We analyze an electronic nanoscale thermal machine driven by time-dependent environment: besides bias and gate voltage variations, we consider also the less prevailing time modulation of the couplings between leads and dot. We provide energy and heat current expressions in such situations, as well as expressions for the power exchanged between the dot+leads system and its outside. Calculations are made in the Keldysh nonequilibrium Green's function framework. We apply these results to design a cyclic refrigerator, circumventing the ambiguity of defining energy flows between subsystems in the case of strong coupling. For fast lead-dot coupling modulation, we observe transient currents which cannot be ascribed to charge tunneling.

We analyze the effective Landé factor $g^{*}$ and its dependence on the orientation of the external magnetic field for a quantum point contact defined in the two-dimensional electron gas. The paper simulates the experimental procedure for evaluation of the effective Landé factors from the transconductance of a biased device in an external magnetic field. The contributions of the orbital effects of the magnetic field, the electron-electron interaction, and spin-orbit (SO) coupling are studied in low-temperature conditions (0.5 K). The anisotropy of the $g^{*}$ factors for the in-plane magnetic field orientation, which seems counterintuitive from the perspective of the effective SO magnetic field, is explained in an analytical model of the constriction as due to the SO-induced subband mixing. The asymmetry of the transconductance as a function of the gate voltage is obtained in agreement with the experimental data and the results are explained as due to depletion of the electron gas within the quantum point contact constriction and the related reduction of the screening as described within the DFT approach. The results for transconductance and the $g^{*}$ factors obtained are in a good agreement with the experimental data [Martin et al., Phys. Rev. B 81, 041303 (2010)].

Scalable and precise nanopatterning of graphene is an essential step for graphene-based device fabrication. Hydrogen-plasma reactions have been shown to narrow graphene only from the edges, or to selectively produce circular or hexagonal holes in the basal plane of graphene, but the underlying plasma-graphene chemistry is unknown. Here, we study the hydrogen-plasma etching of monolayer graphene supported on $\mathrm{Si}{\mathrm{O}}_2$ substrates across the range of plasma ion energies using scale-bridging molecular dynamics (MD) simulations based on reactive force-field potential. Our results uncover distinct etching mechanisms, operative within narrow ion energy windows, which fully explain the differing plasma-graphene reactions observed experimentally. Specific ion energy ranges are demonstrated for stable isotropic ($\sim 2\mathrm{eV}$) versus anisotropic hole growth ($\sim 20–30\mathrm{eV}$) within the basal plane of graphene, as well as for pure edge etching of graphene ($\sim 1\mathrm{eV}$). Understanding the complex plasma-graphene chemistry opens up a means for controlled patterning of graphene nanostructures.

We study spin-orbit torques in two-dimensional hexagonal crystals such as graphene, silicene, germanene, and stanene. The torque possesses two components, a fieldlike term due to inverse spin galvanic effect and an antidamping torque originating from Berry curvature in mixed spin-$k$ space. In the presence of staggered potential and exchange field, the valley degeneracy can be lifted and we obtain a valley-dependent Berry curvature, leading to a tunable antidamping torque by controlling the valley degree of freedom. The valley imbalance can be as high as 100% by tuning the bias voltage or magnetization angle. These findings open new venues for the development of current-driven spin-orbit torques by structural design.

The inelastic tunnel current in the junction formed between the tip of a scanning tunneling microscope (STM) and the sample can electrically generate optical signals. This phenomenon is potentially of great importance for nano-optoelectronic devices. In practice, however, the properties of the emitted light are difficult to control because of the strong influence of the STM tip. In this work, we show both theoretically and experimentally that the sought-after, well-controlled emission of light from an STM tunnel junction may be achieved using a nonplasmonic STM tip and a plasmonic nanoparticle on a transparent substrate. We demonstrate that the native plasmon modes of the nanoparticle may be used to engineer the light emitted in the substrate. Both the angular distribution and intensity of the emitted light may be varied in a predictable way by choosing the excitation position of the STM tip on the particle.

We study the intervalley scattering in defected graphene by low-temperature transport measurements. The scattering rate is strongly suppressed when defects are charged. This finding highlights “screening” of the short-range part of a potential by the long-range part. Experiments on calcium-adsorbed graphene confirm the role of a long-range Coulomb potential. This effect is applicable to other multivalley systems, provided that the charge state of a defect can be electrically tuned. Our result provides a means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials.

The fast decay of carrier inversion in photoexcited graphene has been attributed to optical phonon emission and Auger recombination. Plasmon emission provides another pathway that, as we show here, drives the carrier relaxation dynamics on ultrafast time scales. In studying the nonequilibrium relaxation dynamics we find that plasmon emission effectively converts inversion into hot carriers, whose energy is then extracted by optical phonon emission. This mechanism not only explains the observed femtosecond lifetime of inversion but also offers the prospect for atomically thin ultrafast plasmon emitters.

