We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated $\mathrm{WS}{\mathrm{e}}_2$, ${\mathrm{WS}}_2,\mathrm{MoS}{\mathrm{e}}_2$, and $\mathrm{Mo}{\mathrm{S}}_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $X_{\mathrm{D}}^{-}$, 19 meV below the lowest bright trion, $X_1^{-}$, in $\mathrm{WS}{\mathrm{e}}_2$ and $\mathrm{W}{\mathrm{S}}_2$. In $\mathrm{MoS}{\mathrm{e}}_2$, $X_{\mathrm{D}}^{-}$ lies 6 meV above $X_1^{-}$, while $X_{\mathrm{D}}^{-}$ and $X_1^{-}$ almost coincide in $\mathrm{Mo}{\mathrm{S}}_2$. Our results agree with GW-Bethe-Salpeter equation (GW-BSE) ab initio calculations and quantitatively explain the optical response of doped monolayers with temperature.

Theoretically, a pair of Majorana bound states in a topological superconductor forms a single fermionic level even at large separations, implying that the parity information is stored nonlocally. The nonlocality leads to a long-distance coherence for electrons tunneling through a Coulomb-blockaded Majorana wire [L. Fu, Phys. Rev. Lett. 104, 056402 (2010)], an effect that can be observed, e.g., in an interferometer. Here, we examine theoretically the coherent electron transfer, taking into account that tunneling implies the long-distance transfer of charge, which is carried by one-dimensional plasmons. We show that the charge dynamics does not affect the coherence of the electron tunneling process in a topological superconductor consisting of a semiconductor wire proximitized by a single bulk superconductor. The coherence may be strongly suppressed, however, if the topological superconductivity derives from a semiconductor wire proximitized by a granular superconductor.

By performing exact quantum Monte Carlo simulations of a model of interacting Dirac Fermions with staggered potential, we reveal a novel intermediate phase where the electronic correlations drive a band insulator metallic, and at a larger interaction, drive the metal-to-Mott insulator. We also show that the Mott insulating phase is antiferromagnetic. A complete phase diagram is achieved by studying the phase transitions at large staggered potential and interaction strengths, which shows that the intermediate state is robust and occupies a large part of the phase diagram and that it should be more feasible to be detected experimentally.

We present Co $L_3$-edge resonant inelastic x-ray scattering (RIXS) of bulk ${\mathrm{LaCoO}}_3$ across the thermally induced spin-state crossover around 100 K. Owing to a high energy resolution of 25 meV, we observe unambiguously the dispersion of the intermediate-spin (IS) excitations in the low-temperature regime. Approaching the intermediate temperature regime, the IS excitations are damped and the bandwidth is reduced. The observed behavior can be well described by a model of mobile IS excitons with strong attractive interaction, which we solve by using dynamical mean-field theory for hard-core bosons. Our results provide a detailed mechanism of how high-spin and IS excitations interact to establish the physical properties of cobaltite perovskites.

We consider a Hong-Ou-Mandel interferometer for Lorentzian voltage pulses applied to quantum Hall edge channels at filling factor $\nu =2$. Due to interedge interactions, the injected electronic wave packets fractionalize before partitioning at a quantum point contact. Remarkably enough, differently from what was theoretically predicted and experimentally observed by using other injection techniques, we demonstrate that when the injection occurs through time-dependent voltage pulses (arbitrarily shaped), the Hong-Ou-Mandel noise signal always vanishes for a symmetric device and that a mismatch in the distances between the injectors and the point of collision is needed to reduce the visibility of the dip. We also show that by properly tuning these distances or by applying different voltages on the two edge channels in each arm of the interferometer, it is possible to estimate the intensity of the interedge interaction. Lorentzian-type voltage pulses are chosen because of their experimental relevance.

The structure of amorphous ${\mathrm{Cu}}_2{\mathrm{GeTe}}_3$ is investigated by a combination of anomalous x-ray scattering and extended x-ray absorption fine-structure experiments. The experimental data are analyzed with reverse Monte Carlo modeling, and they are interpreted in terms of short-range-order parameters as well as by using ring statistics and persistent homology to study the intermediate-range order. Based on this information, the structural relationship of the amorphous phase to the corresponding crystal is discussed. It is found that the amorphous network can be rationalized by small atomic displacements of the crystal structure, directed toward the intrinsic void regions. This structural similarity establishes the possibility of a fast phase-change process. On the other hand, the atomic rearrangements also lead to the formation of new chemical bonds and to distortions on the intermediate-range-order level. These are realized by a collapse and contraction of the strict hexagonal ring arrangements of the crystal and by the formation of small, triangular rings as well as Cu cluster configurations. These structural features allow for a new understanding of the phase-change property contrast of this material, especially concerning the density change and the optical contrast.

