There is a close connection between various new phenomena in Weyl semimetals and the existence of linear band crossings in the single particle description. We show, by a full self-consistent mean-field calculation, how this picture is modified in the presence of long-range Coulomb interactions. The chiral symmetry breaking occurs at strong enough interactions and the internode interband excitonic pairing channel is found to be significant, which determines the gap-opened band profile varying with interaction strength. Remarkably, in the resultant interacting phase, finite band Chern number jumps in the three-dimensional momentum space are retained, indicating the robustness of the topologically nontrivial features.

A number of recent experiments have highlighted a remarkable transformation of a large cuprate Fermi surface into small pockets in the underdoped region signaling a breakdown of a conventional Fermi liquid theory in the pseudogap phase. A few phenomenological models have been recently put forward to account for this transformation. However, none of those models have been derived microscopically or are totally compatible with experimental data. In the present work we show that the observed Fermi-surface reconstruction can be accounted for directly within a standard microscopic $t$–$J$ model of correlated electrons, provided strong electron correlations are properly taken into account.

In this paper we investigate the nature of quantum phase transitions between two-dimensional Dirac semimetals and $Z_3$-ordered phases (e.g., Kekule valence-bond solid), where cubic terms of the order parameter are allowed in the quantum Landau-Ginzberg theory and the transitions are putatively first order. From large-$N$ renormalization-group (RG) analysis, we find that fermion-induced quantum critical points (FIQCPs) [Z.-X. Li et al., Nat. Commun. 8, 314 (2017)] occur when $N$ (the number of flavors of four-component Dirac fermions) is larger than a critical value $N_c$. Remarkably, from the knowledge of space-time supersymmetry, we obtain an exact lower bound for $N_c$, i.e., $N_c>1/2$. (Here the “1/2” flavor of four-component Dirac fermions is equivalent to one flavor of four-component Majorana fermions). Moreover, we show that the emergence of two length scales is a typical phenomenon of FIQCPs and obtain two different critical exponents, i.e., $\nu \ne {\nu }^{\prime }$, by large-$N$ RG calculations. We further give a brief discussion of possible experimental realizations of FIQCPs.

The energy and charge fluxes carried by electrons in a two-terminal junction subjected to a random telegraph noise, produced by a single electronic defect, are analyzed. The telegraph processes are imitated by the action of a stochastic electric field that acts on the electrons in the junction. Upon averaging over all random events of the telegraph process, it is found that this electric field supplies, on the average, energy to the electronic reservoirs, which is distributed unequally between them: the stronger is the coupling of the reservoir with the junction, the more energy it gains. Thus the noisy environment can lead to a temperature gradient across an unbiased junction.

We derive force field potentials for CdS, CdSe, CdTe, ZnS, ZnSe, and ZnTe using the vibrational and elastic properties of the bulk materials based on density functional theory calculations and experiment. The potentials have bond-stretching (first- and second-nearest neighbor), bond-bending, as well as long-range Coulomb interactions. The bulk vibrational properties are very well reproduced for all materials considered. The vibrational properties of nanoclusters are compared to the results of large-scale density functional theory calculations with up to 1000 atoms. We obtain a very good agreement for all the core vibrational modes and an overall reasonable agreement over the entire spectrum. However, the low-frequency acoustic-type modes with surface character tend to be too soft in the force field calculations. We demonstrate the capability of the force field to obtain vibrational properties of realistic colloidal quantum dots with sizes up to 60 Å diameter.

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Although much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density-functional theory, to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.

