We investigate the amplitude and phase effects of qubit dynamics and excited-state population under the influence of a biharmonic control field. It is demonstrated that the biharmonic driving field can have a significant effect on the behavior of quasienergy level crossing as well as on multiphoton transitions. Also, the interference pattern for the populations of qubit excited states is sensitive to the signal parameters. We discuss the possibility of using these effects for manipulating qubit states and calibrating nanosecond pulses.

Optical properties from 0.2 to 6.5 eV of epitaxial ${\mathrm{NbO}}_2$ $(4d^1 \text{system})$ films in their insulating states have been investigated by spectroscopic ellipsometry. The optical spectra are compared to that of epitaxial ${\mathrm{VO}}_2$ $(3d^1 \text{system})$ in its insulating phase. Both compounds are insulators at room temperature and undergo temperature-induced metal-insulator transitions. We find a $d_{\left|\right|}-e_{\mathrm{g}}^{\pi }$ orbital splitting energy of $\sim 1.6$ eV in ${\mathrm{NbO}}_2$ compared to $\sim 1.3$ eV in ${\mathrm{VO}}_2$; orbital splitting is a key ingredient that stabilizes the cation-dimerized insulating states of both materials. The edge of the $\text{O}2p$-like valence band is estimated to be 3.2 eV below the Fermi level through x-ray photoelectron spectroscopy measurements, suggesting that electrons from $\text{O}2p$ states do not contribute to absorption below this energy. This allows us to also assign an observed optical peak at $\sim 3.0$ eV to the $\mathrm{Nb}4d_{\left|\right|}\to 4d_{\left|\right|}^{*}$ transition.

We investigate the topological transition in metamaterials considering a structure composed of the uniaxial electric scatterers. Our study is based on the discrete dipole model, which allows us to take into account the effects of spatial dispersion in the structure. The nonlocal effects lead to the existence of the transition region between elliptic and hyperbolic dispersion regimes. The transition region corresponds to the “mixed” regime when quasielliptic and quasihyperbolic isofrequency contours are present simultaneously. In the present work, we show that this regime may occur when the effective permittivity of the structure calculated by the Clausius-Mossotti formula approaches either zero or infinity. We determine the boundaries between the different dispersion regimes for uniaxial metamaterials composed of electric scatterers and perform a detailed study of the metamaterial dispersion properties in the mixed regime. Possible verification of the mixed regime existence is proposed.

The doped Hubbard model is a simple model for high-$T_c$ cuprate superconductors, while its ground state remains a challenge. Here, by performing state-of-the-art variational Monte Carlo calculations for the strong-coupling Hubbard model, we find evidence that the $d$-wave superconducting phase emerges always near the phase separation region and the superconducting order has one-to-one correspondence with the enhancement of charge compressibility. The order as well as the phase separation are vulnerable to realistic intersite Coulomb interaction, while the superexchange interaction enhances both. An appropriate combination of these two widens the stable superconducting phase.

We present the quantum-mechanical calculation of the angular dependence of interlayer conductivity ${\sigma }_{zz}\left(\theta \right)$ in a tilted magnetic field in quasi-two-dimensional (quasi-2D) layered metals. Our calculation is applicable for arbitrary density of electron states and shows that the shape of Landau levels (LLs) is important for this angular dependence. We derive simple analytical formulas for ${\sigma }_{zz}\left(\theta \right)$ in the particular cases of Gaussian and dome-shaped LLs. Since in strongly anisotropic quasi-two-dimensional metals in a high magnetic field the LL shape is closer to dome-like or Gaussian, this analytical formula replaces the traditionally used one, derived for Lorentzian LL shape. The amplitude of angular magnetoresistance oscillations (AMRO) is considerably stronger for the dome-like or Gaussian than for the traditionally used Lorentzian LL shape. The ratio ${\sigma }_{zz}(\theta =0)/{\sigma }_{zz}(\theta \to \pm {90}^{\circ })$ is also several times smaller for the Lorentzian LL shape at the same LL width. The field dependence of ${\sigma }_{zz}(\theta \to \pm {90}^{\circ })$ provides useful information about the electron mean free time. AMRO and Zeeman energy splitting lead to a spin current. For typical organic metals and for a medium magnetic field of 10 T this spin current is only a few percent of the charge current. However, the spin current may almost reach the charge current for special tilt angles of the magnetic field. The spin current has strong angular oscillations, which are phase-shifted as compared to the usual AMRO.