We report the hole doping dependencies of the pseudogap phase energy scale $2{\mathrm{\Delta }}_{\mathrm{PG}}$, the antinodal (nodal) superconducting energy scales $2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{AN}}$ ($2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{N}}$), and the charge-density-wave energy scale $2{\mathrm{\Delta }}_{\mathrm{CDW}}$ extracted from the electronic Raman responses of several copper oxide families. We show for all the cuprates studied that the three energy scales $2{\mathrm{\Delta }}_{\mathrm{PG}}, 2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{AN}}$, and $2{\mathrm{\Delta }}_{\mathrm{CDW}}$ display the same decreasing monotonic behavior with doping. In particular, $2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{AN}}$ and $2{\mathrm{\Delta }}_{\mathrm{CDW}}$ have nearly equal values. This suggests a universal scenario in which $2{\mathrm{\Delta }}_{\mathrm{PG}}, 2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{AN}}$, and $2{\mathrm{\Delta }}_{\mathrm{CDW}}$ are governed by common microscopic interactions that become relevant well above the superconducting transition at $T_c$. This is to be contrasted with the behavior of the nodal superconducting energy scale $2{\mathrm{\Delta }}_{\mathrm{SC}}^{\text{N}}$, which tracks the doping dependence of $T_c$ and, hence, seems to be controlled by different interactions.

We numerically study weak, random, spatial velocity modulation [quenched gravitational disorder (QGD)] in two-dimensional massless Dirac materials. QGD couples to the spatial components of the stress tensor; the gauge-invariant disorder strength is encoded in the quenched curvature. Although it is expected to produce negligible effects, wave interference due to QGD transforms all but the lowest-energy states into a quantum-critical “stack” with universal, energy-independent spatial fluctuations. We study five variants of velocity disorder, incorporating three different local deformations of the Dirac cone: flattening or steepening of the cone, pseudospin rotations, and nematic deformation (squishing) of the cone. QGD should arise for nodal excitations in the $d$-wave cuprate superconductors (SCs) due to gap inhomogeneity. Our results may explain the division between low-energy “coherent” (plane-wave-like) and finite-energy “incoherent” (spatially inhomogeneous) excitations in the SC and pseudogap regimes. The model variant that best matches the cuprate phenomenology possesses quenched random pseudospin rotations and nematic fluctuations. This model variant and another with pure nematic randomness exhibit a robust energy swath of stacked critical states, the width of which increases with increasing disorder strength. By contrast, quenched fluctuations that isotropically flatten or steepen the Dirac cone tend to produce strong disorder effects, with more rarefied wave functions at low and high energies. Our models also describe the surface states of class DIII topological SCs.

We identify quantum many-body scars in the transverse field quantum Ising model on a ladder. We make explicit how the corresponding (mid spectrum, low entanglement) many-body eigenstates can be approximated by injecting quasiparticle excitations into an exact, zero-energy eigenstate, which is of valence bond solid type. Next, we present a systematic construction of product states characterized, in the limit of a weak transverse field, by a sharply peaked local density of states. We describe how the construction of these ‘peak states’ generalizes to arbitrary dimension and we show that on the ladder their number scales with system size as the square of the golden ratio.

While some of the most elegant applications of topological insulators, such as the quantum anomalous Hall effect, require the preservation of Dirac surface states in the presence of time-reversal symmetry breaking, other phenomena such as spin-charge conversion rather rely on the ability for these surface states to imprint their spin texture on adjacent magnetic layers. In this Rapid Communication, we investigate the spin-momentum locking of the surface states of a wide range of monolayer transition metals ($3d-\mathrm{TM}$) deposited on top of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulators using first-principles calculations. We find an anticorrelation between the magnetic moment of the $3d-\mathrm{TM}$ and the magnitude of the spin-momentum locking induced by the Dirac surface states. While the magnetic moment is large in the first half of the $3d$ series, following Hund's rule, the spin-momentum locking is maximum in the second half of the series. We explain this trend as arising from a compromise between intra-atomic magnetic exchange and covalent bonding between the $3d-\mathrm{TM}$ overlayer and the Dirac surface states. As a result, while Cr and Mn overlayers can be used successfully for the observation of the quantum anomalous Hall effect or the realization of axion insulators, Co and Ni are substantially more efficient for spin-charge conversion effects, e.g., spin-orbit torque and charge pumping.

