We obtain a complete and numerically exact in the weak-coupling limit $\left(U\to 0\right)$ ground-state phase diagram of the repulsive fermionic Hubbard model on the square lattice for filling factors $0<n<2$ and next-nearest-neighbor hopping amplitudes $0\le t^{\prime }\le 0.5$. Phases are distinguished by the symmetry and the number of nodes of the superfluid order parameter. The phase diagram is richer than may be expected and typically features states with a high—higher than that of the fundamental mode of the corresponding irreducible representation—number of nodes. The effective coupling strength in the Cooper channel $\lambda$, which determines the critical temperature $T_c$ of the superfluid transition, is calculated in the whole parameter space and regions with high values of $\lambda$ are identified. It is shown that besides the expected increase of $\lambda$ near the Van Hove singularity line, joining the ferromagnetic and antiferromagnetic points, another region with high values of $\lambda$ can be found at quarter filling and $t^{\prime }=0.5$ due to the presence of a line of nesting at $t^{\prime }\ge 0.5$. The results can serve as benchmarks for controlled nonperturbative methods and guide the ongoing search for high-$T_c$ superconductivity in the Hubbard model.

Weyl semimetals typically appear in systems in which either time-reversal ($T$) or inversion ($P$) symmetry is broken. Here we show that in the presence of gauge potentials these topological states of matter can also arise in fermionic lattices preserving both $T$ and $P$. We analyze in detail the case of a cubic lattice model with $\pi$ fluxes, discussing the role of gauge symmetries in the formation of Weyl points and the difference between the physical and the canonical $T$ and $P$ symmetries. We examine the robustness of this $PT$-invariant Weyl semimetal phase against perturbations that remove the chiral sublattice symmetries, and we discuss further generalizations. Finally, motivated by advances in ultracold-atom experiments and by the possibility of using synthetic magnetic fields, we study the effect of random perturbations of the magnetic fluxes, which can be compared to a local disorder in realistic scenarios.

Heterogeneous atomic magnetic chains are built by atom manipulation on a ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}$ (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the ${\mathrm{Cu}}_2\mathrm{N}$ surface. Here, we present results for ${\mathrm{FeMn}}_x$ chains $\left(x=1–6\right)$ emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements $J$ for different chains. For the shorter chains, $x\le 2$, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy $D$ and transversal-anisotropy $E$ constants. These parameters together with the values for $J$ allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data.

We study the laser cooling of a mechanical oscillator through the coupling with a dissipative three-level system. Under a background temperature beyond the Lamb-Dicke regime, we extend the standard cooling analysis by separately studying the classical motion and the quantum dynamics of the oscillator. In ladder-system cooling, the cooling rate degrades by orders of magnitude at large classical motion. This phenomenon causes a critical transition of the final temperature at a hot background. In stark contrast, electromagnetic-induced-transparency (EIT) cooling with a $\mathrm{\Lambda }$ system produces significant negative cooling rate at high motional excitation. At steady state, the oscillator could exhibit both cooling and lasing behaviors. We argue that a successful EIT cooling requires either a poor quality oscillator to suppress the lasing effect, or terminating the cooling process at a transient stage.

Skyrmions and domain walls are significant spin textures of great technological relevance to magnetic memory and logic applications, where they can be used as carriers of information. The unique topology of skyrmions makes them display emergent dynamical properties as compared with domain walls. Some studies have demonstrated that the two topologically inequivalent magnetic objects could be interconverted by using cleverly designed geometric structures. Here, we numerically address the skyrmion domain wall collision in a magnetic racetrack by introducing relative motion between the two objects based on a specially designed junction. An electric current serves as the driving force that moves a skyrmion toward a trapped domain wall pair. We see different types of collision dynamics depending on the driving parameters. Most importantly, the modulation of skyrmion transport using domain walls is realized in this system, allowing a set of domain wall-gated logical NOT, NAND, and NOR gates to be constructed. This work provides a skyrmion-based spin-logic architecture that is fully compatible with racetrack memories.

