Using the Büttiker-Landauer formulation of transport theory in the linear response regime, the valley currents and nonlocal resistances of bilayer graphene nanostructures with broken inversion symmetry are calculated. It is shown that broken inversion symmetry in bilayer graphene nanostructures leads to striking enhancement of the nonlocal four-terminal resistance and to valley currents several times stronger than the conventional electric current when the Fermi energy is in the spectral gap close to the energy of Dirac point. The scaling relation between local and nonlocal resistances is investigated as the gate voltage varies at zero Fermi energy and a power law is found to be satisfied. The valley velocity field and valley accumulation in four-terminal bilayer graphene nanostructures are evaluated in the presence of inversion symmetry breaking. The valley velocity and nonlocal resistance are found to scale differently with the applied gate voltage. The unit cell-averaged valley accumulation is found to exhibit a dipolar spatial distribution consistent with the accumulation arising from the valley currents. We define and calculate a valley capacitance that characterizes the valley accumulation response to voltages applied to the nanostructure's contacts.

We study the quantum dynamics of Majorana and regular fermion bound states coupled to a quasi-one-dimensional metallic lead. The dynamics following the quench in the coupling to the lead exhibits a series of dynamical revivals as the bound state propagates in the lead and reflects from the boundaries. We show that the nature of revivals for a single Majorana bound state depends uniquely on the presence of a resonant level in the lead. When two spatially separated Majorana modes are coupled to the lead, the revivals depend only on the phase difference between their host superconductors. Remarkably, the quench in this case effectively performs a fermion-parity interferometry between Majorana bound states, revealing their unique non-Abelian braiding. Using both analytical and numerical techniques, we find the pattern of fermion parity transfers following the quench, study its evolution in the presence of disorder and interactions, and thus, ascertain the fate of Majorana bound states in a Fermi sea.

The desire to produce high-quality single photons for applications in quantum information science has lead to renewed interest in exploring solid-state emitters in the weak excitation regime. Under these conditions it is expected that photons are coherently scattered, and so benefit from a substantial suppression of detrimental interactions between the source and its surrounding environment. Nevertheless, we demonstrate here that this reasoning is incomplete, as phonon interactions continue to play a crucial role in determining solid-state emission characteristics even for very weak excitation. We find that the sideband resulting from non-Markovian relaxation of the phonon environment is excitation strength independent. It thus leads to an intrinsic limit to the fraction of coherently scattered light and to the visibility of two-photon coalescence at weak driving, both of which are absent for atomic systems or within simpler Markovian treatments.

Among the known transition metal dichalcogenides, monolayer ${\mathrm{VS}}_2$ has attracted particular attention because of its intrinsic ferromagnetism and great application potential as a high-performance functional nanomaterial. Here, using first-principles calculations, we study the structural and electronic phase transitions in monolayer ${\mathrm{VS}}_2$ induced by charge doping. We show that without electron or hole doping, ${\mathrm{VS}}_2$ stabilizes in the 2H phase and is a bipolar magnetic semiconductor (BMS) whose valence and conduction states near the Fermi level carry opposite spin polarization. With the increase of hole doping concentration, ${\mathrm{VS}}_2$ will first experience an electronic phase transition from a BMS to a half metal, followed by a 2H-to-1T structural phase transition (SPT) which concomitantly results in another electronic phase transition from the half metal to a normal metal. Moreover, the reduced reaction barrier from the 2H to 1T phases by hole doping can make the occurrence of the SPT easier. However, electron doping can only induce the BMS-to-half metal electronic phase transition but will not trigger the SPT within the experimentally accessible doping regime. The different effects of hole and electron dopings on the SPT are further explained by the energy band diagram of ${\mathrm{VS}}_2$. These results establish the potential for ${\mathrm{VS}}_2$ utilization in innovative phase-change electronic and spintronic devices.

