We calculate the superfluid weight and the polarization amplitude for the one-dimensional bosonic Hubbard model with focus on the strong-coupling regime via variational, exact diagonalization, and strong coupling calculations. Our variational approach is based on the Baeriswyl wave function, implemented via Monte Carlo sampling. We derive the superfluid weight appropriately in a variational setting. We emphasize the importance of implementing the Peierls phase in position space and to allow for many-body interference effects, rather than implementing the Peierls phase as single particle momentum shifts. At integer filling, the Baeriswyl wave function gives zero superfluid response at any coupling. At half filling our variational superfluid weight is in reasonable agreement with exact diagonalization results. We also calculate the polarization amplitude, the variance of the total position, and the associated size scaling exponent, which corroborate that this variational approach produces an insulating state at integer filling. Our Baeriswyl based variational method is applicable to significantly larger system sizes than exact diagonalization or quantum Monte Carlo.

Recent scanning tunneling microscopy experiments in NbN thin disordered superconducting films found an emergent inhomogeneity at the scale of tens of nanometers. This inhomogeneity is mirrored by an apparent dimensional crossover in the paraconductivity measured in transport above the superconducting critical temperature $T_c$. This behavior was interpreted in terms of an anomalous diffusion of fluctuating Cooper pairs that display a quasiconfinement (i.e., a slowing down of their diffusive dynamics) on length scales shorter than the inhomogeneity identified by tunneling experiments. Here, we assume this anomalous diffusive behavior of fluctuating Cooper pairs and calculate the effect of these fluctuations on the electron density of states above $T_c$. We find that the density of states is substantially suppressed up to temperatures well above $T_c$. This behavior, which is closely reminiscent of a pseudogap, only arises from the anomalous diffusion of fluctuating Cooper pairs in the absence of stable preformed pairs, setting the stage for an intermediate behavior between the two common paradigms in the superconducting-insulator transition, namely, the localization of Cooper pairs (the so-called bosonic scenario) and the breaking of Cooper pairs into unpaired electrons due to strong disorder (the so-called fermionic scenario).

We report the observation of antiferromagnetic (AFM) transitions in 1/1 and 2/1 quasicrystal approximants having slightly different electron-per-atom $\left(e/a\right)$ ratios, in the same Au-Ga-Eu system, at Néel temperatures of 7.0 and 8.5 K, respectively. This clearly demonstrates that an antiferromagnetic order survives when the degree of approximation to the quasicrystals is raised provided that the $e/a$ ratio is kept nearly the same. Since, unlike the 1/1 approximant, the 2/1 approximant has all the building blocks necessary for the construction of the quasicrystal, this finding can be a significant leap toward the long-sought realization of an AFM quasicrystal.

We study Bose-Einstein condensates at the low-energy limit and show that their collective dynamics exhibit interesting quantum dynamical behavior. The system undergoes a dynamical quantum phase transition after a sudden quench into a properly distributed static force, provided its dispersion relation is linear. We corroborate the occurrence of a dynamical phase transition by calculating the Fisher zeros of the Loschmidt amplitude and showing that they cross the real time axis in the thermodynamic limit. A connection is established between the order of nonanalycity in the return rate function and the spectral density of the force. Furthermore, it is shown that a logarithmic or power-law scaling holds at the critical times depending on the displacement spectrum. The scaling behaviors are studied for three different cases. Eventually, a scheme for the quantum simulation of such a dynamical phase transition and its verification is proposed. We show that by taking into account the environmental effects the behavior remains observable.

The recent discovery of superconductivity at high pressure in the two-leg ladder compounds ${\mathrm{BaFe}}_2{X}_3(X$= S, Se) started the novel field of quasi-one-dimensional iron-based superconductors. In this paper, we use density functional theory (DFT) to predict that the previously barely explored ladder compound ${\mathrm{RbFe}}_2{\mathrm{Te}}_3$ should be magnetic with a CX-type arrangement involving ferromagnetic rungs and antiferromagnetic legs, at the realistic density of $n=5.5$ electrons per iron. The magnetic state similarity with ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ suggests that ${\mathrm{RbFe}}_2{\mathrm{Te}}_3$ could also become superconducting under pressure. Moreover, at $n=6.0$ our DFT phase diagrams (with and without lattice tetramerization) reveal that the stable magnetic states could be either a $2\times 2$ magnetic block-type, as for $X$= Se, or a previously never observed before CY-type state, with ferromagnetic legs and antiferromagnetic rungs. In the Te-based studies, electrons are more localized than in S, implying that the degree of electronic correlation is enhanced for the Te case.