We use the field theory description of the fractional quantum Hall states to derive the universal response of these topological fluids to shear deformations and curvature of their background geometry, i.e., the Hall viscosity, and the Wen-Zee term. To account for the coupling to the background geometry, we show that the concept of flux attachment needs to be modified and use it to derive the geometric responses from Chern-Simons theories. We show that the resulting composite particles minimally couple to the spin connection of the geometry. We derive a consistent theory of geometric responses from the Chern-Simons effective field theories and from parton constructions, and apply it to both Abelian and non-Abelian states.

We consider quantum interference effects in carrier and photocurrent excitation in graphene using coherent electromagnetic field components at frequencies $\omega$ and $2\omega$. The response of the material at the fundamental frequency $\omega$ is presented, and it is shown that one-photon absorption at $\omega$ interferes with stimulated electronic Raman scattering (combined $2\omega$ absorption and $\omega$ emission) to result in a net contribution to the current injection. This interference occurs with a net energy absorption of $\hslash \omega$ and exists in addition to the previously studied interference occurring with a net energy absorption of $2\hslash \omega$ under the same irradiation conditions. Due to the absence of a band gap and the possibility to block photon absorption by tuning the Fermi level, graphene is the perfect material to study this contribution. We calculate the polarization dependence of this all-optical effect for intrinsic graphene and show that the combined response of the material at both $\omega$ and $2\omega$ leads to an anisotropic photocurrent injection, whereas the magnitude of the injection current in doped graphene, when transitions at $\omega$ are Pauli blocked, is isotropic. By considering the contribution to coherent current control from stimulated electronic Raman scattering, we find that graphene offers tunable, polarization sensitive applications. Coherent control due to the interference of stimulated electronic Raman scattering and linear absorption is relevant not only for graphene but also for narrow-gap semiconductors, topological insulators, and metals.

Under certain conditions, Hg(Cd)Te quantum wells (QWs) are known to realize a time-reversal symmetric, two-dimensional topological insulator phase. Its low-energy excitations are well described by the phenomenological Bernevig-Hughes-Zhang (BHZ) model that interpolates between Schrödinger and Dirac fermion physics. We study the polarization function of this model in the random phase approximation (RPA) in the intrinsic limit and at finite doping. While the polarization properties in RPA of Dirac and Schrödinger particles are two comprehensively studied problems, our analysis of the BHZ model bridges the gap between these two limits, shedding light on systems with intermediate properties. We gain insight into the screening properties of the system and on its characteristic plasma oscillations. Interestingly, we discover two different kinds of plasmons that are related to the presence of intra- and interband excitations. Observable signatures of these plasmons are carefully analyzed in a variety of distinct parameter regimes, including the experimentally relevant ones for Hg(Cd)Te QWs. We conclude that the discovered plasmons are observable by Raman or electron-loss spectroscopy.

We consider the quantum spin Hall effect in an anisotropic strip of stripes and address both integer and fractional filling factors. The first model is based on a gradient of spin-orbit interaction in the direction perpendicular to the stripes. The second model is based on two weakly coupled strips with reversed dispersion relations. We demonstrate that these systems host helical modes, i.e., modes in which opposite spins propagate in opposite directions. In the integer regime, the modes carry an elementary electron charge whereas in the fractional regime they carry fractional charges, and their excitations possess anyonic braiding statistics. These simple quasi-one-dimensional models can serve as a platform for understanding effects arising due to electron-electron correlations in topological insulators.