It has been recently reported that a superconducting triplet spin valve (TSV) based on the half-metallic ${\mathrm{CrO}}_2$ can show “colossal” variations of the critical temperature, up to more than 1 K. This can be achieved when the magnetic noncollinearity between the mixer $\left({\text{F}}_1\right)$ and the drainage ferromagnetic layer (F) is maximized. In this work we investigate further such TSV devices looking at two aspects: first, we present the dependence of the TSV effect on the thickness of the mixer layer; second, we look at the perpendicular upper critical field $H_{c2\perp }$ as a function of the temperature. The thickness dependence, which is nonmonotonic as expected, represents a further proof that the effect is due to the generation of equal-spin triplet Cooper pairs, while what we observe for the $H_{c2\perp }$ versus $T$ curves is an interesting and peculiar behavior: there is a clear deviation from the universal linear dependence and the average slope is suppressed much more than what can be described with the formalism used for conventional proximized structures.

We report oscillatory magnetoresistance in various superconducting films, with a magnetic-field period $\mathrm{\Delta }B\sim 0.1$ T that is essentially independent of sample dimensions, temperature, transport current, and the magnitude and orientation of the magnetic field, including magnetic fields oriented parallel to the film plane. The characteristics of these oscillations seem hard to reconcile with previously established mechanisms for oscillations in magnetoresistance, suggesting the possibility of another type of physical origin.

Exploring magnetism and magnetic anisotropy in otherwise nonmagnetic two-dimensional materials, such as graphene and transition metal dichalcogenides, is at the heart of spintronics research. Herein, using first-principles calculations we explore the possibility of reaching an atomic-scale perpendicular magnetic anisotropy by carefully exploring the large spin-orbit coupling, orbital magnetism, and ligand field in a suitable choice of a two-dimensional structure with transition metal adatoms. More specifically, we demonstrate perpendicular magnetic anisotropy energies up to an order of 100 meV per atom in individual ruthenium and osmium adatoms at a monosulfur vacancy in molybdenum disulfide. We further propose a phenomenological model where a spin state transition that involves hybridization between molybdenum $a_1$ and adatomic $e^{\prime }$ orbitals is a possible mechanism for magnetization reversal from an in-plane to perpendicular orientation.

We calculate electronic energy transport in inhomogeneous superconductors using a fully self-consistent nonequilibrium quasiclassical Keldysh approach. We develop a general theory and apply it to a superconductor with an order parameter that forms domain walls of the type encountered in the Fulde-Ferrell-Larkin-Ovchinnikov state. The heat transport in the presence of a domain wall is inherently anisotropic and nonlocal. The bound states in the nonuniform region play a crucial role and control heat transport in several ways: (i) they modify the spectrum of quasiparticle states and result in Andreev reflection processes and (ii) they hybridize with the impurity band and produce a local transport environment with properties very different from those in a uniform superconductor. As a result of this interplay, heat transport becomes highly sensitive to temperature, magnetic field, and disorder. For strongly scattering impurities, we find that the transport across domain walls at low temperatures is considerably more efficient than in the uniform superconducting state.

We theoretically study a finite-size $S{F}_{1}N{F}_2$ spin valve, where a normal metal ($N$) insert separates a thin standard ferromagnet ($F_1$) and a thick half-metallic ferromagnet ($F_2$). For sufficiently thin superconductor ($S$) widths close to the coherence length ${\xi }_0$, we find that changes to the relative magnetization orientations in the ferromagnets can result in substantial variations in the transition temperature $T_c$, consistent with experimental results [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Our results demonstrate that, in good agreement with the experiment, the variations are largest in the case where $F_2$ is in a half-metallic phase and thus supports only one spin direction. To pinpoint the origins of this strong spin-valve effect, both the equal-spin $f_1$ and opposite-spin $f_0$ triplet correlations are calculated using a self-consistent microscopic technique. We find that when the magnetization in $F_1$ is tilted slightly out of plane, the $f_1$ component can be the dominant triplet component in the superconductor. The coupling between the two ferromagnets is discussed in terms of the underlying spin currents present in the system. We go further and show that the zero-energy peaks of the local density of states probed on the $S$ side of the valve can be another signature of the presence of superconducting triplet correlations. Our findings reveal that for sufficiently thin $S$ layers, the zero-energy peak at the $S$ side can be larger than its counterpart in the $F_2$ side.

