In this paper, a general formalism for transient current-current correlations and transient noise spectra is presented for noninteracting nanostructures. The exact solution of transient current correlations in both the time and the frequency domains are explicitly carried out for a single-level quantum dot system. We investigate transient current-current correlations with different bias voltages at different finite temperatures, without taking the wide-band limit. Transient noise spectra over the whole frequency range are presented and the corresponding steady-state limit is also reproduced. From transient current-current correlations and noise spectra, we analyze the frequency-dependence of electron transport for the system evolving far away from equilibrium to the steady state. Various time scales associated with the energy structures of the nanosystem are manifested through the transient current-current correlations and transient noise spectra.

We develop a theory of Coulomb drag in ultraclean double layers with strongly correlated carriers. In the regime where the equilibration length of the electron liquid is shorter than the interlayer spacing the main contribution to the Coulomb drag arises from hydrodynamic density fluctuations. The latter consist of plasmons driven by fluctuating longitudinal stresses, and diffusive modes caused by temperature fluctuations and thermal expansion of the electron liquid. We express the drag resistivity in terms of the kinetic coefficients of the electron fluid. Our results are nonperturbative in interaction strength and do not assume Fermi-liquid behavior of the electron liquid.

We performed microscopic imaging of photoluminescence (PL) and photocurrent (PC) on InAs nanostructures including disklike structures (nanodisks) and quantum dots (QDs). The correlation between PL and PC images indicates that the major fraction of upconverted carriers originates from nanodisks. By analyzing the excitation spectra, we find evidence that nanodisks and QDs need to be spatially separated to enhance PC generation via upconversion. The efficient simultaneous use of both QDs and nanodisks is an alternative approach to intermediate-band solar cells, where low-energy photons are upconverted in the QDs and high-energy photons are efficiently upconverted in the nanodisks, resulting in enhanced carrier generation yields. With spatially resolved upconverted PL, we show that PC generation in nanodisks is due to ejection of both electrons and holes.

We experimentally and theoretically investigated the second-harmonic (SH) radiation from spherical gold nanoparticles as function of the particle radius in the range from 40 to 100 nm. By fitting the measured SH intensity to an analytical model, we demonstrated size-dependent second-order susceptibility tensor components, which significantly increase by reducing the size of the particles. Finally, we found new SH polarization diagrams for large particle radii arising from the interference among the different multipolar terms.

Structural, elastic, and thermal properties of cementite (${\mathrm{Fe}}_{3}C$) were studied using a modified embedded atom method (MEAM) potential for iron-carbon (Fe-C) alloys. Previously developed Fe and C single-element potentials were used to develop a Fe-C alloy MEAM potential, using a statistics-based optimization scheme to reproduce structural and elastic properties of cementite, the interstitial energies of C in bcc Fe, and heat of formation of Fe-C alloys in L${}_{12}$ and B${}_1$ structures. The stability of cementite was investigated by molecular dynamics simulations at high temperatures. The nine single-crystal elastic constants for cementite were obtained by computing total energies for strained cells. Polycrystalline elastic moduli for cementite were calculated from the single-crystal elastic constants of cementite. The formation energies of (001), (010), and (100) surfaces of cementite were also calculated. The melting temperature and the variation of specific heat and volume with respect to temperature were investigated by performing a two-phase (solid/liquid) molecular dynamics simulation of cementite. The predictions of the potential are in good agreement with first-principles calculations and experiments.

Ions of the same charge inside confining potentials can form crystalline structures which can be controlled by means of the ion density and of the external trap parameters. In particular, a linear chain of trapped ions exhibits a transition to a zigzag equilibrium configuration, which is controlled by the strength of the transverse confinement. Studying this phase transition in the quantum regime is a challenging problem, even when employing numerical methods to simulate microscopically quantum many-body systems. Here we present a compact analytical treatment to map the original long-range problem into a short-range quantum field theory on a lattice. We provide a complete numerical architecture, based on the density matrix renormalization group, to address the effective quantum ${\varphi }^4$ model. This technique is instrumental in giving a complete characterization of the phase diagram, as well as pinpointing the universality class of the criticality.

We report magnetization $M$, heat capacity $C$, and electrical resistivity $\rho$ for single crystals of the itinerant electron antiferromagnet ${\text{CaCo}}_2$${\text{P}}_2$ ($T_{\mathrm{N}}$ $\approx$ 110 K). Measurements at ambient pressure reveal rich magnetic behavior, where ferromagnetic correlations are present in the paramagnetic state and a subsequent feature is seen at $T_1$ $\approx$ 22 K within the ordered state. Heat-capacity measurements additionally reveal moderately enhanced electronic correlations, as evidenced by the electronic coefficient of the specific heat $\gamma$ $=$ 23 mJ$/$mol$·$K${}^2$, which is large by comparison to closely related 122 analogs and the value predicted by electronic structure calculations. Upon the application of pressure, $T_{\mathrm{N}}$ is suppressed toward zero. For $P$ $\ge$ 0.89 GPa, another phase transition appears at $T_2$ $<$ $T_{\mathrm{N}}$ which is also suppressed by $P$. At $P_{\mathrm{c}}$ $\approx$ 1.4–1.5 GPa, $T_{\mathrm{N}}$ and $T_2$ drop abruptly to zero at a putative quantum phase transition. For $P$ $>$ $P_{\mathrm{c}}$, a broad shoulder in $\rho \left(T\right)$ appears at $T^{*}$, which moves to higher $T$ and broadens with increasing $P$. We discuss possible scenarios to understand the phase diagram and compare to other compounds which show similar $P$-driven behavior.