Using the hydrodynamic model in the electrostatic approximation, we describe the formation of graphene surface plasmons when a nearby charge is in motion either perpendicular or parallel to a graphene sheet. In the first case, the electron-energy loss (EEL) spectrum of the electron is computed, showing that the resonances in the spectrum are linked to the frequency of the graphene surface plasmons. In the second case, we discuss the formation of plasmonic wakes due to the dragging of the surface plasmons induced by the motion of the charge. This effect is similar to Coulomb drag between two electron gases at a distance from each other. We derive simple expressions for the electrostatic potential induced by the moving charge on graphene. We show that there is a transition from a Mach-type wake at high speeds to a Kelvin-type wake at low ones and identify the Froude number for plasmonic wakes. We show that the Froude number can be controlled externally by tunning both the Fermi energy in graphene and the dielectric function of the environment, a situation with no parallel in ship wakes. Using EEL, we propose a source of graphene plasmons, based on a graphene drum built in a metallic waveguide and activated by an electron beam created by the tip of an electronic microscope. We also introduce the notion of a plasmonic billiard.

In graphene and other massless two-dimensional Dirac materials, Klein tunneling compromises electron confinement, and momentum-space contours can be assigned a Berry phase which is either zero or π. Consequently, in such systems the energy spectrum of circular potential wells exhibits an interesting discontinuity as a function of magnetic field $B$: for a given angular momentum the ladder of eigenresonances is split at an energy-dependent critical field $B_{\mathrm{c}}$. Here we show that introducing a mass term $\mathrm{\Delta }$ in the Hamiltonian bridges this discontinuity in such a way that states below $B_{\mathrm{c}}$ are adiabatically connected to states above $B_{\mathrm{c}}$ whose principal quantum number differs by unity depending on the sign of $\mathrm{\Delta }$. In the $B\text{−}\mathrm{\Delta }$ plane, the spectrum of these circular resonators resembles a spiral staircase, in which a particle prepared in the $|n,m\rangle$ resonance state can be promoted to the $|n\pm 1,m\rangle$ state by an adiabatic circuit of the Hamiltonian about $B_{\mathrm{c}}$, the sign depending on the direction of the circuit. We explain the phenomenon in terms of the evolving Berry phase of the orbit, which in such a circuit changes adiabatically by $2\pi$.

Unusual physical properties like large magnetoresistance (MR) and superconductivity occurring in semimetals with Dirac or Weyl points are often linked to their topologically nontrivial band structures. However, there is an increasing number of reports on semimetals that show large MR in the absence of Dirac or Weyl points. Herein we report an experimental and theoretical study on the layered transition-metal dichalcogenide (TMDC) $\mathrm{HfT}{\mathrm{e}}_2$ that shows a large MR of $1350\%$ at $T=2$ K and ${\mu }_{0}H=9\mathrm{T}$ in the absence of Dirac or Weyl points. Moreover, the structure and electrical resistivity under pressure reveal a unique structural transition. These results clearly distinguish $\mathrm{HfT}{\mathrm{e}}_2$ from TMDCs like $\mathrm{MoT}{\mathrm{e}}_2$ or $\mathrm{WT}{\mathrm{e}}_2$ which both exhibit larger MR and are viewed as Weyl semimetals. $\mathrm{HfT}{\mathrm{e}}_2$ is an appealing platform for future investigations on the interplay of particular band-structure features and their connection to emerging physical properties.

We propose a novel theoretical scheme of quantum computation. Nuclear spin pairs are utilized to encode decoherence-free (DF) qubits. A nitrogen-vacancy center serves as a quantum actuator to initialize, readout, and quantum control the DF qubits. The realization of CNOT gates between two DF qubits are also presented. Numerical simulations show high fidelities of all these processes. Additionally, we discuss the potential of scalability. Our scheme reduces the challenge of classical interfaces from controlling and observing complex quantum systems down to a simple quantum actuator. It also provides a novel way to handle complex quantum systems.

