Measurements of the temperature and bias dependence of single-electron transistors (SETs) in a dilution refrigerator show that charge noise increases linearly with refrigerator temperature above a voltage-dependent threshold temperature, and that its low-temperature saturation is due to SET self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with the electrons in the SET, which can be at a much higher temperature than the substrate. We suggest that the noise is caused by electrons tunneling between the SET metal and nearby potential wells.

Ultrafast quantum dynamics of interband transitions in monolayer graphene initiated by strong laser radiation is investigated. A microscopic theory describing nonlinear interaction beyond the Dirac cone approximation and applicable to the full Brillouin zone of the hexagonal nanostructure with tight-binding electronic states is developed. Coherent effects toward the control of macroscopic quantum states in graphene are considered. In particular, we consider Rabi oscillations, rapid adiabatic passage for population transfer, and multiphoton excitation of the Fermi-Dirac sea in a monolayer graphene.

We present a theoretical study of momentum-resolved tunneling between parallel two-dimensional conductors whose charge carriers have a (pseudo)spin-1/2 degree of freedom that is strongly coupled to their linear orbital momentum. Specific examples are single- and bilayer graphene as well as single-layer molybdenum disulfide. Resonant behavior of the differential tunneling conductance exhibited as a function of an in-plane magnetic field and bias voltage is found to be strongly affected by the (pseudo)spin structure of the tunneling matrix. We discuss ramifications for the direct measurement of electronic properties such as Fermi surfaces and dispersion curves. Furthermore, using a graphene double-layer structure as an example, we show how magnetotunneling transport can be used to measure the pseudospin structure of tunneling matrix elements, thus enabling electronic characterization of the barrier material.

The longitudinal spin Seebeck effect (LSSE) and the anomalous Nernst effect (ANE) are investigated in various metal/insulator junction systems and a clear separation of the LSSE from the ANE induced by static magnetic proximity is demonstrated. This separation is realized by comparing transverse thermoelectric voltage in in-plane magnetized (IM) and perpendicularly magnetized (PM) configurations, where the LSSE appears only in the IM configuration while the ANE appears both in the IM and PM configurations. We show that, in Pt/Y${}_3$Fe${}_5$O${}_{12}$ samples, the LSSE voltage in the IM configuration is three orders of magnitude greater than the proximity-ANE contamination estimated from the data in the PM configuration. This quantitative voltage comparison between the IM and PM configurations is corroborated by systematic voltage measurements in Ni${}_{81}$Fe${}_{19}$/Gd${}_3$Ga${}_5$O${}_{12}$, Pt/Gd${}_3$Ga${}_5$O${}_{12}$, Au/Y${}_3$Fe${}_5$O${}_{12}$, and Au/Gd${}_3$Ga${}_5$O${}_{12}$ samples and by our phenomenological model calculation. The LSSE measurements in high magnetic field regions further confirm that the observed voltage in the Pt/Y${}_3$Fe${}_5$O${}_{12}$ and Au/Y${}_3$Fe${}_5$O${}_{12}$ samples is of magnon origin. We apply this voltage comparison method also to a Ni${}_{81}$Fe${}_{19}$/Y${}_3$Fe${}_5$O${}_{12}$ sample and show that both the LSSE and ANE exist in this sample.

Despite intensive research, the underlying mechanisms for ultrafast demagnetization after laser irradiation in transition metals are still not understood. We discuss the possible processes which have been suggested in order to explain the ultrafast demagnetization within several hundreds of femtoseconds and argue that the spin angular momentum has to go to the lattice in the end. Based on this argument, we consider spin-flip electron-phonon scatterings. The demagnetization time ${\tau }_M$ and the demagnetization rate $dM/dt$ due to spin-flip electron-phonon scattering is calculated for fcc Ni and bcc Fe. Thereby, the electronic states and phononic states are calculated ab initio. We find that the demagnetization rates for fcc Ni and bcc Fe are too small to explain experimental demagnetization rates, which is in agreement with earlier publications. In addition, the demagnetization rates for band structures with reduced exchange splitting are calculated, however, also these demagnetization rates are too small. Finally, the phase space for scattering processes which is related to the maximum possible demagnetization is estimated for band structures with ground-state exchange splitting and with reduced exchange splitting. The maximum possible demagnetization is too small for bcc Fe and fcc Co but not necessarily for fcc Ni. We suggest to include magnons and to consider independent combinations of spin-flip electron-phonon and spin-flip electron-magnon scattering processes as a possible explanation for the ultrafast demagnetization.

