It has been theoretically predicted and experimentally confirmed that graphene deposited with atoms of a univalent alkali metal such as Li (${\mathrm{LiC}}_6$) or divalent alkaline-earth metal such as Ca (${\mathrm{CaC}}_6$) can be a superconductor. For atoms of a trivalent metal such as Al, if deposited on graphene, it was predicted that ${\mathrm{AlC}}_8$ can be in a metallic state. Whether this compound is stable and can be made superconducting is an issue which has not been addressed and deserves further investigation. In this work, based on first-principles calculations, it is found that the phonon spectrum of ${\mathrm{AlC}}_8$ shows imaginary frequencies for the two lowest branches, indicating the structure is dynamically unstable. By hole doping, the imaginary frequencies basically disappear and the lattice is stabilized. Besides, biaxial tensile strain was applied to study its effect on phonon and electron-phonon coupling. With the increase of tensile strain, the high-energy phonon spectrum associated with the C-C stretching modes softens greatly and the electron-phonon coupling becomes stronger, resulting in the increase of superconducting transition temperature $T_c$ to a value of more than 22 K for a sample at the experimentally accessible hole doping ($7.5\times {10}^{13}{\mathrm{cm}}^{-2}$) and tensile strain ($12\%$) levels. This is above the liquid hydrogen temperature of 20.3 K. Thus, besides Li and Ca deposited graphene, ${\mathrm{AlC}}_8$ provides another platform for realizing superconductivity in graphene.

We test the analytical formula for the enhancement of the nuclear magnetic resonance rate $T_1^{-1}$ by the critical spin fluctuations, over the simple power-law dependence predicted for a purely one-dimensional spin system, recently derived in the random phase approximation [M. Dupont et al., Phys. Rev. B 98, 094403 (2018)]. This prediction is experimentally confirmed by excellent fits to the published temperature dependence of $T_1^{-1}$ data in the two representative spin compounds, ${\left({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}\right)}_2{\mathrm{CuBr}}_4$ (DIMPY) and ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, providing at the same time a direct and convenient experimental determination of the Tomonaga-Luttinger-liquid parameter $K$, very well in agreement with theoretical predictions.

The hunt for high-temperature superfluidity has received new impetus from the discovery of atomically thin stable materials. Electron-hole superfluidity in coupled ${\mathrm{MoSe}}_2-{\mathrm{WSe}}_2$ monolayers is investigated using a mean-field multiband model that includes band splitting caused by strong spin-orbit coupling. This splitting leads to a large energy misalignment of the electron and hole bands which is strongly modified by interchanging the doping of the monolayers. The choice of doping determines if the superfluidity is tunable from one to two components.

The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.

Mixtures of sodium and potassium differ substantially from the pure elements, while retaining the high compressibility, which is important to the complex behavior of dense alkali metals. We present powder x-ray diffraction of mixtures of Na and K compressed in diamond anvil cells to 48 GPa at 295 K. This reveals two stoichiometric intermetallics: an ${\mathrm{Na}}_2\mathrm{K}$ phase known at ambient pressure and low temperature, and a novel NaK phase formed of interpenetrating sodium and potassium diamond lattices. Density functional theory calculations find the new phase to be dynamically stable and, in contrast to pure alkali metals, reveal decreasing electron localization with applied pressure. Depending on the mixture composition these intermetallics are accompanied by sodium or potassium rich phases suggesting that there are no other intermetallics under the range of P-T conditions studied. Alkali-metal mixtures have seen little study at high pressure and represent an unusual class of materials with very high compressibility and multiple constituents. Such materials exhibit significant compression at experimentally accessible pressures and open a way to measure multispecies structures at high compression. These results challenge structural finding algorithms for mixtures in high-pressure conditions.

