Electronic transport through a material depends on the response to local perturbations induced by defects or impurities in the material. The scattering processes can be described in terms of phase shifts and corresponding cross sections. The multiorbital nature of the spinor states in transition metal dichalcogenides would naturally suggest the consideration of a massive Dirac equation to describe the problem, while the parabolic dispersion of its conduction and valence bands would invite a simpler Schrödinger equation description. Here, we contrast the scattering of massive Dirac particles and Schrödinger electrons, in order to assess different asymptotic regimes (low and high Fermi energy) for each one of the electronic models and describe their regime of validity or transition. At low energies, where the dispersion is approximately parabolic, the scattering processes are dominated by low angular momentum channels, which results in nearly isotropic scattering amplitudes. On the other hand, the differential cross section at high Fermi energies exhibits clear signatures of the linear band dispersion, as the partial phase shifts approach a nonzero value. We analyze the electronic dynamics by presenting differential cross sections for both attractive and repulsive scattering centers. The dissimilar behavior between Dirac and Schrödinger carriers points to the limits and conditions over which different descriptions are required for the reliable treatment of scattering processes in these materials.

Boundary-driven quantum spin chains are paradigmatic nonequilibrium systems featuring the presence of particle currents. In general, it may not be possible to distinguish an incoherent type of particle transport from a truly quantum coherent one through monitoring the mean current, as both ballistic as well as diffusive regimes occur in either setting. Here, we show that genuine coherent features become manifest in large fluctuations which allow a discrimination between incoherent and coherent quantum transport: in the former case, realizations that are characterized by atypically large boundary activity are associated with larger than typical currents, i.e., an enhanced number of events at the boundaries goes together with a large current. Conversely, in the coherent case the Zeno effect leads to the suppression of current in trajectories with large activity at the boundary. We analyze how these different dynamical regimes are reflected in the structure of rare fluctuations. We show moreover that realizations supporting a large current are generated via weak long-range correlations within the spin chain, typically associated with hyperuniformity. We further observe critical time-coexistence behaviors with intermittent currents in rare fluctuations of the strongly interacting XXZ chain for completely asymmetric drivings.

A mechanical analog to the quantum spin Hall insulator (QSHI) is implemented into a simple mass-spring kagome lattice. The transition from the trivial state to the topological one is described by an invariant Chern number function of a contrast parameter. The band diagram and helical edge states characteristic of QSHI are obtained by a combination of numerical and analytical methods. In particular, these states are shown to be Stoneley wave solutions to a set of asymptotic continuous motion equations. Last, scatterless propagation of polarized topological edge waves around sharp corners is demonstrated and robustness is assessed through a parametric study.

We have determined the superconducting and magnetic properties of four samples of ${\mathrm{Lu}}_x{\mathrm{Zr}}_{1-x}{\mathrm{B}}_{12}$ ($x=0.04$, 0.07, 0.17, and 0.8) using muon spin rotation ($\mu \mathrm{SR}$) and magnetometry measurements. We observed a strong magnetic signal in both the $\mu \mathrm{SR}$ and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $\approx 1{\mu }_{\mathrm{B}}$ due to a clustering effect of the ${\mathrm{Lu}}^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $\mu \mathrm{SR}$ data thought to originate from boron nuclei in the ${\mathrm{B}}_{12}$ cages. The superconductivity is found to evolve with $x$, with a decrease in $x$ resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find evidence for the formation of nodes in the superconducting gap for $x\le 0.17$, providing a potential new example of an $s$-to-$s+d$-wave crossover in a superconductor.

Over the past two decades, the magnetic ground states of all rare-earth titanate pyrochlores have been extensively studied, with the exception of ${\mathrm{Sm}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. This is, in large part, due to the very high absorption cross section of naturally occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically enriched single crystal of ${\mathrm{Sm}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Using inelastic neutron scattering, we determine that the crystal field ground state for ${\mathrm{Sm}}^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that ${\mathrm{Sm}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ orders into the all-in, all-out magnetic structure with an ordered moment of $0.44\left(7\right){\mu }_B$ below $T_N=0.35$ K, consistent with expectations for antiferromagnetically coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the ${\mathrm{Sm}}^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

The modification of the magnetization dissipation or Gilbert damping caused by an inhomogeneous magnetic structure and expressed in terms of a wave vector dependent tensor $\underline{\alpha }\left(\vec{q}\right)$ is investigated by means of linear response theory. A corresponding expression for $\underline{\alpha }\left(\vec{q}\right)$ in terms of the electronic Green function has been developed giving in particular the leading contributions to the Gilbert damping linear and quadratic in $q$. Numerical results for realistic systems are presented that have been obtained by implementing the scheme within the framework of the fully relativistic KKR (Korringa-Kohn-Rostoker) band structure method. Using the multilayered system ($\mathrm{Cu}/{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{/\mathrm{Pt})}_n$ as an example for systems without inversion symmetry we demonstrate the occurrence of nonvanishing linear contributions. For the alloy system bcc ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ having inversion symmetry, on the other hand, only the quadratic contribution is nonzero. As it is shown, this quadratic contribution does not vanish even if the spin-orbit coupling is suppressed, i.e., it is a direct consequence of the noncollinear spin configuration.

