We experimentally demonstrate the phenomenon of dynamical many-body freezing in a periodically driven Ising chain. Theoretically [A. Das, Phys. Rev. B 82, 172402 (2010)], for certain values of the drive parameters all fundamental degrees of freedom contributing to the response dynamics freeze for all time and for arbitrary initial state. Also, since the condition of freezing involves only the drive parameters and not the quantization of the momentum (i.e., the system size), our simulation with a small (3-spin) chain captures all salient features of the freezing phenomenon predicted for the infinite chain. Using optimal control techniques, we realize high-fidelity cosine-modulated drive, and observe nonmonotonic freezing of magnetization at specific frequencies of modulation. Time evolution of the excitations in momentum space has been tracked directly through magnetization measurements.

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature $T_c$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1–2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

The recent discovery of an incipient charge-density wave (CDW) instability competing with superconductivity in a class of high-temperature cuprate superconductors has brought the role of charge order in the phase diagram of the cuprates under renewed focus. Here, we take a mean field $Q_1=(2\pi /3,0)$ and $Q_2=(0,2\pi /3)$ biaxial CDW state and calculate the Fermi-surface topology and the resulting Hall and Seebeck coefficients as a function of temperature and hole doping. We establish that, in the appropriate doping ranges where the low-temperature state (in the absence of superconductivity) is a CDW, the Fermi surface consists of electron pockets, resulting in the Hall and Seebeck coefficients becoming negative at low temperatures, as seen in experiments.

Noncentrosymmetric superconductors (NCSs), characterized by antisymmetric spin-orbit coupling and a mixture of spin-singlet and spin-triplet pairing components, are promising candidate materials for topological superconductivity. An important hallmark of topological superconductors is the existence of protected zero-energy states at surfaces or in vortex cores. Here we investigate Majorana vortex-bound states in three-dimensional nodal and fully gapped NCSs by combining analytical solutions of Bogoliubov–de Gennes (BdG) equations in the continuum with exact diagonalization of BdG Hamiltonians. We show that depending on the crystal point-group symmetries and the topological properties of the bulk Bogoliubov-quasiparticle wave functions, different types of zero-energy Majorana modes can appear inside the vortex core. We find that for nodal NCSs with tetragonal point group $C_{4v}$ the vortex states are dispersionless along the vortex line, forming one-dimensional Majorana flat bands, while for NCSs with $D_4$ point-group symmetry the vortex modes are helical Majorana states with a linear dispersion along the vortex line. NCSs with monoclinic point group $C_2$, on the other hand, do not exhibit any zero-energy vortex-bound states. We show that in the case of the $C_{4v}$ $\left(D_4\right)$ point group the stability of these Majorana zero modes is guaranteed by a combination of reflection $(\pi$ rotation), time-reversal, and particle-hole symmetry. Considering continuous deformations of the quasiparticle spectrum in the presence of vortices, we show that the flat-band vortex-bound states of $C_{4v}$ point-group NCSs can be adiabatically connected to the dispersionless vortex-bound states of time-reversal symmetric Weyl superconductors. Experimental implications of our results for thermal transport and tunneling measurements are discussed.

We consider the one-dimensional $XX$ model in a quasiperiodic transverse field described by the Harper potential, which is equivalent to a tight-binding model of spinless fermions with a quasiperiodic chemical potential. For weak transverse field (chemical potential), $h<h_c$, the excitations (fermions) are delocalized, but become localized for $h>h_c$. We study the nonequilibrium relaxation of the system by applying two protocols: a sudden change of $h$ (quench dynamics) and a slow change of $h$ in time (adiabatic dynamics). For a quench into the delocalized (localized) phase, the entanglement entropy grows linearly (saturates) and the order parameter decreases exponentially (has a finite limiting value). For a critical quench the entropy increases algebraically with time, whereas the order parameter decreases with a stretched exponential. The density of defects after an adiabatic field change through the critical point is shown to scale with a power of the rate of field change and a scaling relation for the exponent is derived.

We present a detailed local probe study of the magnetic order in the oxychalcogenide ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2{\mathrm{OSe}}_2$ utilizing ${}^{57}\mathrm{Fe}$ Mössbauer, ${}^{139}\mathrm{La}$ NMR, and muon-spin relaxation spectroscopy. This system can be regarded as an insulating reference system of the Fe arsenide and chalcogenide superconductors. From the combination of the local probe techniques we identify a noncollinear magnetic structure similar to ${\mathrm{Sr}}_2{\mathrm{F}}_2{\mathrm{Fe}}_2{\mathrm{OS}}_2$. The analysis of the magnetic order parameter yields an ordering temperature $T_{\mathrm{N}}=90.1\mathrm{K}$ and a critical exponent of $\beta =0.133$, which is close to the two-dimensional Ising universality class as reported in the related oxychalcogenide family.

