A quantum Monte Carlo approach, considering all the corrugation effects, was used to calculate the complete phase diagram of the second ${}^3\mathrm{He}$ layer adsorbed on graphite. We found that a first-layer triangular solid was in equilibrium with a gas in the second layer. At a surface density $0.166\pm 0.001{\AA }^{-2}$, this fluid changes into two first-layer registered phases: 4/7 and 7/12 solids. The 7/12 arrangement transforms into an incommensurate triangular structure of $\rho =0.189\pm 0.001{\AA }^{-2}$ upon further helium loading. A recently proposed hexatic phase was found to be unstable with respect to those commensurate solids.

We investigate nonequilibrium phase transitions in classical Heisenberg spin chains associated with spontaneous breaking of parity-time ($\mathcal{PT}$) symmetry of the system under the action of Slonczewski spin-transfer torque (STT) modeled by an applied imaginary magnetic field. We reveal the STT-driven $\mathcal{PT}$ symmetry-breaking phase transition between the regimes of precessional and exponentially damped spin dynamics and show that its several properties can be derived from the distribution of zeros of the system's partition function, the approach first introduced by Yang and Lee for studying equilibrium phase transitions in Ising spin chains. The physical interpretation of imaginary magnetic field as describing the action of nonconservative forces opens the possibility of direct observations of Lee-Yang zeros in nonequilibrium physical systems.

We demonstrate a way to polarize and detect nuclear spin in a gate-defined quantum point contact operating at high Landau levels. Resistively detected nuclear magnetic resonance (RDNMR) can be achieved up to the fifth Landau level and at a magnetic field lower than 1 T. We are able to retain the RDNMR signals in a condition where the spin degeneracy of the first one-dimensional (1D) subband is still preserved. Furthermore, the effects of orbital motion on the first 1D subband can be made smaller than those due to electrostatic confinement. This developed RDNMR technique is a promising means to study electronic states in a quantum point contact near zero magnetic field.

Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin-diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a nonuniform spatiotemporal precession pattern is observed. The kinetic-theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba (α) and the Dresselhaus (${\beta }_3$) parameters. Corresponding calculations are further validated by an excitation-density-dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping.

We explore the combined impact of sulfur vacancies and electronic interactions on the optical properties of monolayer ${\mathrm{MoS}}_2$. First, we present a generalized Anderson-Hubbard Hamiltonian that accounts for both randomly distributed sulfur vacancies and the presence of dielectric screening within the material. Second, we parametrize this energy-dependent Hamiltonian from first-principles calculations based on density functional theory and the Green's function and screened Coulomb (GW) method. Third, we apply a first-principles-based many-body typical medium method to determine the single-particle electronic structure. Fourth, we solve the Bethe-Salpeter equation to obtain the charge susceptibility $\chi$ with its imaginary part being related to the absorbance $\mathcal{A}$. Our results show that an increased vacancy concentration leads to decreased absorption both in the band continuum and from exciton states within the band gap. We also observe increased absorption below the band-gap threshold and present an expression, which describes Lifshitz tails, in excellent qualitative agreement with our numerical calculations. This latter increased absorption in the $1.0–2.5\mathrm{eV}$ range makes defect engineering of potential interest for solar cell applications.

Orbital-free density functional theory (OF-DFT) promises to describe the electronic structure of very large quantum systems, being its computational cost linear with the system size. However, the OF-DFT accuracy strongly depends on the approximation made for the kinetic energy (KE) functional. To date, the most accurate KE functionals are nonlocal functionals based on the linear-response kernel of the homogeneous electron gas, i.e., the jellium model. Here, we use the linear-response kernel of the jellium-with-gap model to construct a simple nonlocal KE functional (named KGAP) which depends on the band-gap energy. In the limit of vanishing energy gap (i.e., in the case of metals), the KGAP is equivalent to the Smargiassi-Madden (SM) functional, which is accurate for metals. For a series of semiconductors (with different energy gaps), the KGAP performs much better than SM, and results are close to the state-of-the-art functionals with sophisticated density-dependent kernels.