The magnetic structure of a Cr monolayer on a W(110) substrate is investigated by means of first-principles calculations based on noncollinear spin density functional theory (DFT). As magnetic ground state we find a long-period homochiral left-rotating spin spiral on top of an atomic-scale antiferromagnetic order of nearest-neighbor atoms. The rotation angle of the magnetic moment changes inhomogeneously from atom to atom across the spiral. We predict a propagation direction along the crystallographic $\left[001\right]$ direction with a period length of $\left|\lambda \right|=14.3\mathrm{nm}$, which is in excellent agreement with a modulation of the local antiferromagnetic contrast observed in spin-polarized scanning tunneling microscope experiments by Santos et al. [New J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction as the origin of the homochiral magnetic structure, competing with the Heisenberg-type exchange interaction and magnetocrystalline anisotropy energy. From DFT calculations we extract parameters for a micromagnetic model and thereby determine a considerable inhomogeneity of the spin spiral, increasing the period length by 6% compared to homogeneous spin spirals. The results are compared to the behavior of a Mn and Fe monolayer and Fe double layer on a W(110) substrate.

The charge-ordered state in a quasi-two-dimensional organic superconductor ${\beta }^{\prime \prime }\text{−}\left(\text{BEDT-TTF}\right){}_4\left[\right({\mathrm{H}}_3\mathrm{O}\left)\text{Ga}\right({\mathrm{C}}_2{\mathrm{O}}_4\left){}_3\right]·{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ has been investigated using ${}^{13}\mathrm{C}$ NMR spectroscopy in a high magnetic field. Enhanced resolution of the high-field spectrum allows us to estimate the site charges in the low-temperature charge-ordered state below $8.5$ K. The observed strong increase in the charge imbalance suggests that superconductivity appears on the electronic state with a strong charge modulation. From the NMR intensity ratio of signals from the charge-rich and charge-poor sites, we discuss a possible charge pattern with threefold modulation, which could be stabilized by the effects of electron-electron correlations.

The effects of host isotope mass on the hyperfine interaction of group-V donors in silicon are revealed by pulsed electron nuclear double resonance (ENDOR) spectroscopy of isotopically engineered Si single crystals. Each of the hyperfine-split ${}^{31}\mathrm{P}$, ${}^{75}\mathrm{As}$, ${}^{121}\mathrm{Sb}$, ${}^{123}\mathrm{Sb}$, and ${}^{209}\mathrm{Bi}$ ENDOR lines splits further into multiple components, whose relative intensities accurately match the statistical likelihood of the nine possible average Si masses in the four nearest-neighbor sites due to random occupation by the three stable isotopes ${}^{28}\mathrm{Si}$, ${}^{29}\mathrm{Si}$, and ${}^{30}\mathrm{Si}$. Further investigation with ${}^{31}\mathrm{P}$ donors shows that the resolved ENDOR components shift linearly with the bulk-averaged Si mass.

Spin caloritronics refers to generating spin current by thermal gradient. Here we report a theoretical study demonstrating giant spin caloritronic effects in a new class of materials, called spin semiconductors, which are characterized with a “spin gap,” the energy gap between spin-up and -down channels. Generally, spin Seebeck coefficient ($S_s$) is shown to increase linearly with the spin gap. Specifically, unprecedented large $S_s\sim 3.4$ mV/K and spin figure of merit $Z_{s}T\sim 119$ were found in spin-semiconducting graphene nanoribbons (GNRs) with sawtooth (ST) zigzag edges, based on first-principles calculations. Such giant spin caloritronic effects are shown to originate from a large spin gap of ST GNRs, in addition to two other spin-independent features of large band gap and narrow bandwidth which are commonly known for good thermoelectric materials. Our studies suggest that spin-semiconducting nanostructures, such as ST GNRs, are promising candidates for room-temperature spin caloritronics with high efficiency.