Remarkable electronic transport and magnetotransport properties are found in the intermetallic compounds $R{\mathrm{Co}}_{12}{\mathrm{B}}_6$ ($R=\text{Y}$, Ho, and Gd). Detailed resistivity, magnetoresistance, and Hall effect measurements are reported in the temperature range between 2 and 320 K under applied magnetic fields up to 9 T. Auxiliary magnetization results and structural data are also reported. The magnetic ordering observed in these systems depends on the rare-earth atom. In the ${\mathrm{YCo}}_{12}{\mathrm{B}}_6$ compound, a ferromagnetic-type ordering is observed ($T_C\approx 149$ K) whereas the ${\mathrm{GdCo}}_{12}{\mathrm{B}}_6$ and ${\mathrm{HoCo}}_{12}{\mathrm{B}}_6$ compounds show ferrimagnetic-type ordering ($T_C\approx 162$ and 144 K, respectively) and their magnetization exhibits the compensation phenomenon. All compounds show high residual-resistance ratio (RRR) rates and resistivities typical of magnetically ordered systems, with a characteristic downward inflection at the critical temperature. The resistivity results are described in terms of the interplay between the electron-phonon interaction and a magnetic term. The magnetoresistance is consequence of a complex superposition of effects due to conduction in spin-polarized bands and to spin-disorder suppression. A strong positive contribution, which prevails at low temperatures, is described with a basis on the Campbell-Fert model that assumes conduction by two spin-polarized currents with a spin-mixing term. The usual negative effect related to suppression of spin disorder becomes the dominant contribution at intermediate- and high-temperature regimes. At low temperatures, the Hall resistivity results are also consistent with conduction by two spin-polarized bands. Moreover, a sign reversal observed in the Hall resistivity for all compounds at low temperatures indicates that the majority spin-up band is holelike, while the minority spin band is electronlike. The ordinary and anomalous Hall contributions could be separated. The intrinsic Karplus-Luttinger mechanism is the most relevant contribution to the anomalous Hall resistivity in all studied compounds. However, an additional term due to chiralities related to canting of the Co moments also plays a role in the temperature region near $T_C$.

An excitonic insulator (EI) is an unconventional quantum phase of matter in which excitons, bound pairs of electrons and holes, undergo Bose-Einstein condensation, forming a macroscopic coherent state. While its existence was first hypothesized half a century ago, the EI has eluded experimental observation in bulk materials for decades. In the last few years, a resurgence of interest in the subject has been driven by the identification of several candidate materials suspected to support an excitonic condensate. However, one obstacle in verifying the nature of these systems has been to find signatures of the EI that distinguish it from a normal insulator. To address this, we focus on a clear qualitative difference between the two phases: the existence of Goldstone modes born by the spontaneous breaking of a U(1) symmetry in the EI. Even if this mode is gapped, as occurs in the case of an approximate symmetry, this branch of collective modes remains a fundamental feature of the low-energy dynamics of the EI provided the symmetry-breaking is small. We study a simple model that realizes an excitonic condensate and use mean-field theory within the random-phase approximation to determine its collective modes. We subsequently develop a diagrammatic method to incorporate the effects of disorder perturbatively and use it to determine the scattering rate of the collective modes. We interpret our results within an effective field theory. The collective modes are found to be robust against symmetry-preserving disorder, implying an experimental fingerprint unique to the EI: the ballistic propagation of low-lying modes over mesoscopic distances, at electronic-scale velocities. We suggest this could affect thermal transport at low temperatures and could be observed via spatially resolved pump-probe spectroscopy through the coherent response of phonons that hybridize with the collective modes.

We have developed a theory of collective electron oscillations in two-dimensional semiconductor heterostructures subjected to a high electric field. The effect of the stationary electric field has been taken into account on both steady-state and high-frequency electron transport. The analysis has been conducted by solving Boltzmann-Vlasov equations in the collisionless approach for high-frequency electron transport. Two actual types of heterostructures with ungated and gated two-dimensional (2D) electron gas have been considered. We have found that the collective excitation spectra of 2D electron gas are of the multibranch character with high-quality plasmonic modes and a set of thermal modes. Applied electric field induces the following effects: strong nonreciprocal behavior of both oscillation frequency and damping; interaction of plasmonic and thermal modes; instability of excitations propagating along the electron drift (effect of negative Landau damping). The mechanism of this plasma wave instability is different from early discussed Cherenkov, Dyakonov-Shur, and Ryzhii-Satou-Shur mechanisms of instabilities in plasmonic systems. It has been shown that the electrically induced plasmon instability provides amplification of terahertz (THz) radiation in grating-based plasmonic structures. We suggest that presented results can be important for deeper understanding of THz plasma physics and developing of electrically pumping devices for THz optoelectronics.