The magnetic anisotropy in ${\mathrm{Fe}}_x{\mathrm{Co}}_{100-x}/\text{MgO}/{\mathrm{Fe}}_x{\mathrm{Co}}_{100-x}$ magnetic tunnel junctions as a function of composition is investigated on different substrates (Cu, Ag, and MgO). We use the full relativistic screened-Korringa-Kohn-Rostoker method and employ the coherent potential approximation for description of the alloys. Our findings show that the magnetocrystalline anisotropy (MCA) strongly depends on the CoFe composition and that the MCA decreases with increasing Co concentration. At a certain composition there is a transition at which the MCA changes sign from positive to negative. The origin of the MCA is related to the difference of density of states between $d$ orbitals around Fermi energy, in particular, the difference between $d_{yz}\left(d_{zx}\right)$ and $d_{3z^2-r^2}$ orbitals. We also calculate the shape anisotropy in order to obtain phase diagrams that show at which magnetic layer thickness and at which composition we expect perpendicular anisotropy.

We consider 0-$\pi$ Josephson junctions consisting of 0 and $\pi$ regions of lengths $L_0$ and $L_{\pi }$ with critical current densities $j_{c0}$ and $j_{c\pi }$, respectively. The dependence of the Josephson current on the phase-shift averaged along the junction is derived. We show that these systems exhibit the main features of $\phi$ Josephson junctions—the ground state is doubly degenerate and the current-phase relation can be tuned in situ by applying magnetic field. In the limit of short and long 0 and $\pi$ regions, the current phase relation is derived analytically. In the case of intermediate lengths of 0 and $\pi$ regions, the current-phase relation is calculated numerically.

The few-determinant (FED) methodology, introduced in our previous works [R. Rodríguez-Guzmán et al., Phys. Rev. B 87, 235129 (2013); Phys. Rev. B 89, 195109 (2014)] for one-dimensional (1D) lattices, is here adapted for the repulsive two-dimensional Hubbard model at half filling and with finite doping fractions. Within this configuration mixing scheme, a given ground state with well-defined spin and space group quantum numbers is expanded in terms of a nonorthogonal symmetry-projected basis determined through chains of variation-after-projection calculations. The results obtained for the ground-state and correlation energies of half-filled and doped $4\times 4,6\times 6,8\times 8$, and $10\times 10$ lattices, as well as momentum distributions and spin-spin correlation functions in small lattices, compare well with those obtained using other state-of-the-art approximations. The structure of the intrinsic determinants resulting from the variational strategy is interpreted in terms of defects that encode information on the basic units of quantum fluctuations in the considered 2D systems. The varying nature of the underlying quantum fluctuations, reflected in a transition to a stripe regime for increasing on-site repulsions, is discussed using the intrinsic determinants belonging to a $16\times 4$ lattice with 56 electrons. Such a transition is further illustrated by computing spin-spin and charge-charge correlation functions with the corresponding multireference FED wave functions. In good agreement with previous studies, the analysis of the pairing correlation functions reveals a weak enhancement of the extended $s$-wave and $d_{x^2-y^2}$ pairing modes. Given the quality of results here reported together with those previously obtained for 1D lattices and the parallelization properties of the FED scheme, we believe that symmetry projection techniques are very well suited for building ground-state wave functions of correlated electronic systems, regardless of their dimensionality.

A fundamental property of most single-electron devices with a quasicontinuous quasiparticle spectrum on an island is the periodicity of their transport characteristics in a gate voltage. This property is robust even with respect to placing ferroelectric insulators in the source and drain tunnel junctions. We show that placing a ferroelectric insulator inside the gate capacitance breaks this periodicity. The current-voltage characteristics of this single-electron transistor strongly depends on the ferroelectric polarization and shows a giant memory effect even for negligible ferroelectric hysteresis making this device promising for memory applications.

We investigate the dewetting dynamics of ultrathin solid metal films. In these films, quantum confinement of electrons is known to produce a complex wetting potential, leading to “magic thicknesses” which are strongly favored energetically. We introduce a kinetic Monte Carlo model which accounts for a magic thickness. When the driving forces are small enough, the dewetting proceeds from the edges of the films and we find two regimes: (i) Regime I: for stronger driving forces, no dewetting rim is formed along the edge of the film, and magic-height fingers perpendicular to the film edge invade the film. (ii) Regime II: for smaller driving forces, magic-height fingers form parallel to the edge of the film. In this regime, a magic-height dewetting rim forms, and subsequently breaks up due to the emergence of two instabilities: rim closure failing, and layer-by-layer nucleation of holes in the film behind the rim. In both regimes, the dewetting velocity and the typical wavelength observed in the simulations are found to be in good agreement with a maximum velocity principle based on a simple model accounting for diffusion limited dynamics. Finally, the labyrinthine morphology resulting from the dewetting process and its typical length scales are found to be consistent with experimental observations. For stronger driving forces, we observe homogeneous nucleation. Depending on the relative values of the magic height and the film height, the dewetting process starts with the formation of holes or with the formation of magic-height islands.

