We develop a quantum dynamical field theory for studying phase transitions in driven open systems coupled to Markovian noise, where nonlinear noise effects and fluctuations beyond semiclassical approximations influence the critical behavior. We systematically compare the diagrammatics, the properties of the renormalization group flow, and the structure of the fixed points of the quantum dynamical field theory and of its semiclassical counterpart, which is employed to characterize dynamical criticality in three-dimensional driven-dissipative condensates. As an application, we perform the Keldysh functional renormalization of a one-dimensional driven open Bose gas, where a tailored diffusion Markov noise realizes an analog of quantum criticality for driven-dissipative condensation. We find that the associated nonequilibrium quantum phase transition does not map into the critical behavior of its three-dimensional classical driven counterpart.

We present transport spectroscopy of subgap states in a bottom gated InAs nanowire coupled to a normal lead and a superconducting aluminium lead. The device shows clearly resolved subgap states which we can track as the coupling parameters of the system are tuned and as the gap is closed by means of a magnetic field. We systematically extract system parameters by using numerical renormalization-group theory fits as a level of the quantum dot is tuned through a quantum phase transition electrostatically and magnetically. We also give an intuitive description of subgap excitations.

Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: $\mathrm{Mo}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.1}{\mathrm{W}}_{0.9}{\mathrm{S}}_2$, and $\mathrm{W}{\mathrm{S}}_2$. The quasiparticle band gap, spin-orbit separation of the excitonic transitions at the $K/K^{\prime }$ point in the Brillouin zone, and binding energies of the $A$ exciton are extracted from STS and optical data. The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasiparticle band gaps from $2.17\pm 0.04\mathrm{eV}\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ to $2.38\pm 0.06\mathrm{eV}\left(\mathrm{W}{\mathrm{S}}_2\right)$, with $A$ exciton binding energies ranging from 310 to 420 meV.

Atomic-scale silicon wires, patterned by scanning tunneling microscopy (STM) and degenerately doped with phosphorus (P), have attracted significant interest owing to their exceptionally low resistivity and semiclassical Ohmic conduction at temperatures as low as $T=4.2\mathrm{K}$. Here, we investigate the transition from semiclassical diffusive to quantum-coherent conduction in a 4.6 nm wide wire as we decrease the measurement temperature. By analyzing the temperature dependence of universal conductance fluctuations (UCFs) and one-dimensional (1D) weak localization (WL)—fundamental manifestations of quantum-coherent transport in quasi-1D metals—we show that transport evolves from quantum coherent to semiclassical at $T\sim 4\mathrm{K}$. Remarkably, our study confirms that universal concepts of mesoscopic physics such as UCF and 1D WL retain their validity in quasi-1D metallic conductors down to the atomic scale.

Variable-range hopping conductivity has long been understood in terms of a canonical prescription for relating the single-particle density of states to the temperature-dependent conductivity. Here we demonstrate that this prescription breaks down in situations where a large and long-ranged random potential develops. In particular, we examine a canonical model of a completely compensated semiconductor, and we show that at low temperatures hopping proceeds along self-organized, low-dimensional subspaces having fractal dimension $d=2$. We derive and study numerically the spatial structure of these subspaces, as well as the conductivity and density of states that result from them. One of our prominent findings is that fractal ordering of low energy sites greatly enhances the hopping conductivity and allows Efros-Shklovskii type conductivity to persist up to unexpectedly high temperatures.

The study of quantum quenches in integrable systems has significantly advanced with the introduction of the quench action method, a versatile analytical approach to nonequilibrium dynamics. However, its application is limited to those cases where the overlaps between the initial state and the eigenstates of the Hamiltonian governing the time evolution are known exactly. Conversely, in this work we consider physically interesting initial states for which such overlaps are still unknown. In particular, we focus on different classes of product states in spin-$1/2$ and spin-1 integrable chains, such as tilted ferromagnets and antiferromagnets. We get around the missing overlaps by following a recent approach based on the knowledge of complete sets of quasilocal charges. This allows us to provide a closed-form analytical characterization of the effective stationary state reached at long times after the quench, through the Bethe ansatz distributions of particles and holes. We compute the asymptotic value of local correlations and check our predictions against numerical data.