The recent epitaxial growth of the $1T$ phase of the ${\mathrm{PtSe}}_2$ monolayer (ML) has opened the possibility for novel applications, in particular for a spintronics device. However, in contrast to the $2H$ phase of transition-metal dichalcogenides (TMDs), the absence of spin splitting in the ${\mathrm{PtSe}}_2$ ML may limit the functionality for spintronics application. Through fully relativistic density-functional theory calculations, we show that large spin splitting can be induced in the ${\mathrm{PtSe}}_2$ ML by introducing a substitutional halogen impurity. Depending on the atomic number $Z$ of the halogen dopants, we observe an enhancement of the spin splitting in the localized impurity states (LIS), which is due to the increased contribution of the $p\text{−}d$ orbital coupling. More importantly, we identify very large Rashba splitting in the LIS near the Fermi level around the $\mathrm{\Gamma }$ point characterized by hexagonal warping of the Fermi surface. We show that the Rashba splitting can be controlled by adjusting the doping concentration. Therefore, this work provides a possible way to induce significant Rashba splitting in the two-dimensional TMDs, which is useful for spintronic devices operating at room temperature.

One of the key factors in understanding high-$T_c$ cuprate superconductors is the spatial distribution of holes in the sample. Since x-ray absorption spectroscopy (XAS) and electron-energy-loss spectroscopy (EELS) can directly measure the unoccupied $2p$ states of the oxygen, many experiments have already been performed for a wide range of doping conditions. While polarization-dependent x-ray fluorescence yield absorption cannot spatially resolve different oxygen sites, EELS combined with scanning transmission electron microscopy has the ability to resolve them with atomic resolution. However, since cuprate superconductors are extremely sensitive to electron irradiation, it has not been possible, to date, to characterize them with atomic resolution. Here, we succeeded in atomic-resolution two-dimensional mapping of holes in $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_{4\pm \delta }$ by overcoming the problem of irradiation damage using an advanced integration technique. In addition, we have demonstrated the anisotropic chemical bond related to the difference between $p_x$ and $p_y$ orbitals was observed with atomic resolution. The present approach enables atomic-resolution anisotropic spectroscopy and is expected to give information complementary to the polarization-dependent XAS.

The self-learning Monte Carlo (SLMC) method is a general algorithm to speedup MC simulations. Its efficiency has been demonstrated in various systems by introducing an effective model to propose global moves in the configuration space. In this paper, we show that deep neural networks can be naturally incorporated into SLMC, and without any prior knowledge can learn the original model accurately and efficiently. Demonstrated in quantum impurity models, we reduce the complexity for a local update from $O\left({\beta }^2\right)$ in Hirsch-Fye algorithm to $O(\beta ln\beta )$, which is a significant speedup especially for systems at low temperatures.

We show that the quantum critical point (QCP) between a diffusive metal and ferromagnetic (or antiferromagnetic) phases in the SYK chain has a gravitational description corresponding to the double-trace deformation in an ${\mathrm{AdS}}_2$ chain. Specifically, by studying a double-trace deformation of a $Z_2$ scalar in an ${\mathrm{AdS}}_2$ chain where the $Z_2$ scalar is dual to the order parameter in the SYK chain, we find that the susceptibility and renormalization group equation describing the QCP in the SYK chain can be exactly reproduced in the holographic model. Our results suggest that the infrared geometry in the gravity theory dual to the diffusive metal of the SYK chain is also an ${\mathrm{AdS}}_2$ chain. We further show that the transition in SYK model captures universal information about double-trace deformation in generic black holes with near horizon ${\mathrm{AdS}}_2$ space-time.

Mechanical resonators are a promising way for interfacing qubits in order to realize hybrid quantum systems that offer great possibilities for applications. Mechanical systems can have very long energy lifetimes, and they can be further interfaced to other systems. Moreover, integration of a mechanical oscillator with qubits creates a potential platform for the exploration of quantum physics in macroscopic mechanical degrees of freedom. The utilization of high overtone bulk acoustic resonators coupled to superconducting qubits is an intriguing platform towards these goals. These resonators exhibit a combination of high-frequency and high-quality factors. They can reach their quantum ground state at dilution refrigeration temperatures and they can be strongly coupled to superconducting qubits via their piezoelectric effect. In this paper, we demonstrate our system where bulk acoustic phonons of a high overtone resonator are coupled to a transmon qubit in a planar circuit architecture. We show that the bulk acoustic phonons are interacting with the qubit in a simple design architecture at the quantum level, representing further progress towards the quantum control of mechanical motion.

In a recent paper [Phys. Rev. B 89, 155403 (2014)], the authors investigated the spectrum of a Coulomb impurity in graphene in the presence of magnetic and electric fields using the coupled series expansion approach. In the first part of their paper, they investigated how Coulomb impurity states collapse in the presence of a perpendicular magnetic field. We argue that the obtained spectrum does not give information about the atomic collapse and that their interpretation of the spectrum regarding atomic collapse is not correct. We also argue that the obtained results are only valid up to the dimensionless charge $\left|\alpha \right|=0.5$ and, to obtain correct results for $\alpha >0.5$, a proper regularization of the Coulomb interaction is required. Here we present the correct numerical results for the spectrum for arbitrary values of $\alpha$.

