We present a comprehensive and systematic study of thermal rectification in a prototypical low-dimensional quantum system—a nonlinear resonator: we identify necessary conditions to observe thermal rectification and we discuss strategies to maximize it. We focus, in particular, on the case where anharmonicity is very strong and the system reduces to a qubit. In the latter case, we derive general upper bounds on rectification which hold in the weak system-bath coupling regime, and we show how the Lamb shift can be exploited to enhance rectification. We then go beyond the weak-coupling regime by employing different methods: (i) including cotunneling processes, (ii) using the nonequilibrium Green's function formalism, and (iii) using the Feynman-Vernon path integral approach. We find that the strong coupling regime allows us to violate the bounds derived in the weak-coupling regime, providing us with clear signatures of high-order coherent processes visible in the thermal rectification. In the general case, where many levels participate to the system dynamics, we compare the heat rectification calculated with the equation of motion method and with a mean-field approximation. We find that the former method predicts, for a small or intermediate anharmonicity, a larger rectification coefficient.

During the last decades, quantum dots within the Coulomb blockade regime of transport have been proposed as essential building blocks for a wide variety of nanomachines. This includes thermoelectric devices, quantum shuttles, quantum pumps, and even quantum motors. However, in this regime, the role of quantum mechanics is commonly limited to provide energy quantization while the working principle of the devices is ultimately the same as their classic counterparts. Here, we study quantum-dot-based nanomachines in the Coulomb blockade regime, but in a configuration where the coherent superpositions of the dots' states plays a crucial role. We show that the studied system can be used as the basis for different forms of “true” quantum machines that should only work in the presence of these coherent superpositions. We analyze the efficiency of these machines against different nonequilibrium sources (bias voltage, temperature gradient, and external driving) and the factors that limit it, including decoherence and the role of the different orders appearing in the adiabatic expansion of the charge/heat currents.

Planar metasurfaces for efficient unidirectional conversion of a plane wave into a surface wave are presented. They are designed as a reciprocal device of a decoupler or leaky-wave antenna based on spatially modulated reactance surfaces. In a surface wave-to-leaky wave conversion, the surface reactance is determined from optimization of the complete aperture field over the propagating and evanecent spectra such that the conversion surface becomes locally and globally passive and lossless, while the desired wave conversion is achieved. The coupling efficiency is given by the aperture efficiency of the reciprocal antenna, which can closely approach unity for large apertures. The optimized surface reactance is spatially discretized and translated into physical metasurfaces, comprising an array of subwavelength inclusions. In TM polarization, three lossless metasurface design examples of 40 and 80 wavelengths are provided with performance validation using full-wave simulations: a metallic groove array, a thickness-modulated grounded dielectric substrate, and a width-modulated grounded dielectric ridge array. Deviations of the surface reactance from the optimized distribution reduce the conversion efficiency, but high coupling efficiencies in excess of 98% for the metallic structure and 60% for dielectric structures are predicted.

The topological metal states in electronic systems have been extensively studied, but topological phonons were explored only in a few examples so far. Here, we expose that the topological type-I and type-II Weyl phonons simultaneously appear in a recently realized new carbon allotrope, $T$-carbon, and the topological surface states of phonons are well separated from its bulk phonon spectra that are suitable for experiments. At about 15.2 THz, we find that there exist three mutually orthogonally intersecting nodal loops (named nodal gimbal phonons) centered at the $\mathrm{\Gamma }$ point, and two pairs of type-I Weyl phonons on the boundary of Brillouin zone around each $X$ point. In addition, there exist three pairs of type-II Weyl phonons at about 14.5 THz around each $L$ point. It is shown that these exotic topological phonons are protected by corresponding symmetries, and lead to topologically nontrivial surface states. Our findings not only afford plenty of intriguing topological phonon states in a simple material like $T$-carbon but also provide a platform to study novel properties of topological phonons, which would facilitate further both experimental and theoretical works in future.