The electronic structure of Bi(110) thin films as a function of film thickness is investigated by first-principles calculations, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy. Energy minimization in the calculation reveals significant atomic relaxation and rebonding at the surface. The calculated surface energy for the relaxed structures indicates that films consisting of odd numbers of atomic layers are inherently unstable and tend to bifurcate into film domains consisting of neighboring even numbers of atomic layers. This theoretical trend agrees with experimental observations. The results can be explained by the presence of unsaturated $p_z$ dangling bonds on the surfaces of films of odd-numbered atomic layers only. These $p_z$ dangling bonds form a Dirac-cone feature near the Fermi level at the $\overline{M}$ point as a consequence of the interplay of mirror symmetry and spin-orbit coupling. Films consisting of even numbers of atomic layers exhibit a band gap at $\overline{M}$ instead.

We develop an analytic approach to two-dimensional (2D) holes in a magnetic field that allows us to gain insight into physics of measuring the parameters of holes, such as cyclotron resonance, Shubnikov-de Haas effect and spin resonance. We derive hole energies, cyclotron masses, and the $g$ factors in the semiclassical regime analytically, as well as analyze numerical results outside the semiclassical range of parameters, qualitatively explaining the experimentally observed magnetic field dependence of the cyclotron mass. In the semiclassical regime with large Landau level indices, and for size quantization energy much bigger than the cyclotron energy, the cyclotron mass coincides with the in-plane effective mass, calculated in the absence of a magnetic field. The hole $g$ factor in a magnetic field perpendicular to the 2D plane is defined not only by the constant of direct coupling of the angular momentum of the holes to the magnetic field, but also by the Luttinger constants defining the effective masses of holes. We find that the $g$ factor for quasi-2D holes with heavy mass in the [001] growth direction in GaAs quantum well is $g=4.05$ in the semiclasssical regime. Outside the semiclassical range of parameters, holes behave as a species completely different from electrons. Spectra for size- and magnetic-field-quantized holes are nonequidistant, not fanlike, and exhibit multiple crossings, including crossing in the ground level. We calculate the effect of Dresselhaus terms, which transform some of the crossings into anticrossings, and the effects of the anisotropy of the Luttinger Hamiltonian on the 2D hole spectra. Dresselhaus terms of different symmetries are taken into account, and a regularization procedure is developed for the $k_z^3$ Dresselhaus terms. Control of the nonequidistant levels and crossing structure by the magnetic field can be used to control the Landau level mixing in hole systems, and thereby control hole-hole interactions in the magnetic field.

We study the interplay between topological and conventional long-range order of attractive fermions in a time-reversal-symmetric Hofstadter lattice using quantum Monte Carlo simulations, focusing on the case of one-third flux quantum per plaquette. At half filling, the system is unstable towards $s$-wave pairing and charge-density-wave order at infinitesimally small interactions. At one-third filling, the noninteracting system is a topological insulator, and a nonzero critical interaction strength is needed to drive a transition from the quantum spin Hall insulator to a superfluid. We probe the topological signature of the phase transition by threading a magnetic flux through a cylinder and observe quantized topological charge pumping.

Iridates are characterized by a fascinating interplay of spin-orbit and electron-electron interactions. The honeycomb-lattice materials $A_2{\mathrm{IrO}}_3$ $\left(A=\text{Na},\text{Li}\right)$ have been proposed to realize pseudospin-1/2 Mott insulating states with strongly anisotropic exchange interactions, described by the Heisenberg-Kitaev model, but other scenarios involving longer-range exchange interactions or more delocalized electrons have been put forward as well. Here we study the influence of nonmagnetic doping, i.e., depleted moments, on the magnetic properties of experimentally relevant variants of the Heisenberg-Kitaev and Heisenberg $J_1\text{−}{J}_2\text{−}{J}_3$ models. We generically find that the zigzag order of the clean system is replaced, upon doping, by a spin-glass state with short-ranged zigzag correlations. We determine the spin-glass temperature as a function of the doping level and show that this quantity allows one to assess the importance of longer-range exchange interactions when the doping is driven across the site-percolation threshold of the honeycomb lattice.