We present the results of ${}^{13}\mathrm{C}$ NMR experiments in an organic superconductor with localized Fe spins ${\beta }^{″}-{\left(\text{BEDT-}\mathrm{TTF}\right)}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)\mathrm{Fe}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]·{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Br}$. We reveal the antiferromagnetic coupling between Fe $d$ spins and $\pi$ spins, which creates an exchange field antiparallel to the external field direction at the $\pi$ electrons. In addition to the static effects of Fe spins, we show from the nuclear spin-lattice relaxation rate measurement that the magnetic fluctuations generated by Fe spins are suppressed at low temperatures and high magnetic fields. These conditions are suitable to stabilize the field-induced superconductivity by the field compensation mechanism. After the suppression of Fe-spin dynamics by a magnetic field of 19 T, we observed the underlying $\pi$-electron contribution. We discuss a possible anomaly in the $\pi$-electron system.

We numerically study the fractional quantum Hall effect at filling factors $\nu =12/5$ and 13/5 (the particle-hole conjugate of 12/5) in high-quality two-dimensional GaAs heterostructures via exact diagonalization including finite well width and Landau-level mixing. We find that Landau-level mixing suppresses the $\nu =13/5$ fractional quantum Hall effect relative to $\nu =12/5$. By contrast, we find both $\nu =2/5$ and (its particle-hole conjugate) $\nu =3/5$ fractional quantum Hall effects in the lowest Landau level to be robust under Landau-level mixing and finite well-width corrections. Our results provide a possible explanation for the experimental absence of the 13/5 fractional quantum Hall state as caused by Landau-level mixing effects.

The fact that in organic semiconductors the Hubbard energy is usually positive appears to be at variance with a bipolaron model to explain magnetoresistance (MR) in those systems. Employing percolation theory, we demonstrate that a moderately positive $U$ is indeed compatible with the bipolaron concept for MR in unipolar current flow, provided that the system is energetically disordered, and the density of states (DOS) distribution is partially filled, so that the Fermi level overlaps with tail states of the DOS. By exploring a broad parameter space, we show that MR becomes maximal around $U=0$ and even diminishes at large negative values of $U$ because of spin independent bipolaron dissociation. Trapping effects and reduced dimension enhance MR.

The generation of a quantum fluid of dressed photons at room temperature is experimentally demonstrated in an InGaN microcavity which is divided into two- and one-dimensional sections, resulting in single- and switchable multilevel coherent light emission. Ultra-low-threshold operation is attributed to the slight but robust excitonic fraction of the photonic condensate representing a bosonic laser working below the Mott transition (polariton laser). In contrast to equilibrium Bose-Einstein condensates, the nonequilibrium driven-dissipative nature enables the population of higher orbitals if any confinement potential is present to induce enhanced quantum correlations. Trapping inside microwire spacers leads to a polariton harmonic oscillator resulting in discrete states in an equidistant ladder of photonic orbitals. Level occupation and selection of a specific wave function is managed via optical control, mimicking a quantum emitter on a macroscopic level. It shows that exotic states of matter can be realized in rather simple structures at room temperature directly visible to the human eye. It represents also an excellent opportunity to study basic many-body dynamics in one-dimensional bosonic matter by simultaneously settling an optimized fabrication technique for devices enabling practical Boolean quantum logic gates for optical computing.