We explore the universal behaviors of a magnetic domain wall driven by the spin-transfer torque of an electrical current, in a ferromagnetic (Ga,Mn)(As,P) thin film with perpendicular magnetic anisotropy. For a current transverse to the domain wall, the dynamics of the thermally activated creep regime and the depinning transition are found to be compatible with a self-consistent universal description of magnetic-field-induced domain-wall dynamics. This common universal behavior, characteristic of the so-called quenched Edwards-Wilkinson universality class, is confirmed by an independent analysis of domain-wall roughness. Complementary investigations reveal the directional properties of interaction between current and domain walls which result in the instability of their transverse orientation.

FeSn is an itinerant antiferromagnet that hosts electronic Dirac states and ordered magnetic moments lying within its Fe Kagome-lattice planes. We present magnetization measurements of single crystals of $\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)\text{Sn}$, revealing the evolution and suppression of this magnetic order with Co substitution. We interpret the dramatic changes in magnetic anisotropy to indicate a reorientation of the moments from perpendicular to parallel to the hexagonal $c$ axis and confirm this with neutron diffraction. It has been proposed that the Dirac nodes observed in FeSn should become gapped if the moments rotate as our data suggests. We identify Co-substituted compositions that adopt both antiferromagnetic configurations at different temperatures. This system provides a unique opportunity to study how the details of magnetic order impact Dirac electron states.

Competition between superconducting and ferromagnetic ordering at interfaces between ferromagnets (F) and superconductors (S) gives rise to several proximity effects such as odd-triplet superconductivity and spin-polarized supercurrents. A prominent example of an S/F proximity effect is the spin switch effect (SSE) observed in S/F/N/F superconducting spin-valve multilayers, in which the superconducting transition temperature ${\mathrm{T}}_c$ is controlled by the angle $\varphi$ between the magnetic moments of the F layers separated by a nonmagnetic metallic spacer N. Here we present an experimental study of SSE in $\mathrm{Nb}/\mathrm{Co}/\mathrm{Cu}/\mathrm{Co}/{\mathrm{CoO}}_x$ nanowires measured as a function of bias current flowing in the plane of the layers. These measurements reveal an unexpected dependence of ${\mathrm{T}}_c\left(\varphi \right)$ on the bias current: ${\mathrm{T}}_c\left(\pi \right)$–${\mathrm{T}}_c\left(0\right)$ changes sign with increasing current bias. We attribute the origin of this bias dependence of the SSE to a spin Hall current flowing perpendicular to the plane of the multilayer, which suppresses ${\mathrm{T}}_c$ of the multilayer. The bias dependence of SSE can be important for hybrid F/S devices such as those used in cryogenic memory for superconducting computers as device dimensions are scaled down to the nanometer length scale.

The origin of the positive linear magnetoresistance (PLMR) effect at low temperatures in $\mathrm{CoFe}\left({\mathrm{V}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{Si}$ Heusler alloys is discussed. From anomalous dispersion x-ray diffraction measurements, we can clarify that the crystal structure of the epitaxial CoFeVSi films grown by molecular beam epitaxy (MBE) is $L{2}_1\mathrm{B}$ type. By employing the substitution of Mn for V in the CoFeVSi film in MBE conditions, we experimentally demonstrate homogeneous $\mathrm{CoFe}\left({\mathrm{V}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{Si}$ films and systematic control of the magnetic moments. The PLMR effect gradually decreases with increasing Mn contents but it can be seen even for $\mathrm{CoFe}\left({\mathrm{V}}_{0.5}{\mathrm{Mn}}_{0.5}\right)\mathrm{Si}$. We note that the PLMR effect disappears for $\mathrm{CoFe}\left({\mathrm{V}}_{0.25}{\mathrm{Mn}}_{0.75}\right)\mathrm{Si}$. According to theoretical calculations of the electronic band structures of $\mathrm{CoFe}\left({\mathrm{V}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{Si}$, the gaplesslike electronic band structure due to the V-$d$ state in the minority spins of CoFeVSi is varied to the band gap by increasing the substitution of Mn. This means that the electronic band structure near the Fermi level in $\mathrm{CoFe}\left({\mathrm{V}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{Si}$ can be experimentally controlled by substituting Mn for V. From these considerations, we can conclude that the PLMR effect in $\mathrm{CoFe}\left({\mathrm{V}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{Si}$ is caused by the presence of the V state-induced gaplesslike structure in the minority spin electronic band near the Fermi level.