We report fabrication of ${\mathrm{EuSb}}_2$ single-crystalline films and investigation of their quantum transport. First-principles calculations demonstrate that ${\mathrm{EuSb}}_2$ is a magnetic topological nodal-line semimetal protected by nonsymmorphic symmetry. Observed Shubnikov-de Haas oscillations with multiple frequency components exhibit small effective masses and two-dimensional field-angle dependence even in a 250 nm thick film, further suggesting possible contributions of surface states. This finding of the high-mobility magnetic topological semimetal will trigger further investigation of exotic quantum transport phenomena by controlling magnetic order in topological semimetal films.

We employ the technique of optical detection of magnetic resonance to study dipolar interaction in diamond between nitrogen-vacancy color centers of different crystallographic orientations and substitutional nitrogen defects. We demonstrate optical measurements of resonant spin flip-flips (second Larmor line), and flip-flops between different spin ensembles in diamond. In addition, the strain coupling between the nitrogen-vacancy color centers and bulk acoustic modes is studied using optical detection. Our findings may help optimize cross-polarization protocols, which, in turn, may allow improving the sensitivity of diamond-based detectors.

The tailoring of plasmonic near fields is central to the field of nanophotonics. Detailed knowledge of the field distribution is crucial for design and fabrication of plasmonic sensors, detectors, photovoltaics, plasmon-based circuits, nanomanipulators, electro-optic plasmonic modulators, and atomic devices. We report on a quantitative comparison between near-field observation and numerical calculations, considering the intensity distribution for transverse magnetic (TM) and transverse electric (TE) polarizations, which are necessary for the construction of devices in all these areas. We present the near-field scanning microscopy (NSOM) results of surface plasmon polaritons (SPPs), excited by linearly polarized illumination on a gold, nanofabricated transmission grating. The optimization process is performed for infrared light for applications in cold-atom trapping and plasmonic sensing. We show the in situ processes of buildup and propagation of SPPs and confirm that the out of plane component of the electric field is not coupled to the aperture-type NSOM probe.

Building on quantum Monte Carlo simulations, we study the phase diagram of a one-parameter Hamiltonian interpolating between trivial and topological Ising paramagnets in two dimensions, which are dual to the toric code and the double semion. We discover an intermediate phase with stripe order which spontaneously breaks the protecting Ising symmetry. Remarkably, we find evidence that this intervening phase is gapless due to the incommensurability of the stripe pattern and that it is dual to a $U\left(1\right)$ gauge theory exhibiting Cantor deconfinement.

Chiral topological semimetals host multifold degenerate band crossing points under the protection of crystalline symmetries. In this paper, we suggest that the recently discovered chiral topological semimetals in space group 198, parts of which are superconducting upon doping, can be new candidates of time reversal invariant topological superconductors. By investigating the Fermi surfaces around the band crossing points that carry nonzero Chern numbers, we clarify how the nontrivial topology of chiral topological semimetals affects their superconducting state and show the existence of topological superconductivity in $s_{\pm }$-wave pairing with surface Majorana fermions. We further demonstrate that the topological superconductivity is favored by the interunit-cell phonon-mediated electron-electron interaction.

Motivated by the absence of experimental superconductivity in the metallic $\mathit{Pm}\overline{3}\mathit{n}$ phase of ${\mathrm{AlH}}_3$ despite the predictions, we reanalyze its vibrational and superconducting properties at pressures $P\ge 99$ GPa making use of first-principles techniques. In our calculations based on the self-consistent harmonic approximation method that treats anharmonicity beyond perturbation theory, we predict a strong anharmonic correction to the phonon spectra and demonstrate that the superconducting critical temperatures predicted in previous calculations based on the harmonic approximation are strongly suppressed by anharmonicity. The electron-phonon coupling concentrates on the lowest-energy hydrogen-character optical modes at the X point of the Brillouin zone. As a consequence of the strong anharmonic enhancement of their frequency, the electron-phonon coupling is suppressed by at least 30%. The suppression in $\lambda$ makes $T_c$ smaller than 4.2 K above 120 GPa, which is well consistent with the experimental evidence. Our results underline that metal hydrides with hydrogen atoms in interstitial sites are subject to huge anharmonic effects.

