Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of ${\text{La}}_{2-2x}{\text{Sr}}_{1+2x}{\text{Mn}}_2{\text{O}}_7$ with $x=0.38$, studied using in situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. Upon cooling, the magnetic domain contrast was seen to appear first at the Curie temperature within the a-b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagnetic phase transition. The ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.

We demonstrate that the Higgs mechanism in three-dimensional topological superconductors exhibits unique features with experimentally observable consequences. The Higgs model we discuss has two superconducting components and an axionlike magnetoelectric term with the phase difference of the superconducting order parameters playing the role of the axion field. Due to this additional term, quantum electromagnetic and phase fluctuations lead to a robust topologically nontrivial state that holds also in the presence of interactions. In this sense, we show that the renormalization flow of the topologically nontrivial phase cannot be continuously deformed into a topologically nontrivial one. One consequence of our analysis of quantum critical fluctuations is the possibility of having a first-order phase transition in the bulk and a second-order phase transition on the surface. We also explore another consequence of the axionic Higgs electrodynamics, namely, the anomalous Hall effect. In the low-frequency London regime an anomalous Hall effect is induced in the presence of an applied electric field parallel to the surface. This anomalous Hall current is induced by a Lorentz-like force arising from the axion term, and it involves the relative superfluid velocity of the superconducting components. The anomalous Hall current has a negative sign, a situation reminiscent of but quite distinct in physical origin from the anomalous Hall effect observed in high-$T_c$ superconductors. In contrast to the latter, the anomalous Hall effect in topological superconductors is nondissipative and occurs in the absence of vortices.

The Hubbard and spinless fermion chains are paradigms of strongly correlated systems, very well understood using the Bethe ansatz, density matrix renormalization group (DMRG), and field theory/renormalization group (RG) methods. They have been applied to one-dimensional materials and have provided important insights for understanding higher-dimensional cases. Recently, an interacting fermion model has been introduced, with possible applications to topological materials. It has a single Majorana fermion operator on each lattice site and interactions with the shortest possible range that involve four sites. We present a thorough analysis of the phase diagram of this model in one dimension using field-theory/RG and DMRG methods. It includes a gapped supersymmetric region and a gapless phase with coexisting Luttinger liquid and Ising degrees of freedom. In addition to a first-order transition, three critical points occur: tricritical Ising, Lifshitz, and a generalization of the commensurate-incommensurate transition. We also survey various gapped phases of the system that arise when the translation symmetry is broken by dimerization and find both trivial and topological phases with 0, 1, and 2 Majorana zero modes bound to the edges of the chain with open boundary conditions.

Studying the jellium model in the Hartree-Fock approximation, Overhauser has shown that spin-density waves (SDWs) can lower the energy of the Fermi gas, but it is still unknown whether these SDWs are actually relevant for the phase diagram. In this paper, we give a more complete description of SDW states. We show that a modification of the Overhauser ansatz explains the behavior of the jellium at high density compatibly with previous Hartree-Fock simulations.

Effectively engineering the lattice thermal conductivity of materials is a key interest of the current thermal science community. Pressure or compressive strain is one of the most worthwhile processes to modify the thermal transport property of materials, due to its robust tunability and flexibility of realization. While it is well documented in the literature that the application of hydrostatic pressure normally increases the thermal conductivity of bulk materials, little work has been performed on abnormal pressure-dependent thermal conductivity and the governing mechanism has not been fully understood yet. In this paper, taking bulk telluride systems $X\text{Te}$ $\left(X=\mathrm{Hg},\mathrm{Cd},\mathrm{Zn}\right)$ as examples, we show, by combining first-principle calculation and the phonon Boltzmann transport equation, that the thermal conductivity presents diverse pressure dependence although they belong to the same group. The thermal conductivity of ZnTe is independent of pressure, while abnormal negative pressure dependence of thermal conductivity is observed in HgTe. As for CdTe, the trend falls in between HgTe and ZnTe and relies largely on the temperature. By comparing the key contributors of the lattice thermal conductivity, we find that the diverse pressure dependence of the lattice thermal conductivity is governed by the competition between the enhancement of group velocity of longitudinal acoustic and optic modes and the reduction of phonon relaxation time of transverse acoustic modes, with both effects being fully quantified by our calculation. Comparison with traditional bulk systems such as silicon further underpins the governing mechanism. The correlation between the diverse thermal transport phenomena and the nature of the atomic bonding is also qualitatively established. These findings are expected to deepen our understanding of manipulating phonon transport of bulk materials via simple compressive strain and are also helpful for related applications, such as optimizing thermoelectric performance.

Spin-orbit interaction enables electrical tuning of spins, thus facilitating spintronics applications. It leads to spin precession about a momentum-dependent spin-orbit field. For a diffusive, two-dimensional electron gas the spin orientation at a given spatial position depends on which trajectory the electron travels to that point. Under increasing lateral confinement the spin orientation becomes independent on the trajectory and the formation of a long-lived helical spin mode is predicted. Here we visualize this transition experimentally in a GaAs quantum-well structure with isotropic spin-orbit interaction. Spatially resolved measurements show the formation of a helical mode already for nonquantized and nonballistic channels. We find a spin-lifetime enhancement that is in excellent agreement with theoretical predictions. Lateral confinement of a two-dimensional electron gas provides an easy-to-implement technique for achieving long spin lifetimes in the presence of strong spin-orbit interaction for a wide range of material systems.