We investigate the dynamic structure factor of a system of Bose particles at zero temperature using quantum Monte Carlo methods. Interactions are modeled using a hard-sphere potential of size $a$ and simulations are performed for values of the gas parameter $n{a}^3$ ranging from the dilute regime up to densities $n$ where the thermodynamically stable phase is a solid. With increasing density, we observe a crossover of the dispersion of elementary excitations from a Bogoliubov-type spectrum to a phonon-maxon-roton curve and the emergence of a broad multiphonon contribution accompanying the single-quasiparticle peak. In particular, for $n{a}^3=0.2138$, which corresponds to superfluid ${}^4$He at equilibrium density, the extracted spectrum turns out to be in good agreement with the experimental energy-momentum dispersion relation in the roton region and for higher momenta. The behavior of the spectral function at the same density in the stable solid and metastable gas phase above the freezing point is also discussed.

We present the results of muon spin relaxation/rotation, transmission electron microscopy, and neutron diffraction measurements performed on several specimens of BaTi${}_2$(As${}_{1-x}$Sb${}_x$)${}_2$O, which is known to have either charge density or spin density wave ordering at $T_{\mathrm{DW}}$ for all $x$, and superconductivity below $T_{\mathrm{c}}$ ≈ 1 K for $x$ $=$ 1. Zero-field muon spin relaxation measurements show no significant increase in relaxation rate at the density wave ordering temperature for any composition, indicating that the density wave is of the charge rather than spin type. The absence of any superstructure peaks in selected area electron and high-resolution neutron diffraction measurements below $T_{\mathrm{DW}}$ suggests that the charge density wave does not involve modulation of atomic arrangement. Transverse field muon spin rotation measurements reveal a robust superconducting state below $T_{\mathrm{c}}$ ≈ 1 K for $x$ $=$ 1.

We study the nonequilibrium Seebeck spin transport across metal-magnetic insulator interfaces. The conjugate-converted thermal-spin transport is assisted by the exchange interaction at the interface, between conduction electrons in the metal lead and localized spins in the insulating magnet lead. We predict the rectification and negative differential spin Seebeck effect and resolve their microscopic mechanism, as a consequence of the strongly fluctuated electronic density of states in the metal lead. The rectification of spin Peltier effect is also discussed. The phenomena predicted here are relevant for designing efficient spin/magnon diode and transistor, which could play crucial roles in controlling energy and information in functional devices.

By combining density functional perturbation theory (DFPT) calculations and laser photoemission spectroscopy (LPES) experiments, we have clarified the selective coupling between low-energy photoelectrons and subsurface phonon modes. A step structure resulting from the inelastic scattering of photoelectrons appeared at 14.7 meV below the Fermi level in the LPES of Cu(110). We found that the inelastic step originates from an indirect excitation in which the electron is excited by the low-energy photon near the $\overline{\mathrm{Y}}$ point and then scattered to the $\overline{\Gamma }$ point by phonon modes that are predominantly present in the subsurface region.

We report on the positive magnetoresistance (MR) that accompanies the onset of fan-type magnetic structures in an epitaxially grown Dy/Y superlattice (SL). We find that MR ratios for current perpendicular to plane (CPP) and in-plane (CIP) geometries yield comparable values and a similar nonmonotonic dependence on the applied magnetic field $H$ and temperature, which result in a maximum $\text{MR}\sim 0.45\%$ at $T=110$ K. We demonstrate that the rise in resistance is due to the increase in the number of superzone band gaps that accompanies the magnetization process of the anisotropic spin-spiral magnet, which reflects on the increasing complexity of the field-induced modulated magnetic structures. Furthermore, we find that the suppression of the giant MR in Dy/Y SL is due mainly to the combined action of chemical modulation and epitaxial strain effects. Finally, MR presents a linear (negative slope) and unsaturated scaling with $H$ in the magnetized state, which is a clear fingerprint of magnon MR in highly anisotropic magnetic materials.