$\mathrm{LuC}{\mathrm{o}}_3$ is an itinerant ferromagnet whose magnetic properties strongly depend on the position of the $3d$ electronic states relative to the Fermi level. Here, we report on the magnetization of a $\mathrm{LuC}{\mathrm{o}}_3$ single crystal in pulsed magnetic fields up to 58 T. We find a field-induced phase transition just below 50 T from a low-spin to a high-spin state. The transition shows a pronounced anisotropy of the magnetization jump and hysteresis. A series of ab initio calculations based on the density functional theory show that the transition is due to a significant change in the occupancies of the Co $3d$ electronic states. At the same time, some features in the majority spin density of the Co $3d$ states are slightly modified and pass through the Fermi level when the spin state is changed, which leads to the instability of the $3d$-electron subsystem. Thereby, the applied magnetic field causes a significant redistribution in the majority and minority spin states in the Co $3d$ subsystem, which results in the sharp change in the magnetization.

The effect of pressure on both ferromagnetic (FM) cluster phase and the Griffiths phase (GP) was investigated in disordered cobaltite $\mathrm{G}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$, exhibiting low $T_{\mathrm{C}}=90\mathrm{K}$, high Griffiths temperature $T_{\mathrm{G}}=220\mathrm{K}$, and the exchange bias (EB) effect, which exists exotically inside GP. It was found that applied pressure leads to a decrease of $T_{\mathrm{C}}$ with a coefficient of $d{T}_{\mathrm{C}}/dP=-1\mathrm{K}/\mathrm{kbar}$ and dramatically suppresses GP, resulting in a rapid decrease in $T_{\mathrm{G}}$ at a rate of $d{T}_{\mathrm{G}}/dP=-3.6\mathrm{K}/\mathrm{kbar}$. Similarly, the EB field does not change significantly in the FM phase but it promptly collapses with pressure in GP, e.g., the EB field decreases four times under 10 kbar at 140 K. It appears that external pressure effectively eliminates local structure deformations that are responsible for magnetically ordered clusters existing above $T_{\mathrm{C}}$. It is suggested that the well-known pressure-induced transition from the high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ state to the low-spin state is mainly responsible for the observed decay of the Griffiths phase and simultaneous EB collapse, as well as, for a decrease in $T_{\mathrm{C}}$ under pressure in $\mathrm{G}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$.

We report a temperature-dependent study of Kondo resonance in the heavy-fermion compounds $\mathrm{Yb}X{\mathrm{Cu}}_4$ with $X=\mathrm{Mg}$, Cd, and Sn. A sharp peak of the ${\mathrm{Yb}}^{2+}4f_{7/2}$ state has been observed in photoemission spectra, and its energy position in the limit of zero temperature is in agreement with the energy scale of the Kondo temperature $T_{\mathrm{K}}$. The peak develops in the form of a dispersionless peak with large spectral weight at $T_{\mathrm{coh}}$ well below $T_{\mathrm{K}}$. The onset temperature of the robust Kondo state is comparable to the temperature at a local maximum in magnetic susceptibility. This nontrivial development of changes in the Kondo resonance at $T_{\mathrm{coh}}$ demonstrates the formation of the coherent heavy-fermion state.

ZrSiS-type materials represent a large material family with unusual coexistence of topological nonsymmorphic Dirac fermions and nodal-line fermions. As a special group of ZrSiS family, $\mathit{Ln}\mathrm{SbTe}$ ($Ln=\mathrm{lanthanide}\mathrm{rare}-\mathrm{earth}$) compounds provide a unique opportunity to explore new quantum phases due to the intrinsic magnetism induced by $\mathit{Ln}$. Here we report the single-crystal growth and characterization of NdSbTe, a previously unexplored $\mathit{Ln}\mathrm{SbTe}$ compound. NdSbTe has an antiferromagnetic ground state with field-driven metamagnetic transitions similar to other known $\mathit{Ln}\mathrm{SbTe}$, but exhibits distinct enhanced electronic correlations characterized by large a Sommerfeld coefficient of $115\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$, which is the highest among the known $\mathit{Ln}\mathrm{SbTe}$ compounds. Furthermore, our transport studies have revealed the coupling with magnetism and signatures of Kondo localization. All these findings establish NdSbTe as a platform for observing phenomena arising from the interplay between magnetism, topology, and electron correlations.

We study the RKKY interaction of magnetic impurities in the $\alpha -{\mathcal{T}}_3$ model which hosts pseudospin-1 fermions with two dispersive and one flat bands. By using the effective low-energy Hamiltonian we calculate the RKKY coupling for impurities placed on the same or different sublattices. We find that there are three types of interaction, which depend on the model parameter defining the relative strength of hoppings between sublattices, two of them can be reduced to graphene case while the third one is new and is due to the presence of a flat zero-energy band. We derive general analytical expressions for the RKKY interaction in terms of Mellin-Barnes type integrals and analyze different limiting cases. The cases of finite chemical potential and temperature, as well as asymptotic at large distances, are considered. We show that the interaction between impurities located at different rim sites displays a very strong temperature dependence at small doping being a direct consequence of the flat band. The subtleties of the theorem for signs of the RKKY interaction at zero doping, as applied to the ${\mathcal{T}}_3$ lattice, related to the existence of a dispersionless flat band are discussed.