We studied bulk crystals of wurtzite AlN by means of uniaxial pressure-dependent Raman measurements. As a result, we derive the phonon pressure coefficients and deformation potentials for all zone center optical phonon modes. For the $A_1$ and $E_1$ modes, we further experimentally determined the uniaxial pressure dependence of their longitudinal optical–transverse optical (LO-TO) splittings. Our experimental approach delivers new insight into the large variance among previously reported phonon deformation potentials, which are predominantly based on heteroepitaxial growth of AlN and the ball-on-ring technique. Additionally, the measured phonon pressure coefficients are compared to their theoretical counterparts obtained by density functional theory implemented in the siesta package. Generally, we observe a good agreement between the calculated and measured phonon pressure coefficients but some particular Raman modes exhibit significant discrepancies similar to the case of wurtzite GaN and ZnO, clearly motivating the presented uniaxial pressure-dependent Raman measurements on bulk AlN crystals.

An $N$-channel spinless $p$-wave superconducting wire is known to go through a series of $N$ topological phase transitions upon increasing the disorder strength. Here, we show that at each of those transitions the density of states shows a Dyson singularity $\nu \left(\varepsilon \right)\propto {\varepsilon }^{-1}{|ln\varepsilon |}^{-3}$, whereas $\nu \left(\varepsilon \right)\propto {\varepsilon }^{\left|\alpha \right|-1}$ has a power-law singularity for small energies $\varepsilon$ away from the critical points. Using the concept of “superuniversality” [Gruzberg et al., Phys. Rev. B 71, 245124 (2005)], we are able to relate the exponent $\alpha$ to the wire's transport properties at zero energy and, hence, to the mean free path $l$ and the superconducting coherence length $\xi$.

Single molecules that exhibit narrow optical transitions at cryogenic temperatures can be used as local electric-field sensors. We derive the single-charge sensitivity of aromatic organic dye molecules, based on quantum mechanical considerations. Through numerical modeling, we demonstrate that by using currently available technologies it is possible to optically detect charging events in a granular network with a sensitivity better than ${10}^{-5}e/\sqrt{\mathrm{Hz}}$ and track positions of multiple electrons, simultaneously, with nanometer spatial resolution. Our results pave the way for minimally invasive optical inspection of electronic and spintronic nanodevices and building hybrid optoelectronic interfaces that function at both single-photon and single-electron levels.

We introduce a platform for realizing on-chip quantum electrodynamics using photonic-crystal waveguide structures composed of periodic nanowire arrays with embedded semiconductor quantum dots to act as quantum light sources. These nanowire-based structures, which can now be fabricated with excellent precision, are found to produce waveguide Purcell factors exceeding 100 and on-chip $\beta$ factors up to 99%. We investigate the fundamental optical properties of photonic-crystal waveguides and finite-size structures, using both photonic band structure calculations and rigorous Green function computations which allow us to obtain the modal properties and the local density of photon states. A comparison with slab-based photonic crystals is also made and we highlight a number of key advantages in the nanowire system, including the potential to reduce extrinsic scattering losses and produce high theoretical Purcell factors and $\beta$ factors on-chip. We also demonstrate that these structures exhibit rich photonic Lamb shifts over broadband frequencies.

We develop a theory for quantum phases and quantum multicriticality in bilayer graphene in the presence of an explicit energy gap in the noninteracting spectrum by extending previous renormalization group (RG) analyses of electron-electron interactions in gapless bilayer graphene at finite temperature to include the effect of an electric field applied perpendicular to the sample, which produces an energy gap in the single-particle electron-hole dispersion. We determine the possible outcomes of the resulting RG equations, represented by “fixed rays” along which ratios of the coupling constants remain constant and map out the leading instabilities of the system for an interaction of the form of a Coulomb interaction that is screened by two parallel conducting plates placed equidistant from the electron. We find that some of the fixed rays on the “target plane” found in the zero-field case are no longer valid fixed rays, but that all four of the isolated rays are still valid. We also find five additional fixed rays that are not present in the zero-field case. We then construct maps of the leading instability (or instabilities) of the system for the screened Coulomb-like interaction as a function of the overall interaction strength and interaction range for four values of the applied electric field. We find that the pattern of leading instabilities is the same as that found in the zero-field case, namely, that the system is unstable to a layer antiferromagnetic state for short-ranged interactions, to a nematic state for long-ranged interactions, and to both for intermediate-ranged interactions. However, if the interaction becomes too long ranged or too weak, then the system will exhibit no instabilities. The ranges at which the nematic instability first appears, the antiferromagnetic instability disappears, and the nematic instability disappears all decrease with increasing applied electric field. Our main qualitative finding, that the applied electric field opposes the emergence of symmetry-broken phases in general, suppressing, however, the antiferromagnetc phase more strongly compared with the nematic phase, is directly testable experimentally.