We report on a theoretical study of the spin current excited dynamics of domain walls (DWs) in ferrimagnets in the vicinity of the angular momentum compensation point. For a two-sublattice ferrimagnet effective Lagrangian and nonlinear dynamic equations are derived taking into account both the spin torques and the external magnetic field. The dynamics of the DW is calculated before and after the Walker breakdown for any direction of the spin current polarization. It is shown that for the in-plane polarization of the spin current, the DW mobility reaches a maximum near the temperature of the angular momentum compensation. On the contrary, for the out-of-plane spin polarization, the spin current with densities below the Walker breakdown does not excite DW dynamics. After overcoming the Walker breakdown, the domain wall velocity increases linearly with increasing current density. In this configuration of spin current polarization, the possibility of a gigahertz oscillation dynamics of the quasiantiferromagnetic vector under the action of a dampinglike torque at the angular momentum compensation point is demonstrated. Possible structures for experimental demonstration of the considered effects are discussed.

We perform a thorough first-principles study on superconductivity in yttrium carbide halide ${\mathrm{Y}}_2{X}_2{\mathrm{C}}_2$ ($X=\mathrm{Cl}$, Br, I) whose maximum transition temperature ($T_{\mathrm{c}}$) amounts to $\sim 10$ K. A detailed analysis on the optimized crystal structures reveals that the ${\mathrm{Y}}_2{\mathrm{C}}_2$ blocks are compressed uniaxially upon the halogen substitution from Cl, Br to I, contrary to the monotonic expansion of the lattice vectors. With a nonempirical method based on the density functional theory for superconductors within the conventional phonon mechanism, we successfully reproduce the halogen dependence of $T_{\mathrm{c}}$. An anomalously enhanced coupling of one ${\mathrm{C}}_2$ libration mode is observed in ${\mathrm{Y}}_2{\mathrm{I}}_2{\mathrm{C}}_2$, which implies a possible departure from the conventional pairing picture. Utilizing the Wannier representation of the electron-phonon coupling, we show that the halogen electronic orbitals and ionic vibrations scarcely contribute to the superconducting pairing. The halogen dependence of this system is hence an indirect effect of the halogen ions through the uniaxial compressive force on the superconducting ${\mathrm{Y}}_2{\mathrm{C}}_2$ blocks. We thus establish a quantitatively reliable picture of the superconducting physics of this system, extracting a unique effect of the atomic substitution which is potentially applicable to other superconductors.

The on-the-fly machine learning potential (MLP) generation scheme is combined with the multibaric-isothermal (MUBA) ensemble simulation to construct the MLP applicable for the prediction of the phase stability under various pressures. The MUBA simulation performs a random walk in the volume space and provides an efficient way to sample physically relevant configurations over a wide range of volume. In the present MUBA formulation, the explicit construction of a bias potential to realize a random walk is not required. The sample structures for training are dynamically collected by the simulation using the MLP itself, in which the simultaneous error estimation is utilized to judge whether an updated structure should be added to the sample data set or not. The utility of the method is demonstrated for aluminum nitride. The MUBA ensemble is sampled using the hybrid Monte Carlo (HMC) method at 300 K. Starting from the metastable zinc-blende structure, the simulation correctly reproduces experimentally observed two phases, the wurtzite and rocksalt structures. During 600 000 configuration updates in the HMC sweeps, the total number of density-functional theory (DFT) calculations required is only 107. The constructed MLP shows high accuracy comparable to the DFT calculations.

The gap generation in the dice model with local four-fermion interaction is studied. Due to the presence of two valleys with degenerate electron states, there are two main types of gaps. The intra and intervalley gap describes the electron and hole pairing in the same and different valleys, respectively. We found that while the generation of the intravalley gap takes place only in the supercritical regime, the intervalley gap is generated for an arbitrary small coupling. The physical reason for the absence of the critical coupling is the catalysis of the intervalley gap generation by the flat band in the electron spectrum of the dice model. The completely quenched kinetic energy in the flat band when integrated over momentum in the gap equation leads to extremely large intervalley gap proportional to the area of the Brillouin zone.