We study a new scheme to realize a quantized two-parameter charge pump based on massive Dirac electrons. It is shown that the two time-dependent and out-of-phase staggered potentials introduced in graphene can pump out an integer number of electrons in a pumping cycle as long as the Fermi energy resides in the effective energy gap opened by pumping potentials. The dependence of the pumped charge per mode on the pumping phase or the dynamic phase exhibits a binary alternation from $+e$ to $-e$. This quantization has a topological origin and can be accounted for by adiabatic evolution of the topologically protected interfacial state forming between the two pumping sources.

Tunneling of the spins in the ${\mathrm{Fe}}_8$ molecular magnet from a metastable ground state to an excited state is accompanied by a decay of these spins to the global ground state and an increase of the crystal temperature. We measured this temperature using two thermometers, one strongly coupled and the other weakly coupled to the thermal bath. We found that the temperature increases to no greater than 2.2 K. This upper limit agrees with the flame temperature derived from deflagration theory and previous measurements. In light of this temperature increase we re-examine the Landau, Zener, and Stuckelberg (LZS) theory of spin tunneling in large ${\mathrm{Fe}}_8$ crystals.

We propose a novel scanning tunneling microscope (STM) device in which the tunneling tip is formed by a Majorana bound state (MBS). This peculiar bound state emerges at the boundary of a one-dimensional topological superconductor. Since the MBS has to be effectively spinless and local, we argue that it is the smallest unit that shows itself the properties of odd-frequency superconducting pairing. Odd-frequency superconductivity is characterized by an anomalous Green's function, which is an odd function of the time arguments of the two electrons building the Cooper pair. Interestingly, our Majorana STM can be used as the perfect detector of odd-frequency superconductivity. The reason is that a supercurrent between the Majorana STM and any other superconductor can only flow if the latter system exhibits itself odd-frequency pairing. To illustrate our general idea, we consider the tunneling problem of the Majorana STM coupled to a quantum dot placed on a surface of a conventional superconductor.

Nonuniform strain distributions in a graphene lattice can give rise to uniform pseudomagnetic fields and associated pseudo-Landau levels without breaking time-reversal symmetry. We demonstrate that by inducing superconductivity in a nonuniformly strained graphene sheet, the lowest pseudo-Landau levels split by a pairing gap can be inverted by changing the sign of the pairing potential. As a consequence of this inversion, we predict that a Josephson $\pi$ junction deposited on top of a strained graphene sheet exhibits one-dimensional gapless modes propagating along the junction. These gapless modes mediate single electron tunneling across the junction, giving rise to the $4\pi$-periodic fractional Josephson effect.