Relativistic O($N$) field theories are studied near the quantum-critical point in two space dimensions. We compute dynamical correlations by large-scale Monte Carlo simulations and numerical analytic continuation. In the ordered side, the scalar spectral function exhibits a universal peak at the Higgs mass. For $N=3$ and 4, we confirm its ${\omega }^3$ rise at low frequency. On the disordered side, the spectral function exhibits a sharp gap. For $N=2$, the dynamical conductivity rises above a threshold at the Higgs mass (density gap), in the superfluid (Mott insulator) phase. For charged bosons (Josephson arrays), the power-law rise above the Higgs mass increases from two to four. Approximate charge-vortex duality is reflected in the ratio of imaginary conductivities on either side of the transition. We determine the critical conductivity to be ${\sigma }_{\mathrm{c}}^{*}=0.3(\pm 0.1)\times 4e^2/h$ and describe a generalization of the worm algorithm to $N>2$. We use a singular value decomposition error analysis for the numerical analytic continuation.

The large family of layered transition-metal dichalcogenides is widely believed to constitute a second family of two-dimensional (2D) semiconducting materials that can be used to create novel devices that complement those based on graphene. In many cases these materials have shown a transition from an indirect band gap in the bulk to a direct band gap in monolayer systems. In this work we experimentally show that folding a 1$H$ molybdenum disulfide (MoS${}_2$) layer results in a turbostratic stack with enhanced photoluminescence quantum yield and a significant shift to the blue by ∼90 meV. This is in contrast to the expected 2$H$-MoS${}_2$ band-structure characteristics, which include an indirect gap and quenched photoluminescence. We present a theoretical explanation for the origin of this behavior in terms of exciton screening.

Nanostripes with very high (>20) aspect ratios of a two-dimensional manganese oxide phase are grown on the Ag(100) surface using a physical vapor deposition method and thoroughly characterized via low-energy electron diffraction and scanning tunneling microscopy. Despite the square pattern of the substrate, symmetry breaking is produced by the structural anisotropy of the kinetically stabilized, metastable Mn surface oxide phase. As rationalized by a first-principles analysis, the resulting asymmetry in edge diffusion and attachment energies and the difference in adsorption energetics between Mn and Ag adatoms leads to the kinetic formation of such strikingly anisotropic structural domains: they feature nanoscale width (≈5–20 nm), mesoscopic length (up to ≈1 μ), very low defect density, and grow along the high-symmetry directions of the Ag(100) substrate.

We investigate the electronic structure and electron transport properties of Si nanowires attached to Au electrodes from first principles using density functional theory and the nonequilibrium Green's function method. We systematically study the dependence of the transport properties on the diameter of the nanowires, on the growth direction, and on the length. At the equilibrium Au-nanowire distance we find strong electronic coupling between the electrodes and nanowires, which results in a low contact resistance. With increasing nanowire length we study the transition from metallic to tunneling conductance for small applied bias. For the tunneling regime we investigate the decay of the conductance with the nanowire length and rationalize the results using the complex band structure of the pristine nanowires. The conductance is found to depend strongly on the growth direction, with nanowires grown along the $\langle 110\rangle$ direction showing the smallest decay with length and the largest conductance and current.