We address the calculation of dynamical correlation functions for many fermion systems at zero temperature, using the auxiliary-field quantum Monte Carlo method. The two-dimensional Hubbard hamiltonian is used as a model system. Although most of the calculations performed here are for cases where the sign problem is absent, the discussions are kept general for applications to physical problems when the sign problem does arise. We study the use of twisted boundary conditions to improve the extrapolation of the results to the thermodynamic limit. A strategy is proposed to drastically reduce finite size effects relying on a minimization among the twist angles. This approach is demonstrated by computing the charge gap at half filling. We obtain accurate results showing the scaling of the gap with the interaction strength $U$ in two dimensions, connecting to the scaling of the unrestricted Hartree-Fock method at small $U$ and Bethe ansatz exact result in one dimension at large $U$. An alternative algorithm is then proposed to compute dynamical Green functions and correlation functions which explicitly varies the number of particles during the random walks in the manifold of Slater determinants. In dilute systems, such as ultracold Fermi gases, this algorithm enables calculations with much more favorable complexity, with computational cost proportional to basis size or the number of lattice sites.

For magnetic materials containing many lattice imperfections (e.g., nanocrystalline magnets), the relativistic Dzyaloshinski-Moriya (DM) interaction may result in nonuniform spin textures due to the lack of inversion symmetry at interfaces. Within the framework of the continuum theory of micromagnetics, we explore the impact of the DM interaction on the elastic magnetic small-angle neutron scattering (SANS) cross section. It is shown that the DM interaction gives rise to a polarization-dependent asymmetric term in the spin-flip SANS cross section. Analysis of this feature may provide a means to determine the DM constant.

It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that ${\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$ may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ${\varepsilon }_F$ to show a Weyl semimetal in ${\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$. Then, we study ${\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$ by pump-probe ARPES and we directly access the band structure $>0.2$ eV above ${\varepsilon }_F$ in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of ${\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$ by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in ${\mathrm{Mo}}_x{\mathrm{W}}_{1-x}{\mathrm{Te}}_2$.

Heavily doped semiconductor thin films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in the 5 to $50\mu \mathrm{m}$ wavelength range at least. In this work, we investigate the electrodynamics of heavily $n$-type-doped germanium epilayers at infrared frequencies beyond the assumptions of the Drude model. The films are grown on silicon and germanium substrates, are in situ doped with phosphorous in the ${10}^{17}$ to ${10}^{19}$ ${\mathrm{cm}}^{-3}$ range, then screened plasma frequencies in the 100 to $1200{\mathrm{cm}}^{-1}$ range were observed. We employ infrared spectroscopy, pump-probe spectroscopy, and dc transport measurements to determine the tunability of the plasma frequency. Although no plasmonic structures have been realized in this work, we derive estimates of the decay time of mid-infrared plasmons and of their figures of merit for field confinement and for surface plasmon propagation. The average electron scattering rate increases almost linearly with excitation frequency, in agreement with quantum calculations based on a model of the ellipsoidal Fermi surface at the conduction band minimum of germanium accounting for electron scattering with optical phonons and charged impurities. Instead, we found weak dependence of plasmon losses on neutral impurity density. In films where a transient plasma was generated by optical pumping, we found significant dependence of the energy relaxation times in the few-picosecond range on the static doping level of the film, confirming the key but indirect role played by charged impurities in energy relaxation. Our results indicate that underdamped mid-infrared plasma oscillations are attained in $n$-type-doped germanium at room temperature.