We study Josephson currents between $s$-wave/spin-triplet superconductor junctions by taking into account details of the band structures in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, such as three conduction bands and spin-orbit interactions in the bulk and at the interface. We assume five superconducting order parameters in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$: a chiral $p$-wave symmetry and four helical $p$-wave symmetries. We calculate the current-phase relationship $I\left(\phi \right)$ in these junctions, where $\phi$ is the macroscopic phase difference between the two superconductors. The results for a chiral $p$-wave pairing symmetry show that a $cos\left(\phi \right)$ term appears in the current-phase relation because of time-reversal symmetry (TRS) breaking. On the other hand, this $cos\left(\phi \right)$ term is absent in the helical pairing states that preserve TRS. We also study the dependence of the maximum Josephson current $I_c$ on an external magnetic flux $\mathrm{\Phi }$ in a corner junction. The calculated $I_c\left(\mathrm{\Phi }\right)$ obeys $I_c\left(\mathrm{\Phi }\right)\ne I_c(-\mathrm{\Phi })$ in a chiral state and $I_c\left(\mathrm{\Phi }\right)=I_c(-\mathrm{\Phi })$ in a helical state. We calculate $I_c\left(\mathrm{\Phi }\right)$ in a corner superconducting quantum interference device (SQUID) and a symmetric SQUID geometry. In the latter geometry, $I_c\left(\mathrm{\Phi }\right)=I_c(-\mathrm{\Phi })$ is satisfied for all the pairing states and it is impossible to distinguish a chiral state from a helical one. On the other hand, a corner SQUID always gives $I_c\left(\mathrm{\Phi }\right)\ne I_c(-\mathrm{\Phi })$ and $I_c\left(\mathrm{\Phi }\right)=I_c(-\mathrm{\Phi })$ for a chiral and a helical state, respectively. Experimental tests of these relations in corner junctions and SQUIDs may serve as a tool for unambiguously determining the pairing symmetry in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Motivated by the successful idea of using weakly coupled quantum electronic wires to realize the quantum Hall effects and the quantum spin Hall effects, we theoretically study two systems composed of weakly coupled quantum spin chains within the mean-field approximations, which can exhibit spin analogs of superconductivity and the integer quantum Hall effect. First, a certain bilayer of two arrays of interacting spin chains is mapped, via the Jordan-Wigner transformation, to an attractive Hubbard model that exhibits fermionic superconductivity, which corresponds to spin superconductivity in the original spin Hamiltonian. Secondly, an array of spin-orbit-coupled spin chains in the presence of a suitable external magnetic field is transformed to an array of quantum wires that exhibits the integer quantum Hall effect, which translates into its spin analog in the spin Hamiltonian. The resultant spin superconductivity and spin integer quantum Hall effect can be characterized by their ability to transport spin without any resistance.

Classical molecular dynamics calculations were used to investigate the formation of defects produced during irradiation of energetic ions on silicon. The aim of this study was to characterize the nature of defects and defective regions formed through ion irradiation and to establish a connection between the ion irradiation parameters, lattice defect configurations, and the resulting modified lattice thermal conductivity of silicon. The defective regions were characterized according to the total number of defects generated, the size and the density of the defective region, and the longitudinal and radial distribution of defects along the ion impact path. In addition, the clustering of the defects into amorphous pockets is analyzed and the effect of these processing parameters on the properties of the clusters is also studied. Further, the lattice defect configurations produced during continuous bombardment of multiple ions are directly investigated and compared to the single-ion impact results. A range of irradiation parameters including ion species, ion energies, fluence, and beam width have been explored to elucidate the dependence of the resulting defect configurations on these experimental design parameters. High density defective regions are found to be produced by low-energy ions with high atomic number. Analysis of the defects produced under varying beam diameters indicates that the beam diameter, rather than the beam energy, is the more prominent factor in determining the extent of the defective region. We demonstrate that the thermal conductivity of the material is most significantly influenced by the effective diameter of the defective region, making the beam diameter the most influential experimental parameter for tuning the lattice thermal conductivity. A reduction in thermal conductivity of up to 80% from pristine silicon was achieved with the processing parameters used in this work. This study indicates that ion beam irradiation can be a realizable manufacturing process with high tunability and control to achieve desired material properties.

We analyze the effect of magnetic fluctuations in superconductors with strong spin-orbit coupling and show that they drive a phase transition between two superconducting states: a conventional phase with zero center-of-mass momentum of Cooper pairs, and an exotic phase with nonzero pair momentum. The latter is found to exhibit persistent currents without magnetic field in doubly connected geometries such as rings. Surprisingly, the transition temperature into the superconducting state can be increased by applying a Zeeman magnetic field.

Magnetic properties of metals are investigated through electronic structure calculations based on the recently proposed magnetic-field-containing relativistic tight-binding approximation (MFRTB) method [Phys. Rev. B 91, 075122 (2015)]. It is found that electronic energy bands for a metal immersed in a uniform magnetic field have a cluster structure in which multiple energy bands lie within a small energy width. Each cluster corresponds to an energy level that is derived on the basis of the semiclassical approximation. While the cluster is responsible for the de Haas–van Alphen (dHvA) oscillations, constituent energy bands of the cluster cause additional oscillation peaks of the magnetization that are not explained by the conventional Lifshitz-Kosevich formula. Also, the energy width of the cluster leads to the reduction of the amplitude of the dHvA oscillations, which can be observed as the pseudo Dingle temperature and/or the overestimation of the curvature of the Fermi surface.