Motivated by the reported high dielectric response of monoclinic $\beta$-Ba${}_2$TiO${}_4$ as well as its affinity for absorbing small molecules, we investigate its structural, electronic, and vibrational properties with density functional theory (DFT). DFT-based structural optimization obtains lattice parameters and bond lengths within a few percent of experimentally observed values, with specific details depending on the choice of exchange-correlation functional. Although, for both the local density approximation (LDA) and generalized gradient approximation (GGA) functionals employed, the DFT calculations produce a wide-band-gap insulating state for $\beta$-Ba${}_2$TiO${}_4$ (with an indirect gap of 4.1 eV or greater), they do not agree on the energetic ordering of this phase with respect to its perovskite-like Ruddlesden-Popper (RP) polymorph. Simulations that utilize LDA place the $\beta$ phase 0.30 eV higher, while those using the Perdew-Burke-Ernzerhof GGA functional place it 0.22 eV lower than the RP one, leaving the question of the degree of perovskite-like phase metastability under epitaxial growth conditions unresolved. Comparison of the formula unit volumes of the $A_2$TiO${}_4$ and $A$TiO${}_3$ polymorphs ($A=\text{Sr}$, Ba) reveals that both Ba${}_2$TiO${}_4$ structures possess much more open geometries—more so for the $\beta$ than for the RP phase—than their isostoichiometric Sr-based counterparts and all of the examined $A$TiO${}_3$ compounds, in line with the demonstrated propensity of the Ba-based 2-1-4 oxides to capture molecules like CO${}_2$ and H${}_2$O. However, an analysis of vibrations and their contributions to the static dielectric permittivity tensor of $\beta$-Ba${}_2$TiO${}_4$ indicates that, unlike the perovskite RP phase, this structure does not exhibit strong polar lattice distortions, which results in a rather low value for its average static dielectric constant.

We explore the electrodynamic coupling between a plane wave and an infinite two-dimensional periodic lattice of magnetoelectric point scatterers, deriving a semianalytical theory with consistent treatment of radiation damping, retardation, and energy conservation. We apply the theory to arrays of split ring resonators and provide a quantitative comparison of measured and calculated transmission spectra at normal incidence as a function of lattice density, showing excellent agreement. We further show angle-dependent transmission calculations for circularly polarized light and compare with the angle-dependent response of a single split ring resonator, revealing the importance of cross coupling between electric dipoles and magnetic dipoles for quantifying the pseudochiral response under oblique incidence of split ring lattices.

We propose a scheme of continuous tunable THz emission based on dipolaritons, mixtures of strongly interacting cavity photons and direct excitons, where the latter are coupled to indirect excitons via tunneling. We investigate the property of multistability under continuous wave (CW) pumping, and the stability of the solutions. We establish the conditions of parametric instability, giving rise to oscillations in density between the direct-exciton and indirect-exciton modes under CW pumping. In this way, we achieve continuous and tunable emission in the THz range in a compact single-crystal device. We show that the emission frequency can be tuned in a certain range by varying an applied electric field and pumping conditions. Finally, we demonstrate the dynamic switching between different phases in our system, allowing rapid control of THz radiation.

We report the experimental realization of a nongalvanic, primary thermometer capable of measuring the electron temperature of a two-dimensional electron gas with negligible thermal load. Such a thermometer consists of a quantum dot whose temperature-dependent, single-electron transitions are detected by means of a quantum-point-contact electrometer. Its operating principle is demonstrated for a wide range of electron temperatures from 40 to 800 mK. This noninvasive thermometry can find application in experiments addressing the thermal properties of micrometer-scale mesoscopic electron systems, where heating or cooling electrons require relatively low thermal budgets.

We study the dynamics of a nonlinear electromechanical oscillator with delayed feedback. Compared to their linear counterparts, we find that the dynamics is dramatically different. The well-known Barkhausen stability criterion ceases to exist, and two modes of operation emerge: one characterized by hysteresis in combination with a bistable frequency and amplitude; the other, by self-stabilization of the oscillation frequency and amplitude. The observed features are captured by a model based on a Duffing equation with delayed force feedback. Nonlinear oscillators with delayed force feedback are exemplary for a large class of dynamic systems.

One can demonstrate that a 1D Josephson network containing junctions with different tunneling resistances can be synchronized at frequencies, which are multiples of 2$eV$, where $V$ is the total dc voltage applied across the network. The appearance of such synchronization follows from the law of charge conservation and takes place if charge transfer is dominated by the Josephson channel. One can observe also a subharmonic structure. The result holds for cluster-based arrays as well as for the general case of a tunneling network.