Band structures of topological insulators are characterized by nonlocal topological invariants. Consequently, proposals for the experimental detection using local probes are rare. A recent paper [R.-J. Slager et al., Phys. Rev. B 92, 085126 (2015)] has argued, based on theoretical results for a particular class of models, that insulators with topologically trivial and nontrivial band structures in two space dimensions display a qualitatively different response to pointlike impurities. Here, we present a comprehensive investigation of the impurity response of a large set of models of noninteracting electrons on the honeycomb lattice, driven insulating by either broken inversion, broken time-reversal, broken $C_3$, or broken translation symmetry. These cases include Hofstadter bands, strain-induced pseudo-Landau levels, and higher-order topological insulators. Our results confirm that for hopping models respecting the lattice symmetries, the response to a single impurity can indeed distinguish between trivial and nontrivial band topology. However, for modulated or inhomogeneous host systems we find that trivial states of matter can display an impurity response akin to that of topologically nontrivial states, and thus the diagnostic fails.

A general understanding of the mechanism underlying the pressure-induced Mott insulator-metal transition in strongly correlated materials is still lacking. Here we explore the pressure-induced electronic reconstruction in ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, a potential two-leg ladder system for unconventional (non-BCS) superconductivity. We stress the importance of multiorbital Coulomb interactions in concert with first-principles band-structure calculations for a consistent understanding of its intrinsic Mott-Hubbard insulating state both at ambient and under pressure. We elucidate the nature of pressure-induced insulator-metal transition seen in experiment, showing that it is driven by bandwidth broadening under pressure. We reveal an orbital-selective electronic state where Mott localized and itinerant electrons coexist in compressed ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, which incorporates orbital-resolved scattering rates and renormalization factors hidden in the normal state at high pressures.

Motivated by the results of recent thermoelectric effect studies, we show the effects of Coulomb interactions on the Seebeck coefficient based on an extended Hubbard model that describes the electronic states of a slightly doped organic Dirac electron system, $\alpha {\text{−}\left(\text{BEDT-TTF}\right)}_2{\mathrm{I}}_3$. Our results indicate that the Hartree terms of the Coulomb interactions enhance the electron-hole asymmetry of the energy band structure and change the energy dependence of the relaxation time from impurity scattering, which reflects the shape of the density of states. Thus, the Seebeck coefficient exhibits a nonmonotonic $T$ dependence which qualitatively agrees with the experimental results. Furthermore, we also show that the signs of the Seebeck coefficient and the Hall coefficient calculated by linear response theory do not necessarily correspond to the sign of the chemical potential using a modified Weyl model with electron-hole asymmetry. These results point out that changing the electron-hole asymmetry by strong Coulomb interaction has the potential to control the sign and value of the Seebeck coefficient in the Dirac electron systems.

In the present paper we study magneto-intersubband oscillations (MISO) in $\mathrm{HgTe}/{\mathrm{Hg}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ single quantum well with “inverted” and “normal” spectra and in ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}/{\mathrm{In}}_{1-y}{\mathrm{Al}}_y\mathrm{As}$ quantum wells with normal band ordering. For all the cases when two branches of the spectrum arise due to spin-orbit splitting, the mutual arrangement of the antinodes of the Shubnikov-de Haas oscillations and the maxima of MISO occurs opposite to that observed in double quantum wells and in wide quantum wells with two subbands occupied and does not agree with the theoretical predictions. A “toy” model is proposed that explains qualitatively this unusual result.

We present an evolutionary construction technique of formation-energy convex hull to search for thermodynamically stable compounds. In this technique, candidates with a wide variety of chemical compositions and crystal structures are created by systematically applying evolutionary operators, “mating,” “mutation,” and “adaptive mutation,” to two target compounds, and the convex hull is directly updated through the evolution. We applied the technique to carbon-hydrogen binary system at 10 GPa and obtained 15 hydrocarbons within the convex hull distance less than 0.5 mRy/atom: graphane, polybutadiene, polyethylene, butane, ethane, methane, three molecular compounds of ethane and methane, and six molecular compounds of methane and hydrogen. These results suggest that our evolutionary construction technique is useful for the exploration of stable phases under extreme conditions and the synthesis of new compounds.

Quantum magnonics is a new and active research field, leveraging the strong collective coupling between microwaves and magnetically ordered spin systems. To date work in quantum magnonics has focused on transition metals and almost entirely on ferromagnetic resonances in yttrium iron garnet. Antiferromagnetic systems have gained interest as they produce no stray field, and are therefore robust to magnetic perturbations with narrow, shape independent resonant linewidths. Rare-earth ion spins are also of particular interest as they can exhibit narrow optical and microwave transitions and have large magnetic dipole moments. Here we show experimental evidence of ultrastrong-coupling between a microwave cavity and collective antiferromagnetic resonances (magnons) in a fully concentrated rare-earth crystal. Using a loop-gap microwave resonator a coupling strength of 1.75 GHz is obtained between the cavity and the antiferromagnetic resonances in a ${\mathrm{GdVO}}_4$ sample. Furthermore we measure the linewidth of the antiferromagnetic magnon to be 35 MHz, proving that narrow magnon linewidths can be achieved in rare-earth crystals. The combination of the unique optical and spin properties of the rare earths and collective antiferromagnetic order paves the way for quantum magnonic applications.