The origin of Rashba spin splitting in the two-dimensional electron gas at the (001) surface of ${\mathrm{SrTiO}}_3$ is studied using first-principles calculations and tight-binding model. Calculations of oxygen vacancies under virtual crystal approximation reveal a two-dimensional electron-gas subband structure similar to polar materials, consistent with observations on ${\mathrm{SrTiO}}_3$. Our studies also confirm that $k$ dependence of the spin splitting is predominantly cubic in the surface $\mathrm{Ti}-t_{2g}$ states, even though structural relaxations diminish the effect in $d_{xy}$ bands. A tight-binding model, explicitly including $\mathrm{Ti}-d$ and $\mathrm{O}-p$ states as well as next-nearest-neighbor interactions, is derived to understand the first-principles results. Effective Rashba Hamiltonians for the surface bands are derived using quasidegenerate perturbation theory and scenarios in which linear $k$ contribution may be suppressed are discussed. However, the cubic terms in the Hamiltonian are found to be different from the model derived using $k·p$ theory, leading to different pseudospin symmetry in the Brillouin zone.

Self-consistent (non-)Abelian statistics in 2+1 dimensions (2+1D) are classified by modular tensor categories (MTCs). In recent works, a simplified axiomatic approach to MTCs, based on fusion coefficients $N_k^{ij}$ and spins $s_i$, was proposed. A numerical search based on these axioms led to a list of possible (non-)Abelian statistics, with rank up to $N=7$. However, there is no guarantee that all solutions to the simplified axioms are consistent and can be realized by bosonic physical systems. In this paper, we use simple-current algebra to address this issue. We explicitly construct many-body wave functions, aiming to realize the entries in the list (i.e., realize their fusion coefficients $N_k^{ij}$ and spins $s_i$). We find that all entries can be constructed by simple-current algebra plus conjugation under time-reversal symmetry. This supports the conjecture that simple-current algebra is a general approach that allows us to construct all (non-)Abelian statistics in 2+1D. It also suggests that the simplified theory based on $(N_k^{ij},s_i)$ is a classifying theory at least for simple bosonic 2+1D topological orders (up to invertible topological orders).

Using the Kadanoff-Baym-Keldysh formalism, we employ nonequilibrium dynamical mean-field theory to exactly solve for the nonlinear response of an electron-mediated charge-density-wave-ordered material. We examine both the dc current and the order parameter of the conduction electrons as the ordered system is driven by the electric field. Although the formalism we develop applies to all models, for concreteness, we examine the charge-density-wave phase of the Falicov-Kimball model, which displays a number of anomalous behaviors including the appearance of subgap density of states as the temperature increases. These subgap states should have a significant impact on transport properties, particularly the nonlinear response of the system to a large dc electric field.

The key ingredient of high critical currents in a type-II superconductor is defect sites that pin vortices. Contrary to earlier understanding on nanopatterned artificial pinning, here we show unequivocally the advantages of a random pinscape over an ordered array in a wide magnetic field range. We reveal that the better performance of a random pinscape is due to the variation of its local density of pinning sites (LDOPS), which mitigates the motion of vortices. This is confirmed by achieving even higher enhancement of the critical current through a conformally mapped random pinscape, where the distribution of the LDOPS is further enlarged. The demonstrated key role of LDOPS in enhancing superconducting critical currents gets at the heart of random versus commensurate pinning. Our findings highlight the importance of random pinscapes in enhancing the superconducting critical currents of applied superconductors.

Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ has strong electron-phonon coupling: Our neutron and x-ray scattering experiments show strong softening and broadening of transverse acoustic phonons on heating through the Curie temperature $T_C=350\mathrm{K}$. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic metal phase between $200\mathrm{K}$ and $T_C$ is characterized by quasielastic scattering from dynamic CE-type polarons with the relatively short lifetime of $\tau \approx 1\mathrm{ps}$. This scattering is greatly enhanced above $T_C$ in the paramagnetic polaronic metal phase. Our results suggest that the strength of magnetoresistance in manganites scales with the inverse of the polaron lifetime, not the strength of electron-phonon coupling.

While changes in resonant Raman scattering measurements are commonly used to measure the effect of chemical functionalization on single-walled carbon nanotubes, the precise effects of functionalization on these spectra have yet to be clearly identified. In this density functional theory study, we explore the effects of functionalization on both the nanotube resonance energy and frequency shifts in radial breathing mode. Charge transfer effects cause a shift in the first Van Hove singularity spacings, and hence resonance excitation energy, and lead to a decrease in the radial breathing mode frequency, notably when the Fermi level decreases. By varying stochastically the effective mass of carbon atoms in the tube, we simulate the mass effect of functionalization on breathing mode frequency. Finally, full density functional calculations are performed for different nanotubes with varying functional group distribution and concentration using fluorination and hydrogenation, allowing us to determine overall effect on radial breathing mode and charge transfer. The results concur well with experiment, and we discuss the importance when using Raman spectroscopy to interpret experimental functionalization treatments.