Recent numerical studies have provided strong evidence for a gapped $Z_2$ quantum spin liquid in the kagome lattice spin-1/2 Heisenberg model. A special feature of spin liquids is that symmetries can be fractionalized, and different patterns of symmetry fractionalization imply distinct phases. The symmetry fractionalization pattern for the kagome spin liquid remains to be determined. A popular approach to studying spin liquids is to decompose the physical spin into partons obeying either Bose (Schwinger bosons) or Fermi (Abrikosov fermions) statistics, which are then treated within mean-field theory. A long-standing question has been whether these two approaches are truly distinct, or describe the same phase in complementary ways. Here we show that all $8Z_2$ spin liquid phases in the Schwinger-boson mean-field (SBMF) construction can also be described in terms of Abrikosov fermions, unifying pairs of theories that seem rather distinct. The key idea is that for $Z_2$ spin liquid states that admit a SBMF description on the kagome lattice, the symmetry fractionalization of visions is uniquely fixed. Two promising candidate states for the kagome Heisenberg model, Sachdev's $Q_1=Q_2$ SBMF state and the Lu-Ran-Lee $Z_2[0,\pi ]\beta$ Abrikosov-fermion state, are found to describe the same symmetric spin liquid phase. We expect these results to aid in a complete specification of the numerically observed spin liquid phase. We also discuss a set of $Z_2$ spin liquid phases in the fermionic parton approach, where spin rotation and lattice symmetries protect gapless edge states that do not admit a SBMF description.

Klein tunneling, the perfect transmission of normally incident Dirac electrons across a potential barrier, has been widely studied in graphene and explored to design switches, albeit indirectly. We show an alternative way to directly measure Klein tunneling for spin-momentum locked electrons crossing a PN junction along a three-dimensional topological insulator surface. In these topological insulator PN junctions (TIPNJs), the spin texture and momentum distribution of transmitted electrons can be measured electrically using a ferromagnetic probe for varying gate voltages and angles of current injection. Based on transport models across a TIPNJ, we show that the asymmetry in the potentiometric signal between PP and PN junctions and its overall angular dependence serve as a direct signature of Klein tunneling.

We study the transport properties of an arbitrary two-terminal Hermitian system within a tight-binding approximation and derive an expression for the transparency in a form that enables one to determine the exact energies of the perfect (unity) transmittance, zero transmittance (Fano resonance), and bound state in the continuum (BIC). These energies correspond to the real roots of two energy-dependent functions that are obtained from two non-Hermitian Hamiltonians: the Feshbach effective Hamiltonian and the auxiliary Hamiltonian, which can be easily deduced from the effective one. BICs and scattering states are deeply interconnected. We show that the transformation of a scattering state into a BIC can be formally described as a “phase transition” with a divergent generalized response function. Design rules for quantum conductors and waveguides are presented. These rules describe the structures exhibiting coalescence of both resonances and antiresonances resulting in the formation of almost rectangular transparency and reflection windows. The results can find applications in the construction of molecular conductors, broad-band filters, quantum heat engines, and waveguides with controllable BIC formation.

We study a one-dimensional chain of $2N$ Majorana bound states, which interact through a local quartic interaction. This model describes for example the edge physics of a quasi-one-dimensional (1D) stack of $2N$ Kitaev chains with modified time-reversal symmetry $T{\gamma }_i{T}^{-1}={\gamma }_i$, which precludes the presence of quadratic coupling. The ground state of our 1D Majorana chain displays a fourfold periodicity in $N$, corresponding to the four distinct topological classes of the stacked Kitaev chains. We analyze the transport properties of the 1D Majorana chain, when probed by local conductors located at its ends. We find that for finite but large $N$, the scattering matrix partially reflects the fourfold periodicity, and the chain exhibits strikingly different transport properties for different chain lengths. In the thermodynamic limit, the 1D Majorana chain hosts a robust many-body zero mode, which indicates that the corresponding stacked two-dimensional bulk system realizes a weak topological phase.