The anisotropic properties of thermal transport in insulating or conducting ferromagnets are derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the nonmagnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magnetovoltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.

Energy spectra of superconducting nanospheres observed via tunneling spectroscopy are theoretically discussed. In the energy gap we find small peaks which correspond to the two-particle dynamics beyond the mean-field BCS approximation. These peaks promise to be a sensitive test of theories of superconductivity as their number and positions depend on the approximation used.

We show that the recombination of Cooper pairs in semiconductors can be used as a natural source of polarization-entangled photons, making use of the inherent angular momentum entanglement in the superconducting state. Our proposal is not based on opposite spin population of discrete energy levels and thus does not require isolated emitters such as single atoms or quantum dots. We observe that in bulk materials, the photon entanglement would be degraded due to the variety of decay channels available in the presence of light-hole (LH)–heavy-hole (HH) degeneracy. However, we show that the lifting of this degeneracy by use of a semiconductor quantum well should lead to faithful conversion of the Cooper-pair entanglement into photon entanglement. The second-order decay of two-electron states in Cooper-pair luminescence leaves no which-path information, resulting in perfect coherence between two pathways and hence, in principle, perfect entanglement. We calculate the purity of the entangled-photon state and find that it increases for larger LH–HH energy splitting and for lower temperatures. Moreover, the superconducting macroscopic coherence offers an enhancement to the emission rate, making this a promising scheme for efficient generation of entangled photons in simple electrically driven structures.

The noncentrosymmetric superconductors LaPdSi${}_3$ and LaPtSi${}_3$ have been studied with magnetization, specific-heat, resistivity, and $\mu$SR measurements. These crystallize in the tetragonal BaNiSn${}_3$ structure and superconductivity is observed at $T_{\mathrm{c}}=2.65\left(5\right)$ K for LaPdSi${}_3$ and $T_{\mathrm{c}}=1.52\left(6\right)$ K for LaPtSi${}_3$. The results are consistent with both compounds being weakly coupled, fully gapped superconductors but $\mu$SR measurements reveal that LaPdSi${}_3$ is a bulk type-I superconductor while LaPtSi${}_3$ is a type-II material with a Ginzburg-Landau parameter of $\kappa =2.49\left(4\right)$. This is further supported by specific-heat measurements, where the transition in an applied field is first order in LaPdSi${}_3$ but second order in LaPtSi${}_3$. The electronic specific heat in the superconducting state was analyzed using an isotropic $s$-wave model that gave ${\Delta }_0/k_{\mathrm{B}}{T}_{\mathrm{c}}=1.757\left(4\right)$ for LaPdSi${}_3$ and 1.735(5) for LaPtSi${}_3$. The temperature dependence of the effective penetration depth [${\lambda }_{\mathrm{eff}}\left(T\right)$] of LaPtSi${}_3$ was extracted from $\mu$SR measurements and was fitted giving ${\Delta }_0/k_{\mathrm{B}}{T}_{\mathrm{c}}=1.60\left(8\right)$ and ${\lambda }_{\mathrm{eff}}\left(0\right)=239\left(3\right)$ nm. A critical field of $B_{\mathrm{c}}$(0) = 182.7 G was obtained for LaPdSi${}_3$ from $\mu$SR measurements, which is in good agreement with the calculated thermodynamic critical field.

The damping rates of high quality factor nanomechanical resonators are well beyond intrinsic limits. Here, we explore the underlying microscopic loss mechanisms by investigating the temperature-dependent damping of the fundamental and third harmonic transverse flexural mode of a doubly clamped silicon nitride string. It exhibits characteristic maxima reminiscent of two-level defects typical for amorphous materials. Coupling to those defects relaxes the momentum selection rules, allowing energy transfer from discrete long-wavelength resonator modes to the high frequency phonon environment.

