We investigate tunneling dynamics of electrons from an ensemble of self-assembled InAs quantum dots into the subbands of a two-dimensional electron gas (2DEG). LO-phonon-assisted tunneling processes and tunneling into higher subbands of the 2DEG electronic structure cause distinct resonances in the evolution of the tunneling rate as a function of the energy detuning between quantum dot and 2DEG ground state. By devising a semiquantitative model, we identify the momentum mismatch between the quantum dot and 2DEG wave function as the crucial quantity governing the evolution of the tunneling rate. In particular, we demonstrate that this mechanism along with the availability of tunneling into the second 2DEG subband allows for tuning of the tunneling rate by more than two orders of magnitude.

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$. The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

We study the mechanical and electronic properties of heterobilayers composed of black phosphorus (BP) on hexagonal boron nitride (hBN) and of blue phosphorus (${\mathrm{P}}_{\mathrm{blue}}$) on hBN by means of ab initio density functional theory. Emphasis is put on how the stress applied on the constituent layers impact their structural and electronic properties. For this purpose, we adopt a specific scheme of structural relaxation which allows us to distinguish between the energy cost of distorting each layer and the gain in stacking them together. In most cases we find that the BP tends to contract along the softer armchair direction, as already reported for similar structures. This contraction can attain up to 5% of strain, which might deteriorate its very good transport properties along the armchair direction. To prevent this, we propose a twisted-bilayer configuration where the largest part of the stress applies on the zigzag axis, resulting in a lower impact on the transport properties of BP. We also investigated a ${\mathrm{P}}_{\mathrm{blue}}$/hBN bilayer. A peculiar hybridization between the valence states of the two layers lets us suggest that electron-hole pairs excited in the bilayer will exhibit a mixed character, with electrons localized solely in the ${\mathrm{P}}_{\mathrm{blue}}$ layer and holes spread onto the two layers.

Using single-crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in ${\mathrm{UPt}}_2{\mathrm{Si}}_2$. The modulation wave vector, ${\mathit{Q}}_{\mathrm{mod}}$, is intriguingly similar to the incommensurate wave vector of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. ${\mathit{Q}}_{\mathrm{mod}}$ shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at aproximately (0.42 0 0) near 180 K. Bulk measurements indicate a crossover toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at $T_{\mathrm{N}}=35$ K.

A ${30}^{\circ }$ twisted bilayer graphene demonstrates the quasicrystalline electronic states with 12-fold symmetry. These states are, however, far away from the Fermi level, which makes conventional Dirac fermion behavior dominating the low-energy spectrum in this system. By using a tight-binding approximation, we study the effect of external pressure and electric field on the quasicrystalline electronic states. Our results show that, by applying the pressure perpendicular to the graphene plane, one can push the quasicrystalline electronic states towards the Fermi level. Then, the electron or hole doping on the order of $\sim 4\times {10}^{14}{\mathrm{cm}}^{-2}$ is sufficient for the coincidence of the Fermi level with these quasicrystalline states. Moreover, our paper indicates that applying the electric field perpendicular to the graphene plane can destroy the 12-fold symmetry of these states, and it is easier to reach this in the conduction band than in the valence band. Importantly, the application of the pressure can partially recover the 12-fold symmetry of these states against the electric field. We propose a hybridization picture that can explain all these phenomena.

We present a general theory of quantum information propagation in chaotic quantum many-body systems. The generic expectation in such systems is that quantum information does not propagate in localized form; instead, it tends to spread out and scramble into a form that is inaccessible to local measurements. To characterize this spreading, we define an information speed via a quench-type experiment and derive a general formula for it as a function of the entanglement density of the initial state. As the entanglement density varies from zero to one, the information speed varies from the entanglement speed to the butterfly speed. We verify that the formula holds both for a quantum chaotic spin chain and in field theories with an AdS/CFT gravity dual. For the second case, we study in detail the dynamics of entanglement in two-sided Vaidya-AdS-Reissner-Nordstrom black branes. We also show that, with an appropriate decoding process, quantum information can be construed as moving at the information speed, and, in the case of AdS/CFT, we show that a locally detectable signal propagates at the information speed in a spatially local variant of the traversable wormhole setup.

We systematically generate the perturbative expansion for the two-particle spin susceptibility in the Feynman diagrammatic formalism and apply this expansion to a model system—the single-band Hubbard model on a square lattice. We make use of algorithmic Matsubara integration (AMI) [A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlanc, Phys. Rev. B 99, 035120 (2019)] to analytically evaluate Matsubara frequency summations, allowing us to symbolically impose analytic continuation to the real-frequency axis. We minimize our computational expense by applying graph invariant transformations [A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlanc, Phys. Rev. B 101, 125109 (2020)]. We highlight extensions of the random-phase approximation and T-matrix methods that, due to AMI, become tractable. We present results for weak interaction strength where the direct perturbative expansion is convergent, and verify our results on the Matsubara axis by comparison to other numerical methods. By examining the spin susceptibility as a function of real frequency via an order-by-order expansion, we can identify precisely what role higher-order corrections play on spin susceptibility and demonstrate the utility and limitations of our approach.