Breaking inversion symmetry in chiral graphene systems, e.g., by applying a perpendicular electric field in chirally stacked rhombohedral multilayer graphene or by introducing staggered sublattice potentials in monolayer graphene, opens up a bulk band gap that harbors a quantum valley-Hall state. When the gap size is allowed to vary and changes sign in space, a topologically confined one-dimensional (1D) zero-line mode (ZLM) is formed along the zero lines of the local gap. Here, we show that gapless ZLM with distinguishable valley degrees of freedom $K$ and $K^{\prime }$ exist for every propagation angle except for the armchair direction that exactly superpose the valleys. We further analyze the role of different geometries of top-bottom gated device setups that can be realized in experiments, discuss the effects of their edge misalignment, and analyze three common forms of topological defects that could influence the 1D ZLM transport properties in actual devices.

We study the effects of a phase difference on Yu-Shiba-Rusinov (YSR) states in a spinful Coulomb-blockaded quantum dot contacted by a superconducting loop. In the limit where charging energy is larger than the superconducting gap, we determine the subgap excitation spectrum, the corresponding supercurrent, and the differential conductance as measured by a normal-metal tunnel probe. In absence of a phase difference only one linear combination of the superconductor lead electrons couples to the spin, which gives a single YSR state. With finite phase difference, however, it is effectively a two-channel scattering problem and therefore an additional state emerges from the gap edge. The energy of the phase-dependent YSR states depend on the gate voltage and one state can cross zero energy twice inside the valley with odd occupancy. These crossings are shifted by the phase difference towards the charge degeneracy points, corresponding to larger exchange couplings. Moreover, the zero-energy crossings give rise to resonant peaks in the differential conductance with magnitude $4e^2/h$. Finally, we demonstrate that the quantum fluctuations of the dot spin do not alter qualitatively any of the results.

A recent study [Rosati, Dolcini, and Rossi, Appl. Phys. Lett. 106, 243101 (2015)] has predicted that, while in semiconducting single-walled carbon nanotubes (SWNTs) an electronic wave packet experiences the typical spatial diffusion of conventional materials, in metallic SWNTs, its shape remains essentially unaltered up to micrometer distances at room temperature, even in the presence of the electron-phonon coupling. Here, by utilizing a Lindblad-based density-matrix approach enabling us to account for both dissipation and decoherence effects, we test such a prediction by analyzing various aspects that were so far unexplored. In particular, accounting for initial nonequilibrium excitations, characterized by an excess energy $E_0$, and including both intra- and interband phonon scattering, we show that for realistically high values of $E_0$ the electronic diffusion is extremely small and nearly independent of its energetic distribution, in spite of a significant energy-dissipation and decoherence dynamics. Furthermore, we demonstrate that the effect is robust with respect to the variation of the chemical potential. Our results thus suggest that metallic SWNTs are a promising platform to realize quantum channels for the nondispersive transmission of electronic wave packets.

We study the exciton valley relaxation dynamics in single-layer ${\mathrm{MoS}}_2$ by a combination of two nonequilibrium optical techniques: time-resolved Faraday rotation and time-resolved circular dichroism. The depolarization dynamics, measured at 77 K, exhibits a peculiar biexponential decay, characterized by two distinct time scales of 200 fs and 5 ps. The fast relaxation of the valley polarization is in good agreement with a model including the intervalley electron-hole Coulomb exchange as the dominating mechanism. The valley relaxation dynamics is further investigated as a function of temperature and photoinduced exciton density. We measure a strong exciton density dependence of the transient Faraday rotation signal. This indicates the key role of exciton-exciton interactions in ${\mathrm{MoS}}_2$ valley relaxation dynamics.

A circularly polarized light can induce a dissipationless dc current in a quantum nanoring which is responsible for a resonant helicity-driven contribution to magnetic moment. This current is not suppressed by thermal averaging despite its quantum nature. We refer to this phenomenon as the quantum resonant inverse Faraday effect. For weak electromagnetic field, when the characteristic coupling energy is small compared to the energy level spacing, we predict narrow resonances in the circulating current and, consequently, in the magnetic moment of the ring. For strong fields, the resonances merge into a wide peak with a width determined by the spectral curvature. We further demonstrate that weak short-range disorder splits the resonances and induces additional particularly sharp and high resonant peaks in dc current and magnetization. In contrast, long-range disorder leads to a chaotic behavior of the system in the vicinity of the separatrix that divides the phase space of the system into regions with dynamically localized and delocalized states.