The lattice dynamics of ${\mathrm{TiO}}_2$ in the rutile crystal structure was studied by a combination of thermal diffuse scattering, inelastic x-ray scattering, and density functional perturbation theory. We experimentally confirm the existence of a predicted anomalous soft transverse acoustic mode with energy minimum at $q=\left(\frac{1}{2}\frac{1}{2}\frac{1}{4}\right)$. The phonon energy landscape of this particular branch is reported and compared to the calculation. The harmonic calculation underestimates the phonon energies but despite this the shape of both the energy landscape and the scattering intensities are well reproduced. We find a significant temperature dependence in the energy of this transverse acoustic mode over an extended region in reciprocal space, which point to a strong role of anharmonicity in line with a substantially anharmonic mode potential-energy surface. The reported low-energy branch is quite different from the ferroelectric mode that softens at the Brillouin zone center and may help explain anomalous convergence behavior in calculating ${\mathrm{TiO}}_2$ surface properties and is potentially relevant for real behavior in ${\mathrm{TiO}}_2$ thin films.

The magnetic properties of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ (NFB)/transition metal (TM = Fe, Co) multilayer systems are studied on the basis of first-principles density functional calculations. Assuming a collinear spin structure, we optimize the model structure under a variety of crystallographic alignments of the NFB layer, and analyze the mechanism of interface magnetic coupling. Improvements in remanent magnetization compared to that of single NFB are observed in NFB(001)/Fe, NFB(110)/Fe, and NFB(100)/Co. On the other hand, in NFB(100)/Fe, remanence degradation due to the antiparallel magnetization alignment between NFB and Fe layers is observed. In this system, which has the shortest optimized interlayer distance among all considered systems, an itinerant electron magnetism is required around the interface to lower the total energy, and accordingly, antiferromagnetic coupling is preferred. The significant difference in property between NFB(100)/Fe and NFB(100)/Co is attributed to the difference between their interface structures, optimized interlayer distances, and magnetic stiffness of TM layers.

High-resolution neutron powder diffraction is used to demonstrate that the ${\mathrm{Eu}}^{2+}$ moments in ${\mathrm{EuZrO}}_3$ are oriented along the $c$ axis, in contrast to ${\mathrm{EuTiO}}_3$ where the moments lie within the $ab$ plane. By applying a Landau theory analysis to the Ti and Zr system we are able to contrast the differing magnetoelectric coupling symmetries resulting from the magnetic space groups of the two ordered structures. The observed order parameter is consistent with the asymptotic three-dimensional Ising model.

Inherently layered magnetic materials, such as magnetic $M_{n+1}A{X}_n$ (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition ${M_n}_{+1}A{X}_n$ consists of $M_{n+1}{X}_n$ blocks separated by atomically thin $A$-layers where $M$ is a transition metal, $A$ an A-group element, $X$ refers to carbon and/or nitrogen, and $n$ is typically 1, 2, or 3. Here, we show that the recently discovered magnetic $\mathrm{M}{\mathrm{n}}_2\mathrm{GaC}$ MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of $\mathrm{M}{\mathrm{n}}_2\mathrm{GaC}$ MAX phases in future magnetoelectric and magnetocaloric applications.

The magnetic structure and phase diagram of the layered ferromagnetic compound ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high-resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single-crystal neutron diffraction finds ferromagnetic order with moments of $1.11\left(5\right){\mu }_B/\mathrm{Fe}$ aligned along the $c$ axis at 4 K. These flux-grown crystals have a lower Curie temperature $T_{\text{c}}\approx 150$ K compared to crystals previously grown by vapor transport ($T_{\text{c}}=220$ K). The difference is a reduced Fe content in the flux-grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ and structurally similar ${\mathrm{Ni}}_{3-x}{\mathrm{GeTe}}_2$.

We report on a source of in-plane anisotropy in nanomagnets due to the presence of Dzyaloshinskii-Moriya interaction (DMI). This anisotropy depends on the shape of the magnet, and is orthogonal to the demagnetization shape anisotropy. This effect originates from the DMI energy reduction due to an out-of-plane tilt of the spins at edges oriented perpendicular to the magnetization. Our numerical and analytical investigations demonstrate that this energy reduction can compensate the demagnetization shape anisotropy energy in magnets of elongated shape, provided that their volumes are small enough and thus that their magnetization is quasiuniform. Our model agrees quantitatively with magnetic force microscopy observations on preliminary sub-100-nm structures.