We report on magnetotransport measurements up to 30 T performed on a bilayer graphene Hall bar, enclosed by two thin hexagonal boron nitride flakes. In the quantum Hall regime, our high-mobility sample exhibits an insulating state at the neutrality point which evolves into a metallic phase when a strong in-plane field is applied, as expected for a transition from a canted antiferromagnetic to a ferromagnetic spin-ordered phase. We individuate a temperature-independent crossing in the four-terminal resistance as a function of the total magnetic field, corresponding to the critical point of the transition. We show that the critical field scales linearly with the perpendicular component of the field, as expected from the underlying competition between the Zeeman energy and interaction-induced anisotropies. A clear scaling of the resistance is also found and a universal behavior is proposed in the vicinity of the transition.

We study ground state properties and particle excitation spectra across a commensurate charge density wave transition in a system of strongly interacting fermions, using series expansion methods and mean-field theory. We consider a 1/3-filled system of spinless fermions on a triangular lattice, with hopping parameter $t$, nearest-neighbor repulsion $V$, and a sublattice dependent chemical potential ${\mu }_s$. The phase transition is found to be first order for ${\mu }_s=0$, but becomes continuous with increasing ${\mu }_s$. The particle and hole excitation spectra exhibit dramatic changes in the vicinity of the phase transitions and in the charge density wave ordered phase. We discuss the relevance of this study to the pinball Fermi liquid phase postulated theoretically in earlier studies as well as to various strongly correlated triangular lattice materials.

We investigate the effect of the Coulomb interaction, $U_{cf}$, between the conduction and $f$ electrons in the periodic Anderson model using the density matrix renormalization group algorithm. We calculate the excitation spectrum of the half-filled symmetric model with an emphasis on the spin and charge excitations. In the one-dimensional version of the model, it is found that the spin gap is smaller than the charge gap below a certain value of $U_{cf}$ and the reversed inequality is valid for stronger $U_{cf}$. This behavior is also verified by the behavior of the spin and density correlation functions. We also perform a quantum information analysis of the model and determine the entanglement map of the $f$ and conduction electrons. It is revealed that for a certain $U_{cf}$ the ground state is dominated by the configuration in which the conduction and $f$ electrons are strongly entangled, and the ground state is almost a product state. For larger $U_{cf}$, the sites are occupied alternatingly dominantly by two $f$ electrons or by two conduction electrons.

Symmetry is a powerful concept in physics, and its recent application to understand nonequilibrium behavior is providing deep insights and groundbreaking exact results. Here we show how to harness symmetry to control transport and statistics in open quantum systems. Such control is enabled by a first-order-type dynamic phase transition in current statistics and the associated coexistence of different transport channels (or nonequilibrium steady states) classified by symmetry. Microreversibility then ensues, via the Gallavotti-Cohen fluctuation theorem, a twin dynamic phase transition for rare current fluctuations. Interestingly, the symmetry present in the initial state is spontaneously broken at the fluctuating level, where the quantum system selects the symmetry sector that maximally facilitates a given fluctuation. We illustrate these results in a qubit network model motivated by the problem of coherent energy harvesting in photosynthetic complexes, and introduce the concept of a symmetry-controlled quantum thermal switch, suggesting symmetry-based design strategies for quantum devices with controllable transport properties.

Chalcopyrite ${\mathrm{ZnSnP}}_2$ is an alternative photoabsorber material for solar cells because of its controllable band gap, high absorption coefficient, and earth abundant constituents. In this study we systematically investigate its native point defects including vacancies, interstitials, and antisites using first-principles calculations with the Heyd-Scuseria-Ernzerhof hybrid functional. We evaluate the defect formation energies and defect single-particle levels at the dilute limit using finite-size image-charge corrections and compare them with those reported for ${\mathrm{CuInSe}}_2$ and ${\mathrm{CuGaSe}}_2$. The most likely donors and acceptors are cation antisites, Sn-on-Zn and Zn-on-Sn, respectively. Because of their significantly low formation energies, they lead to Fermi level pinning in the band gap under any growth condition, and constrain the carrier concentration. The Sn-on-Zn antisite in the neutral charge state becomes an intrinsic DX center, a complex of the Sn interstitial and Zn vacancy, and shows a deep donor level as reported for ${\mathrm{CuGaSe}}_2$.