We investigate theoretically and numerically the coupling between elastic and localized surface plasmon modes in a system of gold nanocylinders separated from a thin gold film by a dielectric spacer of few nanometers thickness. That system supports plasmon modes confined in between the bottom of the nanocylinder and the top of the gold film, which arise from the formation of interference patterns by short-wavelength metal-insulator-metal propagating plasmon. First, we present the plasmonic properties of the system though computer-simulated extinction spectra and field maps associated to the different optical modes. Next, a simple analytical model is introduced, which allows to correctly reproduce the shape and wavelengths of the plasmon modes. This model is used to investigate the efficiency of the coupling between an elastic deformation and the plasmonic modes. In the last part of the paper, we present the full numerical simulations of the elastic properties of the system, and then compute the acousto-plasmonic coupling between the different plasmon modes and five acoustic modes of very different shape. The efficiency of the coupling is assessed first by evaluating the modulation of the resonance wavelength, which allows comparison with the analytical model, and finally in term of time-modulation of the transmission spectra on the full visible range, computed for realistic values of the deformation of the nanoparticle.

The friction coefficient relates friction forces to normal loads and plays a key role in fundamental and applied areas of science and technology. Despite its importance, the relationship between the friction coefficient and the properties of the materials forming a sliding contact is poorly understood. We illustrate how simple relationships regarding the changes in energy that occur during slip can be used to develop a quantitative model relating the friction coefficient to atomic-level features of the contact. The slip event is considered as an activated process and the load dependence of the slip energy barrier is approximated with a Taylor series expansion of the corresponding energies with respect to load. The resulting expression for the load-dependent slip energy barrier is incorporated in the Prandtl-Tomlinson (PT) model and a shear-based model to obtain expressions for friction coefficient. The results indicate that the shear-based model reproduces the static friction coefficients ${\mu }_s$ obtained from first-principles molecular dynamics simulations more accurately than the PT model. The ability of the model to provide atomistic explanations for differences in ${\mu }_s$ amongst different contacts is also illustrated. As a whole, the model is able to account for fundamental atomic-level features of ${\mu }_s$, explain the differences in ${\mu }_s$ for different materials based on their properties, and might be also used in guiding the development of contacts with desired values of ${\mu }_s$.

Ground-state properties of the Hubbard model on a two-dimensional square lattice are studied by the auxiliary-field quantum Monte Carlo method. Accurate results for energy, double occupancy, effective hopping, magnetization, and momentum distribution are calculated for interaction strengths of $U/t$ from 2 to 8, for a range of densities including half-filling and $n=0.3,0.5,0.6,0.75$, and $0.875$. At half-filling, the results are numerically exact. Away from half-filling, the constrained path Monte Carlo method is employed to control the sign problem. Our results are obtained with several advances in the computational algorithm, which are described in detail. We discuss the advantages of generalized Hartree-Fock trial wave functions and its connection to pairing wave functions, as well as the interplay with different forms of Hubbard-Stratonovich decompositions. We study the use of different twist angle sets when applying the twist averaged boundary conditions. We propose the use of quasirandom sequences, which improves the convergence to the thermodynamic limit over pseudorandom and other sequences. With it and a careful finite size scaling analysis, we are able to obtain accurate values of ground-state properties in the thermodynamic limit. Detailed results for finite-sized systems up to $16\times 16$ are also provided for benchmark purposes.

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean-field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT transition in two-dimensional inhomogeneous superconductors.

Quantized Hall conductance and de Haas–van Alphen (dHvA) oscillation are studied theoretically in the tight-binding model for ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{NO}}_3$, in which there are small pockets of electrons and holes due to the periodic potentials of anion ordering in the $a$ direction. The magnetic field is treated by hoppings as complex numbers due to the phase caused by the vector potential, i.e., Peierls substitution. In realistic values of parameters and the magnetic field, the energy as a function of the magnetic field (Hofstadter butterfly diagram) is obtained. It is shown that the energy levels are broadened and the gaps are closed or almost closed periodically as a function of the inverse magnetic field, which is not seen in the semiclassical theory of the magnetic breakdown. The Hall conductance is quantized with an integer obtained by the Diophantine equation when the chemical potential lies in an energy gap. When electrons or holes are doped in this system, the Hall conductance is quantized in some regions of a magnetic field but it is not quantized in other regions of a magnetic field due to the broadening of the Landau levels. The amplitude of the dHvA oscillation at zero temperature decreases as the magnetic field increases, while it is constant in the semiclassical Lifshitz Kosevich formula.