A typical $f$-electron Kondo lattice system Ce exhibits a well-known isostructural transition, the so-called $\gamma -\alpha$ transition, accompanied by an enormous volume collapse. Most interestingly, we have discovered that a topological phase transition also takes place in elemental Ce, concurrently with the $\gamma -\alpha$ transition. Based on the dynamical mean-field theory approach combined with density functional theory, we have unraveled that the nontrivial topology in $\alpha$-Ce is driven by the $f-d$ band inversion, which arises from the formation of a coherent $4f$ band around the Fermi level. We captured the formation of the $4f$ quasiparticle band that is responsible for the Lifshitz transition and the nontrivial ${\mathrm{Z}}_2$ topology establishment across the phase boundary. This discovery provides a concept of a “topology switch” for topological Kondo systems. The “on” and “off” switching knob in Ce is versatile in a sense that it is controlled by the available pressure ($\lesssim 1$ GPa) at room temperature.

We investigate the transport properties in an 8-$Pmmn$ borophene-based $n\text{−}p\text{−}n$ junction. The valley-dependent electron retroreflection is generated by changing the direction of junction, which breaks up the conventional wisdom that only the electron specular reflection occurs at an interface. Furthermore, the valley-dependent anomalous Klein tunneling, i.e., the perfect transmission at a nonzero incident angle of electron regardless of the width and height of potential barrier, is found. Interestingly, this anomalous Klein tunneling depends on the direction of junction and the tilted velocity. These two findings arise from the same fact that the anisotropic band structure, induced by tilted and anisotropic velocities, leads to the release of locking relation between the transverse wave vector and velocity component along the interface. Our work gives insight into the understanding of the electron retroreflection and the Klein tunneling.

We study the energy spectrum of Weyl fermions in a topological semimetal in the presence of a central charged impurity. When the strength of the charge exceeds a critical value, an infinite family of quasibound states appear that show characteristic signatures of discrete scale invariance. The invariance is broken by the presence of screening or a finite mass. If these terms are small enough the quasibound states still exist, but there can be perturbations of the shallowest quasibound states. These special quasibound states manifest themselves in the local density of states as well as the relative phase, which can be observed by scanning tunneling microscopy. Such effects that are usually associated with the relativistic quantum electrodynamics of strong fields become observable in topological materials.

We study the competition between stripe states with different periods and a uniform $d$-wave superconducting state in the extended two-dimensional Hubbard model at 1/8 hole doping using infinite projected entangled-pair states (iPEPS). With increasing strength of negative next-nearest neighbor hopping $t^{\prime }$, the preferred period of the stripe decreases. For the values of $t^{\prime }$ predicted for cuprate high-$T_c$ superconductors, we find stripes with a period 4 in the charge order, in agreement with experiments. Superconductivity in the period 4 stripe is suppressed at $1/8$ doping. Only at larger doping, $0.18\lesssim \delta <0.25$, the period 4 stripe exhibits coexisting $d$-wave superconducting order. The uniform $d$-wave state is only favored for sufficiently large positive $t^{\prime }$.

The recently proposed density functional theory for steady-state transport (i-DFT) is extended to include temperature gradients between the leads. Within this framework, a general and exact expression is derived for the linear Seebeck coefficient which can be written as the sum of the Kohn-Sham coefficient and an exchange-correlation contribution. The formalism is applied to the single-impurity Anderson model for which approximate exchange-correlation functionals are suggested for temperatures both above and below the Kondo temperature. A certain structural property of the exchange-correlation potentials in the Coulomb blockade regime allows us to recover an earlier result expressing the Seebeck coefficient in terms of quantities of equilibrium density functional theory. The numerical i-DFT results are compared to calculations with the numerical renormalization group over a wide range of temperatures finding a reasonable agreement while i-DFT comes at a much lower computational cost.

The single-particle eigenstates of the Aubry-André-Harper model are known to show a delocalization-localization transition at a finite strength of the quasiperiodic disorder. In this work, we point out that an intimate relationship exists between the subband structure of the spectrum and transport properties of the model. To capture the transport properties, we have not only used a variety of single-particle measures like inverse participation ratio, and von Neumann entropy, but also many-particle measures such as persistent current and its variance, and many-body entanglement entropy. The many-particle measures are very sensitive to the subband structure of the spectrum. Even in the delocalized phase, surprisingly the entanglement entropy is substantially suppressed when the Fermi level is in the band gaps, whereas the persistent current is vanishingly small for the same locations of the Fermi level. The entanglement entropy seems to follow area law exclusively for these special locations of Fermi level or filling fractions of free fermions. A study of the standard deviation of persistent current offers further distinguishing features for the special fillings. In the delocalized phase, the standard deviation vs mean persistent current curves are discontinuous for the nonspecial values of filling fractions and continuous (closed) for the special values of filling fractions, whereas in the localized phase, these curves become straight lines for both types of filling fractions. We have also discussed how the results depend on the system size. Our results, especially on the persistent current, can potentially be tested experimentally using the present day setups based on ultracold atoms.