Identifying and characterizing the parent phases of iron-based superconductors is an important step towards understanding the mechanism for their high-temperature superconductivity. We present an investigation into the magnetic interactions in the Mott insulator La${}_2$O${}_2$Fe${}_2$OSe${}_2$. This iron oxyselenide adopts a 2-$k$ magnetic structure with low levels of magnetic frustration. This magnetic ground state is found to be dominated by next-nearest-neighbor interactions $J_2$ and $J_{2^{\prime }}$ and the magnetocrystalline anisotropy of the Fe${}^{2+}$ site, leading to 2D-Ising-like spin $S=2$ fluctuations. In contrast to calculations, the values are small and confine the spin excitations below $\sim$25 meV. This is further corroborated by sum rules of neutron scattering. This indicates that superconductivity in related materials may derive from a weakly coupled and unfrustrated magnetic structure.

To address unsolved fundamental problems of the intermediate state (IS), the equilibrium magnetic flux structure and the critical field in a high-purity type-I superconductor (indium film) are investigated using magneto-optical imaging with a three-dimensional vector magnet and electrical transport measurements. The least expected observation is that the critical field in the IS can be as small as nearly 40% of the thermodynamic critical field $H_c$. This indicates that the flux density in the bulk of normal domains can be considerably less than $H_c$, in apparent contradiction with the long-established paradigm, stating that the normal phase is unstable in fields below $H_c$. Here we present a theoretical model consistently describing this and all other properties of the IS. Moreover, our model, based on a rigorous thermodynamic treatment of the observed equilibrium flux structure in a tilted field, allows for a quantitative determination of the domain-wall parameter and the coherence length, and provides new insight into the properties of superconductors.

Using molecular dynamics simulation, we study nanoindentation into an Fe (100) surface. We use the so-called magnetic interaction potentials provided by Dudarev and Derlet [J. Phys.: Condens. Matter 17, 7097 (2005)] and Chiesa et al. [J. Phys.: Condens. Matter 23, 206001 (2011)] in three different parametrizations; these allow us to extract information about the local magnetic moments from the molecular-dynamics simulation. Our simulations are performed at low temperature; the simulation cell is assumed to consist of a single ferromagnetic domain. Material hardness as well as the density of dislocations generated coincide satisfactorily with previous studies using conventional potentials. We observe the generation of both $\mathit{b}=\frac{1}{2}\langle 111\rangle$ and $\mathit{b}=\langle 100\rangle$ dislocations, which contribute to the plastic deformation. Besides dislocations, the induced microstructure features twins and vacancies. The local magnetic moments change considerably due to the strain field induced by the developing microstructure and assume values between 83 and 112% of the equilibrium bulk value in Fe. These changes are, however, quite localized and quickly decay away from the highly strained regions of dislocation cores.

The effect of high pressure exceeding 6 GPa on magnetism and multiferroicity was investigated for the olivine ${\text{Mn}}_2$${\text{GeO}}_4$ that shows successive magnetic transitions at ambient pressure and a ferroelectric ground state driven by spin-spiral order. We measured heat capacity, dielectric constant, and electric polarization at various pressures using a diamond anvil cell. The pressure evolution of the magnetic structures was also investigated by powder neutron diffraction measurements using a Paris-Edinburgh press. We found that all of the magnetic transition temperatures are enhanced monotonically by applying pressure. Furthermore, the spin-driven ferroelectricity persists up to about 6 GPa but suddenly vanishes by 6.3 GPa. A consistent description of all data is achieved if an incommensurate-commensurate phase transition occurs with pressurization and causes the suppression of ferroelectricity. We discuss the origin of the observed pressure effects on the magnetism and multiferroicity in ${\text{Mn}}_2$${\text{GeO}}_4$ in terms of a pressure-induced change in the superexchange magnetic interactions.

Raman scattering is employed to explore the effects of hole doping in single crystals of ${\text{Sr}}_2$${\text{Ir}}_{1-x}$${\text{Ru}}_x$${\text{O}}_4$ ($x=0$, 0.01, 0.03, 0.05, 0.1, and 0.2). Introducing a few percentages of holes has a strong impact on magnetic excitations and lattice dynamics. With increasing $x$ the well structured two-magnon continuum turns into diffusive magnetic scattering. Furthermore one- and two-phonon scatterings are rapidly suppressed. Remarkably, the two Ir(Ru)-O-Ir(Ru) bond angle modes with different Ir(Ru)${\text{O}}_6$ octahedral rotations coexist and compete upon hole doping. This is ascribed to the difference of electronic properties between Ir${}^{4+}$ and Ru${}^{4+}$ ions. The doping and temperature dependence of the bond angle modes suggests that an electronically phase separated state develops upon Ru doping.

We study the excitation spectrum and dynamical response functions for several quasi-one-dimensional spin systems in magnetic fields without dipolar spin order transverse to the field. This includes both nematic phases, which harbor “hidden” breaking of spin-rotation symmetry about the field and have been argued to occur in high fields in certain frustrated chain systems with competing ferromagnetic and antiferromagnetic interactions, and spin density wave states, in which spin-rotation symmetry is truly unbroken. Using bosonization, field theory, and exact results on the integrable sine-Gordon model, we establish the collective mode structure of these states, and show how they can be distinguished experimentally.

Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.