Finding efficient methods for controlling entanglement is an important issue in quantum information processing. Here, we inspect theoretically possible ways for tuning orbital and spin entanglement of two interacting particles confined to a double quantum dot with spin-orbit coupling (SOC) which renders possible the coupling of spin to an external electric field. The entanglement in the spin part is sensitive to a transverse electric field. We show that through the field-induced SOC the entanglement of the spin subsystem can be slightly increased; the effect of a longitudinal electric field is more subtle: For a small interdot distance, a longitudinal electric field enhances substantially the entanglement of spins, while for a large interdot distance it has no impact. An external longitudinal electric field has a minor effect on the entanglement in the orbital part irrespective of the interdot distance. We trace back this observed phenomenon to a combined effect of the applied external electric field and the overlap of wave functions which is enhanced for decreasing interdot distances. As a result, at large interdot separation, the concurrence of the two-electron states turns insensitive to the applied electric field.

We study the quantum hydrodynamical features of exciton polaritons flowing circularly in a ring-shaped geometry. We consider a resonant-excitation scheme in which the spinor polariton fluid is set into motion in both components by spin-to-orbital angular momentum conversion. We show that this scheme allows us to control the winding number of the fluid and to create two circulating states differing by two units of the angular momentum. We then consider the effect of a disorder potential, which is always present in realistic nanostructures. We discuss how a smooth disorder can be efficiently screened by the polariton-polariton interactions, yielding a signature of polariton superfluidity. This effect is reminiscent of supercurrent in a superconducting loop.

We investigate the anisotropic superconducting and magnetic properties of single-crystal ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of $T_m=15\mathrm{K}$ and a superconducting transition temperature of $T_c=36.8\mathrm{K}$. The superconducting phase diagram is characterized by high upper critical field slopes of $-70$ and $-42$ kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $\mathrm{\Gamma }=1.7$. Ginzburg-Landau parameters of ${\kappa }_c\sim 67$ and ${\kappa }_{ab}\sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu magnetism is quasi-two dimensional (2D), as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<0.04$ K. A consequence of the quasi-2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 and 4 kG.

Long wavelength descriptions of a half-filled lowest Landau level ($\nu =1/2$) must be consistent with the experimental observation of particle-hole (PH) symmetry. The traditional description of the $\nu =1/2$ state pioneered by Halperin, Lee, and Read (HLR) naively appears to break PH symmetry. However, recent studies have shown that the HLR theory with weak quenched disorder can exhibit an emergent PH symmetry. We find that such inhomogeneous configurations of the $\nu =1/2$ fluid, when described by HLR mean-field theory, are tuned to a topological phase transition between an integer quantum Hall state and an insulator of composite fermions with a dc Hall conductivity ${\sigma }_{xy}^{\left(\mathrm{cf}\right)}=-\frac{1}{2}\frac{e^2}{h}$. Our observations help explain why the HLR theory exhibits PH symmetric dc response.

A prime characterization of many-body localized (MBL) systems is the entanglement of their eigenstates; in contrast to the typical ergodic phase whose eigenstates are volume law, MBL eigenstates obey an area law. In this paper, we show that a spin-disordered Hubbard model has both a large number of area-law eigenstates as well as a large number of eigenstates whose entanglement scales logarithmically with system size (log-law). This model provides a microscopic Hamiltonian which is neither ergodic nor MBL. We establish these results through a combination of analytic arguments based on the eta-pairing operators combined with a numerical analysis of eigenstates. In addition, we describe and simulate a dynamic time evolution approach starting from product states through which one can separately probe the area-law and log-law eigenstates in this system.

We study Cooper pairing in the Dirac composite fermion (CF) system. The presence of the mass term in the Dirac CF description (which may simulate Landau level mixing), i.e., breaking of particle-hole (PH) symmetry in this system, is a necessary condition for the existence of a PH Pfaffian-like topological state. In the scope of the random-phase approximation (RPA) and hydrodynamic approach, we find some signatures of pairing at finite frequencies. Motivated by this insight, we extend our analysis to the case of a different but still Dirac quasiparticle (CF) representation, appropriate in the presence of a mass term, and discuss the likelihood of PH Pfaffian and Pfaffian pairings in general. On the basis of gauge field effects, we find for a small Dirac mass, an anti-Pfaffian or Pfaffian instability depending on the sign of mass, while for large mass (Landau level mixing), irrespective of its sign, we find a PH Pfaffian-like instability.