Transition metal oxides are considered promising thermoelectric materials for harvesting high-temperature waste heat due to their chemical and thermal stability, abundance, and low toxicity. Despite their typically strong ionic character, they can exhibit surprisingly high power factors $\sigma S^2$, as in $n$-type ${\mathrm{SrTiO}}_3$ for instance. Thus, it is worth examining other transition metal oxides that might equal or even surpass the performances of strontium titanate. This theoretical paper investigates the thermoelectric properties of $n$-type rutile ${\mathrm{TiO}}_2$, which is the most stable phase of titanium oxide up to $2000\mathrm{K}$. The electronic structure is obtained through density functional theory calculations, while the prominent features of strong electron-phonon interaction and defects states are modeled using a small number of parameters. The thermoelectric transport properties are computed by solving the Boltzmann transport equation with the relaxation time approximation. The theoretical results are compared with a wealth of experimental data from the literature, yielding very good agreements over a wide range of carrier concentrations. This validates the hypothesis of band conduction in rutile ${\mathrm{TiO}}_2$ and allows the prediction of the high-temperature thermoelectric properties.

Multiterminal multidot devices have been put forward as versatile and high-performing setups for thermoelectric energy harvesting at the nanoscale. With a technique that encompasses and overtakes several of the usual theoretical tools used in this context, we explore a three-terminal Coulomb-coupled-dot device for refrigeration purposes. The refrigerator is monitored by either a voltage or a thermal bias. This comparative study shows that the heat-driven refrigerator is underperforming relative to the power-driven one, due to scarce on-dot charge fluctuations.

Direct current in clean semiconductors and metals was recently shown to obey the laws of hydrodynamics in a broad range of temperatures and sample dimensions. However, the determination of frequency window for hydrodynamic phenomena remains challenging. Here, we reveal a phenomenon being a hallmark of high-frequency hydrodynamic transport, the Cerenkov emission of plasmons by drifting Dirac electrons. The effect appears in a hydrodynamic regime only due to reduction of plasmon velocity by electron-electron collisions below the velocity of carrier drift. To characterize the Cerenkov effect quantitatively, we analytically find the high-frequency nonlocal conductivity of drifting Dirac electrons across the hydrodynamic-to-ballistic crossover. We find the growth rates of hydrodynamic plasmon instabilities in two experimentally relevant setups: parallel graphene layers and graphene covered by subwavelength grating, further showing their absence in ballistic regime. We argue that the possibility of Cerenkov emission is linked to singular structure of nonlocal conductivity of Dirac materials and is independent on specific dielectric environment.

Topological states require the presence of extended bulk states, as usually found in the picture of energy bands and topological states bridging the bulk gaps. But in driven systems this can be circumvented, and one can get topological states coexisting with fully localized bulk states, as in the case of the anomalous Floquet-Anderson insulator. Here, we show the fingerprints of this peculiar topological phase in the transport properties and their dependence on the disorder strength, geometrical configuration (two-terminal and multiterminal setups), and details of the driving protocol.

The single-dot photoluminescence properties of $\sim 7\mathrm{nm}$ perovskite $\mathrm{MAPb}{\mathrm{I}}_3\left(\mathrm{MA}=\mathrm{C}{\mathrm{H}}_3\mathrm{N}{{\mathrm{H}}_3}^{+}\right)$ nanocrystals (NCs) were investigated in the $5\text{–}295\mathrm{K}$ temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at $140\text{–}160\mathrm{K}$, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of $\sim 0.6\mathrm{meV}$ was measured, which is the narrowest ever recorded for this perovskite material system. A $\sim 4.0\mathrm{meV}$ phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of $\mathrm{M}{\mathrm{A}}^{+}$ leads to broader spectra than for $\mathrm{C}{\mathrm{s}}^{+}$ or $\mathrm{F}{\mathrm{A}}^{+}$ containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to $\mathrm{C}{\mathrm{s}}^{+}$ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.