We define a structural parameter, called atomic bond length deviation ($\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$), to characterize structural heterogeneity of CuZr melt and metallic glass (MG). Molecular dynamics simulations have been performed to explore the average $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ of the system evolution with temperature during $\mathrm{C}{\mathrm{u}}_{64}\mathrm{Z}{\mathrm{r}}_{36}$ and $\mathrm{C}{\mathrm{u}}_{50}\mathrm{Z}{\mathrm{r}}_{50}$ MGs formation and the correlation between $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ and thermal relaxation/local atomic shear strain upon compressive loading. The results indicate that $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ contains both symmetrical characteristic and volumetric information of the short-range order clusters while symmetry seems to play a more important role in relaxation and deformation events; the fast decreasing of average $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ near above the glass transition temperature $T_{\mathrm{g}}$ with decreasing temperature corresponds to the sharp increase of the number of full icosahedra while the shear transformation zones or single jump events have a high propensity to originate from those regions with the higher $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ clusters. Additionally, the system average $\mathrm{BL}{\mathrm{D}}_{\mathrm{i}}$ can also be accessed experimentally, through the radial distribution function.

From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available high-frequency dielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a full picture of the soft- and central-mode behavior in the classical relaxor ferroelectric $\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3$ (PMN) is suggested. A detailed comparison is made with the recent hyper-Raman spectroscopy data [Hehlen et al., Phys. Rev. Lett. 117, 155501 (2016)] and also with other available experiments based on inelastic light and neutron scattering. It is revealed that each type of experiment provides slightly different data. The closest agreement is with the hyper-Raman data: both techniques yield the same number of soft-mode components and the same high-temperature softening towards the temperature $T^{*}\approx 400\mathrm{K}$. In addition to evaluation of the IR-terahertz data using fitting with the standard factorized form of the dielectric function, we performed a successful fitting of the same data using the effective medium approach (EMA), originally based on the assumption that the mesoscopic structure of PMN consists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhat harder transverse optical polar modes in the direction along the local PND dipole [Hlinka et al., Phys. Rev. Lett. 96, 027601 (2006)]. Evaluation using Bruggeman EMA modeling has been successfully applied in the entire investigated temperature range. These results suggest that the response perpendicular to the local dipole moment, at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from high temperatures with permittivity obeying the Curie-Weiss law, ${\varepsilon }_{\perp }=C/(T-T_C)$, where $C=1.7\times {10}^5\mathrm{K}$ and $T_C=380\mathrm{K}$, whereas the response parallel to it shows no softening. Below the Burns temperature, $\sim 620\mathrm{K}$, a gigahertz relaxation ascribed to flipping of the PNDs emerges from the soft-mode response, slows down, and broadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations of the PND boundaries.

We report the results of ambient temperature high-pressure synchrotron-based x-ray diffraction, Raman, and electrical resistance study of $\mathrm{Cs}{\mathrm{I}}_3$ up to 29, 25, and 8 GPa, respectively, and confirm three-phase transitions under quasihydrostatic conditions. The ambient orthorhombic (space group (SG): Pnma) phase of $\mathrm{Cs}{\mathrm{I}}_3$ is stable up to a pressure of $\sim 1.3\mathrm{GPa}$, above which a phase transition to a trigonal ($\mathrm{SG}:P\text{−}3c1$) phase is observed. The stability region of the trigonal phase has been found to be up to 22.6 GPa, above which the trigonal phase transforms to a cubic ($\mathrm{SG}:Pm\text{−}3n$) phase which remains stable until the maximum pressure of 29 GPa achieved in this study. A third-order Birch-Murnaghan equation of state fit to the pressure volume ($P\text{−}V$) data yields a bulk modulus of 17.7(9) GPa for the trigonal phase. Raman spectroscopic measurements however indicate three-phase transitions at $\sim 1.3,4.0$, and 22.6 GPa, respectively. The electrical resistance measured in the low-pressure region up to 8 GPa indicates an electronic transition at around 4 GPa confirming the Raman result observed at 4.0 GPa. The $P\text{−}V$ data when transformed to the universal equation of state (UEOS) show a deviation from linearity around 4.0 GPa confirming the electronic transition. The present study has thus revealed a three-phase structural sequence in alkali trihalides, viz., orthorhombic (SG: Pnma) to trigonal ($\mathrm{SG}:P\text{−}3c1$) to a cubic ($\mathrm{SG}:Pm\text{−}3n$) phase.