. Our results show that Bogoliubov Fermi surfaces of experimentally relevant size can be thermodynamically stable.

Using the framework of the density-matrix renormalization group (DMRG), we study a quantum dot coupled to a superconducting nanowire with strong Rashba spin-orbit coupling. Regarding the singlet-to-doublet “0-$\pi$” transition that takes place when the Kondo effect is overcome by the superconducting gap, we show that the Rashba coupling modifies the critical values at which the transition occurs, favoring the doublet phase. In addition, using a generalized Haldane's formula for the Kondo temperature $T_K$, we show that it is lowered by the Rashba coupling. We benchmark our DMRG results, comparing them with previous numerical renormalization group results. The good agreement obtained opens the possibility of studying chains or clusters of impurities coupled to superconductors by means of DMRG.

The discovery of quantum oscillations in hole- and electron-doped cuprate families has underscored the importance of the Fermi surface in cuprate superconductivity. While the observed quantum oscillations in both families have revealed the presence of reconstructed Fermi surfaces, there remains an important distinction between the two. In hole-doped cuprates the oscillations are thought to arise from the effects of a charge density wave, while in the electron-doped cuprates it is thought that these oscillations occur from an antiferromagnetically reconstructed Fermi surface, despite the fact that the oscillations are observed in overdoped compounds, far from the putative antiferromagnetic critical point. In this work we study thin films of ${\mathrm{Pr}}_2{\mathrm{CuO}}_{4\pm \delta }$, whose apparent doping can be finely tuned by annealing, allowing studies of quantum oscillations in samples straddling the critical point. We show that even though there is a mass enhancement of the quasiparticles, there are only small changes to the Fermi surface itself, suggesting that charge order is a more likely origin, with electronic correlations that are strongly dependent on the structural parameters.

We generalize the construction of time-reversal symmetry-breaking triple-component semimetals, transforming under the pseudospin-1 representation, to arbitrary (anti-)monopole charge $2n$, with $n=1,2,3$ in the crystalline environment. The quasiparticle spectra of such systems are composed of two dispersing bands with pseudospin projections $m_s=\pm 1$ and energy dispersions $E_{\mathbf{k}}=\pm \sqrt{{{\alpha }_n^2{k}_{\perp }^{2n}+v_z^2{k}_z^2}^{\phantom{\int }}}$, where $k_{\perp }=\sqrt{{k_x^2+k_y^2}^{\phantom{\int }}}$, and one completely flat band at zero energy with $m_s=0$. We construct simple tight-binding models for such spin-1 excitations on a cubic lattice and address the symmetries of the generalized triple-component Hamiltonian. In accordance to the bulk-boundary correspondence, triple-component semimetals support $2n$ branches of topological Fermi arc surface states and also accommodate a large anomalous Hall conductivity (in the $xy$ plane), given by ${\sigma }_{xy}^{3\mathrm{D}}\propto 2n\times$ the separation of the triple-component nodes (in units of $e^2/h)$. Furthermore, we compute the longitudinal magnetoconductivity, planar Hall conductivity, and magnetothermal conductivity in these systems, which increase as $B^2$ for sufficiently weak magnetic fields $\left(B\right)$ due to the nontrivial Berry curvature in the medium. A generalization of our construction to arbitrary integer spin systems is also highlighted.

Hybrid quantum systems exhibiting coupled optical, spin, and mechanical degrees of freedom can serve as a platform for sensing, or as a bus to mediate interactions between qubits with disparate energy scales. These systems are also creating opportunities to test foundational ideas in quantum mechanics, including direct observations of the quantum regime in macroscopic objects. Here, we make use of angular momentum conservation to study the dynamics of a pair of paramagnetic centers featuring different spin numbers in the presence of a properly tuned external magnetic field. We examine the interplay between optical excitation, spin evolution, and mechanical motion, and theoretically show that in the presence of continuous optical illumination, interspin cross relaxation must induce rigid rotation of the host crystal. The system dynamics is robust to scattering of spin-polarized phonons, a result we build on to show this form of angular momentum transfer should be observable using state-of-the-art torsional oscillators or trapped nanoparticles.