The importance of type-II superconductors with strong pinning comes from their ability to carry large electrical currents in the presence of a magnetic field. We report on the results of the bulk magnetization measurements in the superconducting state in high-quality single crystals of ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ at various doping levels ranging from the underdoped to the overdoped regimes. The zero-temperature superconducting critical current density $J_c$ at optimal composition $x$ = 0.10, where the superconducting transition temperature $T_c$ reaches a maximum of 19.9(0.4) K, displays a pronounced sharp peak in the doping dependence. Thus the observed doping dependence of the critical current implies that pinning becomes stronger upon initial doping. In addition, the best pinning conditions are realized in the presence of structural and magnetic domains. Our results strongly suggest that the high $J_c$ values are mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. The experimental results of the normalized $J_c$ present a remarkably good agreement with the $\delta l$ pinning theoretical curve, confirming that pinning in our samples originates from spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for $H\parallel c$.

Finite-size scaling analysis is a well-accepted method for identification and characterization of quantum phase transitions (QPTs) in superconducting, magnetic, and insulating systems. We formally apply this analysis in the form suitable for QPTs in two-dimensional (2D) superconducting films to magnetic-field-driven superconductor-metal transition in one-dimensional (1D) MoGe nanowires. Despite being obviously inapplicable to nanowires, the 2D scaling equation leads to a high-quality scaling collapse of the nanowire resistance in the temperature and resistance ranges comparable or better to what is accepted in the analysis of the films. Our results suggest that the appearance and the quality of the scaling collapse by itself is not a definitive indicator of a QPT. We have also observed a sign change of the zero-bias anomaly (ZBA) in the nonlinear resistance, occurring exactly at the critical field of the accidental QPT. This behavior is often taken as an additional confirmation of the transition. We argue that in nanowires, the nonlinearity is caused by electron heating and has no relation to the critical fluctuations. Our observation suggests that similar to the scaling collapse, the sign change of ZBA can be a misleading indicator of QPT.

Electron beams in two-dimensional systems can provide a useful tool to study energy-momentum relaxation of electrons and to generate microwave radiation stemming from plasma-beam instabilities. Naturally, these two applications cannot coexist: if instability exists, it strongly distorts the distribution function of beam electrons; if scattering is strong, it typically suppresses plasma instabilities. Here, we study the role of inevitable electron-electron ($e-e$) collisions on possible plasma beam instabilities in graphene and show that scattering effects are far less trivial. We find that an unstable plasma mode associated with beam bunching is stabilized already by weak $e-e$ collisions. Quite surprisingly, further enhancement of $e-e$ collisions results in loss compensation and self-excitation of an ordinary graphene plasmon mode. Such instability is interpreted as viscous transfer of momentum from an electron beam to two-dimensional plasmons. Its growth rate reaches its maximum at hydrodynamic-to-ballistic crossover, when plasmon wavelength and electron mean free path are of the same order of magnitude.

We theoretically study a scheme to distinguish the two ground states of a one-dimensional topological superconductor, which could serve as a basis for the readout of Majorana qubits. The scheme is based on parity-to-charge conversion, i.e., the ground-state parity of the superconductor is converted to the charge occupation on a tunnel-coupled auxiliary quantum dot. We describe how certain error mechanisms degrade the quality of the parity-to-charge conversion process. We consider (i) leakage due to a strong readout tunnel pulse, (ii) incomplete charge Rabi oscillations due to slow charge noise, and (iii) charge relaxation due to phonon emission and absorption. To describe these effects, we use simple model Hamiltonians based on the ideal Kitaev chain, and draw conclusions to generic one-dimensional topological superconductors wherever possible. In general, the effects of the error mechanisms can be minimized by choosing a smooth shape and an optimal strength for the readout tunnel pulse. In a case study based on InAs heterostructure device parameters, we estimate that the parity-to-charge conversion error is mainly due to slow charge noise for weak tunnel pulses and leakage for strong tunnel pulses.

We report density functional theory based simulations of the final stages in the formation process of oxidized Ni nanocontacts along with their measurable conductance under mechanical strain. Not surprisingly, the presence of O atoms drastically changes the overall conductance as well as its spin polarization character. We have considered up to two O atoms in our simulations and found that their placement near the atomic constriction largely determines the conductance values. An overall picture emerges where the stability of the nanocontact improves with respect to that of pure Ni ones, supporting longer elongations as seen in the experiments. Also a remarkable effect takes place: the emergence of half metallicity (a fully spin polarized current), which seems to be a robust effect, but not necessarily translated into a very large magnetoresistance.