We investigate the functional renormalization group (FRG) flow of the two-particle vertex function of a model for x-ray absorption in metals. Concerning the appearance of logarithmic divergences, the model is prototypical for an important class of mostly zero- and one-dimensional systems which includes the Kondo model and the interacting one-dimensional Fermi gas. For our analysis, we formulate the FRG in the framework of the real-time zero-temperature formalism, in which the model was studied before with a parquet-based approach. We establish that a reasonably crafted, purely fermionic one-loop FRG approximation is fully equivalent on a detailed level to the leading-logarithmic parquet approximation. These two approximation schemes are thus found to just represent different perspectives on the same technical steps. This finding also reconfirms the traditional understanding of the capabilities of one-loop RG approximations for such models, which was recently put into question by an investigation of the x-ray-absorption model with multiloop FRG.

Twisting van der Waals heterostructures to induce correlated many body states provides a novel tuning mechanism in solid state physics. In this work, we theoretically investigate the fate of the surface Dirac cone of a three-dimensional topological insulator subject to a superlattice potential. Using a combination of diagrammatic perturbation theory, lattice model simulations, and ab initio calculations we elucidate the unique aspects of twisting a single Dirac cone with an induced moiré potential and the role of the bulk topology on the reconstructed surface band structure. We report a dramatic renormalization of the surface Dirac cone velocity as well as demonstrate a topological obstruction to the formation of isolated minibands. Due to the topological nature of the bulk, surface band gaps cannot open; instead additional satellite Dirac cones emerge, which can be highly anisotropic and made quite flat. We discuss the implications of our findings for future experiments.

The interplay of strain engineering and photon-assisted tunneling of electrons in graphene is considered for giving rise to atypical transport phenomena. The combination of uniaxial strain and a time-periodic potential barrier helps to control the particle transmission for a wide range of tunable parameters. With the use of the tight-biding approach, the elasticity theory, and the Floquet scattering, we find an angular shift of the transmission maximum in the sidebands for uniaxial strains breaking the mirror symmetry with respect to the normal incidence, which is called anomalous Floquet tunneling. This strain also modulates the transmission maximum in the sidebands and favors photoinduced currents. From Floquet scattering theory, we derive a generalized Snell's law for electrons in the presence of time-periodic potentials and uniaxial strain. We show that electron tunneling depends strongly on the barrier width, incident angle, uniaxial strain, and the tuning of the time-periodic potential parameters. An adequate modulation of the barrier width and oscillation amplitude serves to select the transmission in the sidebands. These findings can be useful for controlling the electron current through the photon-assisted tunneling being used in multiple nanotechnological applications.

The topological nodal-line semimetal (TNLS) $\mathrm{ZrSi}M$ $(M=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ is a promising platform to study the TNLS phase by tuning chalcogens. In this work, we study the evolution of the Fermi surface (FS) by tuning the Se/Te ratio in ${\mathrm{ZrSiSe}}_{1-x}{\mathrm{Te}}_x$ compounds. Transport properties and magnetometry results present signatures of Fermi surface topology change by the sudden changes in symmetry and carrier densities, as well as singularity of FSs at the critical chemical potentials. A $2\frac{1}{2}$-order Lifshitz transition occurs when 0.20 $\le x\le$ 0.33. Another $3\frac{1}{2}$-order electron topological transition is revealed by a large diamagnetic anomaly of susceptibility at $x\sim$ 0.80. In combination with first-principles calculations, this study reveals the vital roles of spin-orbit coupling, charge transfer, and shifts of the chemical potential in the evolution of FSs in this system. Our results demonstrate how the FSs evolve in ${\mathrm{ZrSiSe}}_{1-x}{\mathrm{Te}}_x$ compounds, providing fundamental clues for designing topological state switchable devices.