We report electrical conductivity $\sigma$ measurements on a range of two-dimensional electron gases (2DEGs) of varying linear extent. Intriguingly, at low temperatures ($T$) and low carrier density ($n_{\mathrm{s}}$) we find the behavior to be consistent with $\sigma \sim L^{\alpha }$, where $L$ is the length of the 2DEG along the direction of transport. Importantly, such scale-dependent behavior is precisely in accordance with the scaling hypothesis of localization [Abrahams et al., Phys. Rev. Lett. 42, 673 (1979)] which dictates that in systems where the electronic wave function $\xi$ is localized, $\sigma$ is not a material-specific parameter but depends on the system dimensions. From our data we are able to construct the “$\beta$ function” $\equiv (h/e^2)dln\sigma /dlnL$ and show this to be strongly consistent with theoretically predicted limiting values. These results suggest, remarkably, that the electrons in the studied 2DEGs preserve phase coherence over lengths $\sim 10\mu \mathrm{m}$. This suggests the utility of the 2DEGs studied towards applications in quantum information as well as towards fundamental investigations into many-body localized phases.

A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of the free carrier response in pyrochlore ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-semimetal transition at $T_N=110$ K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase and decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below $T_N$ is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity $v_F\approx 4\times {10}^7$ cm/s, a logarithmic renormalization scale ${\mathrm{\Lambda }}_L\approx 600$ K, and require a Fermi temperature of $T_F\approx 100$ K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.

Unidirectional transports have been observed in two-dimensional systems, however, so far they have not been experimentally observed in three-dimensional bulk materials. In this theoretical work, we show that the recently discovered Weyl materials provide a platform for unidirectional transports inside bulk materials. With high experimental feasibility, a complex Dirac mass can be generated and manipulated in photonic Weyl crystals, creating unidirectionally propagating modes observable in transmission experiments. A possible realization in (electronic) Weyl semimetals is also studied. We show in a lattice model that, with a short-range interaction, the desired form of the Dirac mass can be spontaneously generated in a first-order transition.

Long-lived population storage in optically pumped levels of rare-earth ions doped into solids, referred to as persistent spectral hole burning, is of significant fundamental and technological interest. However, the demonstration of deep and persistent holes in rare-earth ion doped amorphous hosts, e.g., glasses, has remained an open challenge for many decades—a fact that motivates our work towards a better understanding of the interaction between impurities and vibrational modes in glasses. Here we report the observation and detailed characterization of such holes in an erbium-doped silica glass fiber cooled to below 1 K. We demonstrate population storage in electronic Zeeman sublevels of the erbium ground state with lifetimes up to 30 seconds and 80% spin polarization. In addition to its fundamental aspect, our investigation reveals a potential technological application of rare-earth ion doped amorphous materials, including at telecommunication wavelength.

Bismuth-derived resonant states with $T_2$ symmetry are detected in the valence band of $\mathrm{GaA}{\mathrm{s}}_{1\text{−}x}\mathrm{B}{\mathrm{i}}_x$ using electromodulated reflectance. A doublet is located 42 meV below the valence-band edge of GaAs that is split by local strain around isolated Bi impurity atoms. A transition associated with a singlet is also observed just above the GaAs spin-orbit split-off band. These states move deeper into the valence band with increasing Bi concentration but at a much slower rate than the well-known giant upward movement of the valence-band edge in $\mathrm{GaA}{\mathrm{s}}_{1\text{−}x}\mathrm{B}{\mathrm{i}}_x$. Our results provide key insight for clarifying the mechanisms by which isovalent impurities alter the band structure of the host semiconductor.

The stability of the $\nu =1/3$ fractional Chern insulator (FCI) phase is analyzed on the example of a checkerboard lattice undergoing a transition into a Lieb lattice. The transition is performed by the addition of a second sublattice, whose coupling to the checkerboard sites is controlled by sublattice staggered potential. We investigate the influence of these sites on the many-body energy gap between three lowest energy states and the fourth state. We consider cases with different complex phases acquired in hopping and a model with a flattened topologically nontrivial band. We find that an interaction with the additional sites either open the single-particle gap or enlarge the existing one, which translates into similar effect on the many-particle gap. By looking at Berry curvature flatness we notice its strong correlation with the magnitude of the many-body energy gap, suggesting that the main mechanism of the FCI stabilization by additional atoms is via their influence on the Berry curvature. Evidence of the FCI phase for a region in a parameter space with larger energy gap is shown by looking at momenta of the threefold degenerate ground state, spectral flow, and quasihole excitation spectrum.

We theoretically investigate the transport and magnetotransport properties of three-dimensional Weyl semimetals. Using the random phase approximation–Boltzmann transport scattering theory for electrons scattering off randomly distributed charged impurities, together with an effective medium theory to average over the resulting spatially inhomogeneous carrier density, we smoothly connect our results for the minimum conductivity near the Weyl point with known results for the conductivity at high carrier density. In the presence of a nonquantizing magnetic field, we predict that for both high and low carrier densities, Weyl semimetals show a transition from quadratic magnetoresistance (MR) at low magnetic fields to linear MR at high magnetic fields, and that the magnitude of the $\text{MR}\gtrsim 10$ for realistic parameters. Our results are in quantitative agreement with recent unexpected experimental observations on the mixed-chalcogenide compound TlBiSSe.