We demonstrate that the spin Hall magnetoresistance (SMR) of epitaxial Pt/Co bilayers on MgO(110) single crystal substrates is strongly anisotropic and depends on the applied current direction with respect to the two primary in-plane crystal directions [001] and $\left[1\overline{1}0\right]$, in the Pt layer. SMR results at different temperatures qualitatively suggest the D'yakonov-Perel spin relaxation mechanism through the invariance of the spin diffusion length ${\lambda }_{sf}$ for a given current direction, while also suggesting an anisotropic Rashba-Edelstein effect from the ratio of ${\lambda }_{sf}^{\left[001\right]}$ to ${\lambda }_{sf}^{\left[1\overline{1}0\right]}$. Finally, deviation from the standard SMR model spurs the need for new theory that properly characterizes and quantifies the anisotropies in such epitaxial systems.

We investigate the macroscopic and microscopic physical properties of the solid solution of ${\mathrm{Ce}}_{1-x}{\mathrm{Pr}}_x\mathrm{AlGe}$. The series tunes from CeAlGe with its multi-$\vec{k}$ structure and a major moment in the $ab$ plane, to PrAlGe with an easy $c$-axis ferromagnetic ground state coexisting with a low density of nanoscale textured magnetic domain walls. Using AC, DC susceptibility, resistivity, specific heat, muon spin relaxation/rotation, and neutron scattering we analyze the magnetic ground state of the series. We provide further evidence supporting our previous claim for spin-glass-like properties in pure PrAlGe. With the introduction of Pr to CeAlGe the finite magnetic field required to stabilize the topological multi-$\vec{k}$ magnetic phase for $x=0$ becomes suppressed. The crossover between the two end-member ground states occurs in the vicinity of $x=0.3$, a region where we further anticipate the field-induced topological magnetic phase for $x<0.3$ to become the zero field ground state.

In a graphene-based Josephson junction, the Andreev reflection can become specular, which gives rise to propagating Andreev modes. These propagating Andreev modes are essentially charge neutral and therefore they transfer energy but not electric charge. One main result of this work is that when the Dirac theory of graphene is deformed into a tilted Dirac cone, the breaking of charge conjugation symmetry of the Dirac equation renders the resulting Andreev modes electrically charged. We calculate an otherwise zero charge conductance arising solely from the tilt parameters $\vec{\zeta }=({\zeta }_x,{\zeta }_y)$. The distinguishing feature of such a form of charge transport from the charge transport caused by normal electrons is their dependence on the phase difference $\varphi$ of the two superconductors which can be experimentally extracted by employing a flux bias. Another result concerns the enhancement of Josephson current in a regime where instead of propagating Andreev modes, localized Andreev levels are formed. In this regime, we find enhancement by orders of magnitude of the Josephson current when the tilt parameter is brought closer and closer to $\zeta =1$ limit. We elucidate that the enhancement is due to a combination of two effects: (i) enhancement of number of transmission channels by flattening of the band on tilting to $\zeta \approx 1$ and (ii) a nontrivial dependence on the angle $\theta$ of the the tilt vector $\vec{\zeta }$.

In a previous work, we predicted that a thermally biased tunnel junction between two different superconductors can display a thermoelectric effect of nonlinear nature in the temperature gradient under proper conditions. In this work we give a more extended discussion, and we focus on the two main features of the nonlinear contributions: (i) the linear-in-bias thermoelectricity, which can be associated to a spontaneous breaking of electron-hole symmetry, and (ii) the strong contribution at the matching peak singularity, which is typically associated to the maximum output power and efficiency. We discuss the nonlinear origin of the thermoelectricity and its relationship with the nonlinear cooling mechanism in superconducting junctions previously discussed in the literature. Finally, we design and characterize the performance of the system as a heat engine for a realistic design and experimental parameter values. We discuss possible nonidealities demonstrating that the system is amenable to current experimental realization.