Recently, Zacharias et al. [Phys. Rev. Lett. 115, 177401 (2015)] developed an ab initio theory of temperature-dependent optical absorption spectra and band gaps in semiconductors and insulators. In that work, the zero-point renormalization and the temperature dependence were obtained by sampling the nuclear wave functions using a stochastic approach. In the present work, we show that the stochastic sampling of Zacharias et al. can be replaced by fully deterministic supercell calculations based on a single optimal configuration of the atomic positions. We demonstrate that a single calculation is able to capture the temperature-dependent band-gap renormalization including quantum nuclear effects in direct-gap and indirect-gap semiconductors, as well as phonon-assisted optical absorption in indirect-gap semiconductors. In order to demonstrate this methodology, we calculate from first principles the temperature-dependent optical absorption spectra and the renormalization of direct and indirect band gaps in silicon, diamond, and gallium arsenide, and we obtain good agreement with experiment and with previous calculations. In this work we also establish the formal connection between the Williams-Lax theory of optical transitions and the related theories of indirect absorption by Hall, Bardeen, and Blatt, and of temperature-dependent band structures by Allen and Heine. The present methodology enables systematic ab initio calculations of optical absorption spectra at finite temperature, including both direct and indirect transitions. This feature will be useful for high-throughput calculations of optical properties at finite temperature and for calculating temperature-dependent optical properties using high-level theories such as $GW$ and Bethe-Salpeter approaches.

In this paper, we discuss the connections between pseudospin, real spin of electrons in a material, and spin polarization of photoemitted electrons out of a material. By investigating these three spin textures for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{SmB}}_6$ compounds, we find that the spin orientation of photoelectrons for ${\mathrm{SmB}}_6$ has a different correspondence to pseudospin and real spin compared to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, due to the different symmetry properties of the photoemission matrix between the initial and final states. We calculate the spin polarization and circular dichroism spectra of photoemitted electrons for both compounds, which can be detected by spin-resolved and circular dichroism angle-resolved photoemission spectroscopy experiments.

We show by first-principles calculations that the skew-scattering anomalous Hall and spin Hall angles of $L{1}_0$-ordered FePt drastically depend on different types of disorder. A different sign of the anomalous Hall angle is obtained when slightly deviating from the stoichiometric ratio towards the Fe-rich side as compared to the Pt-rich side. For stoichiometric samples, short-range ordering of defects has a profound effect on the Hall angles and can change them by a factor of 2 as compared to the case of uncorrelated disorder. This might explain the vast range of anomalous Hall angles measured in experiments, which undergo different preparation procedures and thus might differ in their crystallographic quality.

The Dzyaloshinskii-Moriya interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for future spintronic technologies. Here we employ a scanning nanomagnetometer based on a single nitrogen-vacancy defect in diamond to locally probe the strength of the interfacial DMI in $\mathrm{CoFeB}|\mathrm{MgO}$ ultrathin films grown on different heavy metal underlayers $X=\text{Ta}$, TaN, and W. By measuring the stray field emanating from domain walls in micron-long wires of such materials, we observe deviations from the Bloch profile for TaN and W underlayers that are consistent with a positive DMI value favoring right-handed chiral spin structures. Moreover, our measurements suggest that the DMI constant might vary locally within a single sample, illustrating the importance of local probes for the study of magnetic order at the nanoscale.

Quasiparticle poisoning and diabatic transitions may significantly narrow the window for the experimental observation of the $4\pi$-periodic $\text{dc}$ Josephson effect predicted for topological Josephson junctions. Here, we show that switching-current measurements provide accessible and robust signatures for topological superconductivity which persist in the presence of quasiparticle poisoning processes. Such measurements provide access to the phase-dependent subgap spectrum and Josephson currents of the topological junction when incorporating it into an asymmetric SQUID together with a conventional Josephson junction with large critical current. We also argue that pump-probe experiments with multiple current pulses can be used to measure the quasiparticle poisoning rates of the topological junction. The proposed signatures are particularly robust, even in the presence of Zeeman fields and spin-orbit coupling, when focusing on short Josephson junctions. Finally, we also consider microwave excitations of short topological Josephson junctions which may complement switching-current measurements.