In this reply to R. Van Pottelberge, M. Zarenia, and F. M. Peeters's comment on our original paper, “Impurity spectra of graphene under electric and magnetic fields,” we discuss the existence of bound states of a Coulomb impurity in graphene through the regular and irregular solution of the Dirac equation, and clarify its relation to the term “atom collapse” in our original paper.

We investigated the vibrational property of lead telluride (PbTe) with a focus on lattice anharmonicity at moderate temperatures $(300<T<800$ K) using the phonon quasiparticle approach which combines first-principles molecular dynamics and lattice dynamics. The calculated anharmonic phonon dispersions are strongly temperature dependent and some phonon modes adopt giant frequency shifts, e.g., transverse optical modes in the long-wavlength regime. As a result, we witness the avoided crossing between transverse optical modes and longitudinal acoustic modes at elevated temperature, in good agreement with experimentation and available theoretical studies. These results, together with the large root-mean-square displacements of atoms, reveal a strong anharmonic effect in PbTe. The obtained phonon lifetimes allow studies of transport properties. For considered temperatures, the phonon mean free paths can be shorter than lattice constants at relatively high temperature, especially for optical modes. This finding goes against the widely employed minimal phonon mean free path concept. As such, the calculated lattice thermal conductivity of PbTe, which is indeed relatively small, does not have the prescribed minima at high temperature, showcasing the breakdown of the minimal mean free path theory. Our study provides a basis for delineating vibrational and transport properties of PbTe and other thermoelectric materials within the framework of the phonon gas model.

It is demonstrated that thermally induced atomic displacements from ideal lattice positions can produce considerable effect on magnetic exchange interactions and, consequently, on the Curie temperature of Fe. Thermal lattice distortion should, therefore, be accounted for in quantitatively accurate theoretical modeling of the magnetic phase transition. At the same time, this effect seems to be not very important for magnetic exchange interactions and the Curie temperature of Co and Ni.

Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI's exotic potential. Recently, we reported the doping of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ remained elusive. Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.

A continuum density field formulation with particle-scale resolution is constructed to simultaneously incorporate the orientation dependence of interparticle interactions and the rotational invariance of the system, a fundamental but challenging issue in modeling the structure and dynamics of a broad range of material systems across variable scales. This generalized phase field crystal-type approach is based upon the complete expansion of particle direct correlation functions and the concept of isotropic tensors. Through applications to the modeling of various two- and three-dimensional crystalline structures, our study demonstrates the capability of bond-angle control in this continuum field theory and its effects on the emergence of ordered phases, and provides a systematic way of performing tunable angle analyses for crystalline microstructures.

The effective screening of the polarization bound charge is a prerequisite to stabilize the ferroelectricity in ferroelectric thin films. Here, by combining annular bright field imaging and electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope with phase-field simulations, we investigate the screening mechanism by quantitatively measuring the structural relaxation at $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}\right){\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interfaces. We find that the thickness of the interfacial layer is $\sim 3.5$ unit cells ($\sim 1.4$ nm) in a domain with upward polarization and $\sim 5.5$ unit cells ($\sim 2.2$ nm) in a domain with downward polarization. Phase-field simulations, an EELS analysis, and a lattice parameter analysis verify the existence of interfacial oxygen vacancies accounting for the narrower interfacial layer in the domain with upward polarization. Our study indicates the internal structural relaxation at the interface is the dominant mechanism for the polarization charge screening for ferroelectric films grown on insulating substrates and has implications for our understanding of domain switching in ferroelectric devices.

Josephson junctions made from aluminum and its oxide are the most commonly used functional elements for superconducting circuits and qubits. It is generally known that the disordered thin film ${\mathrm{AlO}}_x$ contains atomic tunneling systems. Coherent tunneling systems may couple strongly to a qubit via their electric dipole moment, giving rise to spectral level repulsion. In addition, slowly fluctuating tunneling systems are observable when they are located close to coherent ones and distort their potentials. This interaction causes telegraphic switching of the coherent tunneling systems' energy splitting. Here, we measure such switching induced by individual fluctuators on timescales from hours to minutes using a superconducting qubit as a detector. Moreover, we extend the range of measurable switching times to millisecond scales by employing a highly sensitive single-photon qubit swap spectroscopy and statistical analysis of the measured qubit states.