Projected entangled pair states (PEPS) methods have been proven to be powerful tools to solve strongly correlated quantum many-body problems in two dimensions. However, due to the high computational scaling with the virtual bond dimension $D$, in a practical application, PEPS are often limited to rather small bond dimensions, which may not be large enough for some highly entangled systems, for instance, frustrated systems. Optimization of the ground state using the imaginary time evolution method with a simple update scheme may go to a larger bond dimension. However, the accuracy of the rough approximation to the environment of the local tensors is questionable. Here, we demonstrate that by combining the imaginary time evolution method with a simple update, Monte Carlo sampling techniques and gradient optimization will offer an efficient method to calculate the PEPS ground state. By taking advantage of massive parallel computing, we can study quantum systems with larger bond dimensions up to $D=10$ without resorting to any symmetry. Benchmark tests of the method on the $J_1-J_2$ model give impressive accuracy compared with exact results.

The adsorption of hydrogen at nonpolar $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of ${\mathrm{H}}_2$ and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the $\mathrm{GaN}\left(1\overline{1}00\right)$ surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga–N surface dimer buckling. The signatures of the Ga–H and N–H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean $\mathrm{GaN}\left(1\overline{1}00\right)$ surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion.

Based on the nonequilibrium Green's function formalism, we show a numerically efficient method to treat inelastic scattering in multidimensional atomistic codes. Using a simple rescaling approach, we detail the calculations of the lowest-order approximation (LOA) [Y. Lee et al., Phys. Rev. B 93, 205411 (2016)] series to the usual, computationally intensive, self-consistent Born approximation (SCBA). This, combined with the analytic continuation technique of Padé approximants, is applied to an atomistic code based on a tight-binding $s{p}^3{d}^5{s}^{*}$ model for electrons and holes, and a modified valence-force-field method for phonons. Currents in Si and Ge gate-all-around nanowire transistors are then computed considering the main crystallographic transport directions ($\langle 100\rangle$, $\langle 110\rangle$, $\langle 111\rangle$) for both $n$-type and $p$-type devices. Our results show that in most configurations, third-order LOA currents are enough to achieve a high agreement with SCBA results, while reducing the calculation time by about one order. In addition, we propose a criterion to determine the validity of such expansion techniques.

Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5–295 K under pressures to 210 GPa. The magnetic ordering temperature $T_{\text{o}}$ of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking $\mathrm{\Delta }{T}_{\text{c}}$ as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30–40 GPa, the exchange coupling $J$ between Nd ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. The rise and fall of $T_{\text{o}}$ and $\mathrm{\Delta }{T}_{\text{c}}$ with pressure can be accounted for in terms of an increase in the Kondo temperature.

We demonstrate the efficient generation of second harmonic light in zinc oxide (ZnO) by utilizing double resonant plasmonic nanoantenna arrays. The antenna design is based on two gold dipole rods with plasmonic resonances at $\omega$ and $2\omega$, enabling strong localization of light at the fundamental frequency $\omega$ within the ZnO, as well as improved reemission of the second harmonic generation (SHG) at $2\omega$ into the far field. Wavelength-dependent SHG measurements show that the intensity of the far-field signal strongly depends on the properties of the ZnO substrate: While a bulk-ZnO substrate causes the SHG signal emitted from the nanoantennas to decrease, a thin-film ZnO substrate provides a strongly enhanced signal from the double resonant antennas. Comparing the wavelength-dependent results from the double resonant antennas with single dipole rods, the enhancement of the SHG intensity is more than twofold. Our experimental results confirm theoretical calculations of the SHG obtained from the double resonant antenna arrays.