We report on high-field angle-dependent magnetotransport measurements on epitaxial thin films of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, a three-dimensional topological insulator. At low temperature, we observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. The magnetoresistance of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is found to be highly anisotropic. In the presence of a parallel electric and magnetic field, we observe a strong negative longitudinal magnetoresistance that has been considered as a smoking gun for the presence of chiral fermions in a certain class of semimetals due to the so-called axial anomaly. Its observation in a three-dimensional topological insulator implies that the axial anomaly may be in fact a far more generic phenomenon than originally thought.

Vacancies in graphene have been proposed to give rise to $\pi$-like magnetism in carbon materials, a conjecture which has been supported by recent experimental evidence. A key element in this “vacancy magnetism” is the formation of magnetic moments in vacancy-induced electronic states. In this work we compute the charging energy $U$ of a single-vacancy-generated localized state for bulk graphene and graphene ribbons. We use a tight-binding model to calculate the dependency of the charging energy $U$ on the amplitudes of the localized wave function on the graphene lattice sites. We show that for bulk graphene $U$ scales with the system size $L$ as ${(lnL)}^{-2}$, confirming the predictions in the literature, based on heuristic arguments. In contrast, we find that for realistic system sizes $U$ is of the order of eV, a value that is orders of magnitude higher than the previously reported estimates. Finally, when edges are considered, we show that $U$ is very sensitive to the vacancy position with respect to the graphene flake boundaries. In the case of armchair nanoribbons, we find a strong enhancement of $U$ in certain vacancy positions as compared to the value for vacancies in bulk graphene.

Long known for its peculiar resistivity, showing a thus far unexplained anomalous peak as a function of temperature, ${\mathrm{ZrTe}}_5$ has recently received rising attention in a somewhat different context. While both theoretical and experimental results seem to point to a nontrivial topology of the low-energy electronic states, there is no agreement on the nature of their topological character. Here, by an angle-resolved photoemission study of the evolution of the band structure with temperature and surface doping, we show that (i) the material presents a van Hove singularity close to the Fermi level, and (ii) no surface states exist at the (010) surface. These findings reconcile band structure measurements with transport results and establish the topology of this puzzling compound.

Topologically protected magnetic structures such as skyrmions and domain walls (DWs) have drawn a great deal of attention recently due to their thermal stability and potential for manipulation by spin current, which is the result of chiral magnetic configurations induced by the interfacial Dzyaloshinskii-Moriya interaction (DMI). Designing devices that incorporate DMI necessitates a thorough understanding of how the interaction presents and can be measured. One approach is to measure growth asymmetry of chiral bubble domains in perpendicularly magnetized thin films, which has been described elsewhere by thermally activated DW motion. Here, we demonstrate that the anisotropic angular dependence of DW energy originating from the DMI is critical to understanding this behavior. Domains in Co/Ni multilayers are observed to preferentially grow into nonelliptical teardrop shapes, which vary with the magnitude of an applied in-plane field. We model the domain profile using energetic calculations of equilibrium shape via the Wulff construction, which serves as a new paradigm for describing chiral domains that explains both the teardrop shape and the reversal of growth symmetry at large fields.

Motivated by the recent report of superconductivity above 200 K in ultra-dense hydrogen sulfide, we search for high-$T_C$ conventional superconductivity in the phase diagram of the binary Li-S system, using ab initio methods for crystal structure prediction and linear response calculations for the electron-phonon coupling. We find that at pressures higher than 20 GPa, several new compositions, besides the known ${\mathrm{Li}}_2\mathrm{S}$, are stabilized; many exhibit electride-like interstitial charge localization observed in other alkali-metal compounds. Of all predicted phases, only an fcc phase of ${\mathrm{Li}}_3\mathrm{S}$, metastable before 640 GPa, exhibits a sizable $T_C$, in contrast to what is observed in sulfur and phosphorus hydrides, where several stoichiometries lead to high $T_C$. We attribute this difference to $2s-2p$ hybridization and avoided core overlap, and predict similar behavior for other alkali-metal compounds.