We perform ultrafast pump-probe spectroscopy of plasmonic structures beyond the perturbative excitation regime. As a prototypical system we choose gold nanorods dispersed in water, which display both longitudinal and transverse plasmon resonances. A broadband investigation reveals a complex scenario for the saturation of the transient optical response as a function of the pump fluence, with strong dependence on the probe wavelength. In particular, we observed stronger saturation effects for the high energy transverse plasmonic resonance as compared to the low energy longitudinal one. This behavior is well captured by a three-temperature model and is understood in terms of the nonlinearity of the Fermi-smearing mechanism, presiding over the optical nonlinearity of noble metal nanomaterials. Our results highlight a universal saturation dynamics in the transient optical response of plasmon-enhanced photonic structures excited by intense light beams, with potential impact on many applications, from the all-optical modulation of light to photovoltaics and photocatalysis.

We study edge state transport for the ${\nu }_{\text{bulk}}=2/3$ edge in the fully incoherent transport regime. To do so, we use a hydrodynamic approximation for the calculation of voltage and temperature profiles along the edge of the sample. Within this formalism, we study two different bare mode structures with tunneling: the edge model (1) consisting of two counterpropagating modes with filling factor discontinuities (from the bulk to the edge) $\delta \nu =-1/3$ and $\delta \nu =+1$, and the more complicated model (2) consisting of four modes with $\delta \nu =-1/3,+1,-1/3,$ and $+1/3$. We find that the topological characteristics of transport (quantized electrical and heat conductance) within these models are intact, with finite-size corrections which are determined by the extent of equilibration. In particular, our calculation of conductance for edge model (2) in a double quantum point contact geometry reproduces conductance results of a recent experiment [R. Sabo et al., Edge reconstruction in factional quantum Hall states, Nat. Phys. 13, 491 (2017)], which are inconsistent with the edge model (1). Our results can be explained in the charge/neutral mode picture, with incoherent analogs of the renormalization fixed points of the edge models. Additionally, we find diffusive $(\sim 1/L)$ conductivity corrections to the heat conductance in the fully incoherent regime for both models of the edge.

Studying basic physical effects sustained in metamaterials characterized by specific constitutive relation is a research topic with a long-standing tradition. Besides intellectual curiosity, it derives its importance from the ability to predict observable phenomena that are, if found with an actual metamaterial, a clear indication on its properties. Here, we consider a nonlocal (strong spatial dispersion), lossy, and isotropic metamaterial and study the impact of the nonlocality on the dispersion relation of surface plasmon polaritons sustained at an interface between vacuum and such metamaterial. For that, Fresnel coefficients are calculated and appropriate surface plasmon polaritons existence conditions are being proposed. Predictions regarding the experimentally observable reflection from a frustrated internal reflection geometry are being made. A different behavior for TE and TM polarization is observed. Our work unlocks opportunities to seek for traces of the nonlocality in experiments made with nowadays metamaterials.

The dual fermion (DF) method allows for calculating corrections due to nonlocal correlations relative to an effective impurity model. Choosing the impurity as that of a dynamical mean-field theory (DMFT) solution at self-consistency is popular, and the corrections from dual fermion theory are physically meaningful. We investigate the effect of choosing the impurity instead in a self-consistent manner and find for the two-dimensional Hubbard model an exponential increase of the correlation length and susceptibility at low temperatures. There are pronounced differences for the two self-consistency schemes that are discussed in the literature; the self-consistent DF solution can even be more metallic than the DMFT solution.

The pairing mechanism and gap structure in Ba122 pnictides have been hotly discussed for a long time as one of the central issues in Fe-based superconductors. Here, we attack this problem by taking account of the vertex corrections (VCs) for the Coulomb interaction $U(U-\mathrm{VCs})$, which are totally dropped in conventional Migdal-Eliashberg formalism. The $U-\mathrm{VC}$ in the charge susceptibility induces strong orbital fluctuations, and the $U-\mathrm{VC}$ enlarges the orbital-fluctuation-driven attractive interaction. By analyzing the effective multiorbital Hubbard model for Ba122 pnictides, we find that the orbital fluctuations develop in all four $d$ orbitals ($t_{2g}$ and $z^2$ orbitals) by which the Fermi surfaces (FSs) are composed. For this reason, nearly isotropic gap functions appear on all the hole-type FSs, including the outer hole FS around the $Z$ point composed of the $z^2$ orbital. In contrast, nodal gap structure appears on the electron FSs for the wide parameter range. The obtained nodal $s$-wave state changes to a fully gapped $s$-wave state without sign reversal ($s_{++}$-wave state) by introducing a small amount of impurities, accompanied by a small reduction in $T_{\mathrm{c}}$. The present microscopic theory naturally explains the important characteristics of the gap structure of both hole and electron FSs in Ba122 pnictides without introducing any phenomenological pairing interaction.