The effect of a lateral electric current on the photoluminescence H band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H band are studied by means of continuous-wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of a lateral electric field of ∼100–200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ∼100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (>40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons.

In this paper, we investigate the transport properties of spinful electrons tunnel coupled to a finite-length Majorana nanowire on one end which is further tunnel coupled to a quantum dot (QD) at the other end. Using a full counting statistics approach, we show that Andreev reflection can happen in two separate channels that can be associated with the two spin states of the tunneling electrons. In a low-energy model for the nanowire that is represented by two overlapping Majorana bound states (MBSs) localized at the ends of the wire, analytical formulas for conductance and noise reveal their crucial dependence on the spin-canting angle difference of the two MBSs in the absence of the QD if the spinful lead couples to both MBSs. We further investigate the influence of a finite temperature on the observation of the coupling to both MBSs. In the presence of the QD, the interference of different tunneling paths gives rise to Fano resonances and the symmetry of those provide decisive information about the coupling to both MBSs. We contrast the low-energy model with a tight-binding model of the Majorana nanowire and treat the Coulomb interaction on the QD with a self-consistent mean field approach. Using the scattering matrix approach, we thereby extend the transport results obtained in the low-energy model including also higher excited states in the nanowire.

It is of fundamental importance but challenging to simultaneously identify atomic and magnetic configurations of two-dimensional van der Waals materials. In this work, we show that the nonreciprocal second-harmonic generation (SHG) can be a powerful tool to answer this challenge. Despite the preserved lattice inversion symmetry, the interlayer antiferromagnetic order and spin-orbit coupling generate and enhance SHG in parity-time (PT) -symmetric bilayer chromium triiodide (${\mathrm{CrI}}_3$). Importantly, the in-plane polarization-resolved SHG is sensitive to the subtly different interlayer structures that cannot be differentiated by linear optical spectra. Beyond bilayer, we further predict that the intensity and angle-resolved SHG can be employed to identify both interlayer atomic and magnetic configurations of trilayer ${\mathrm{CrI}}_3$. Our first-principles results agree with available measurements and show the potential of SHG as a noncontact approach to explore correlations between interlayer structures and magnetic orders of emerging ultrathin magnetic materials.

Previous single-pulse extreme ultraviolet and x-ray coherent diffraction studies revealed that superfluid ${}^4\mathrm{He}$ droplets obtained in a free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluid ${}^3\mathrm{He}$ droplets may help elucidate the origin of the large degree of rotational excitation and highlight similarities and differences of dynamics in normal and superfluid droplets. Here, we present a comparison of the shapes of isolated ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ droplets following expansion of the corresponding fluids in vacuum at temperatures as low as ∼2 K. Large ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ droplets with average radii of ∼160 and ∼350 nm, respectively, were produced. We find that the majority of the shapes of ${}^3\mathrm{He}$ droplets in the beam correspond to rotating oblate spheroids, in agreement with previous observations for ${}^4\mathrm{He}$ droplets. The aspect ratio of the droplets is related to the degree of their rotational excitation, which is discussed in terms of reduced angular momenta (Λ) and reduced angular velocities (Ω), the average values of which are found to be similar in both isotopes. This similarity suggests that comparable mechanisms induce rotation regardless of the isotope. We hypothesize that the observed distribution of droplet sizes and angular momenta originates from processes in the dense region close to the nozzle, where a significant velocity spread and frequent collisions between droplets induces excessive rotation followed by droplet fission.

Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${\mathbf{q}}_{\parallel }=(0,0)$ for various doping rates in electron-doped cuprates ${\text{La}}_{2-x}{\text{Ce}}_x{\text{CuO}}_4$. We find that those new experimental data are well captured by acousticlike plasmon excitations in a microscopic study of the layered $t-J$ model with the long-range Coulomb interaction. The acousticlike plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.

The Haldane-Shastry model is one of the most studied interacting spin systems. The Yangian symmetry makes it exactly solvable, and the model has semionic excitations. We introduce disorder into the Haldane-Shastry model by allowing the spins to sit at random positions on the unit circle and study the properties of the eigenstates. At weak disorder, the spectrum is similar to the spectrum of the clean Haldane-Shastry model. At strong disorder, the long-range interactions in the model do not decay as a simple power law. The eigenstates in the middle of the spectrum follow a volume law, but the coefficient is small, and the entropy is hence much less than for an ergodic system. In addition, the energy level spacing statistics is neither Poissonian nor of the Wigner-Dyson type. The behavior at strong disorder hence serves as an example of a nonergodic phase, which is not of the many-body localized kind, in a model with long-range interactions and SU(2) symmetry.

Superconductivity has recently been observed in Sr-doped ${\mathrm{NdNiO}}_2$ films grown on ${\mathrm{SrTiO}}_3$. Whether it is caused by or related to the interface remains an open question. To address this issue, we use density-functional theory calculation and charge-transfer self-consistent model to study the effects of polar discontinuity on the electronic and atomic reconstruction at the ${\mathrm{NdNiO}}_2/{\mathrm{SrTiO}}_3$ interface. We find that sharp interface with pure electronic reconstruction only is energetically unfavorable, and atomic reconstruction is unavoidable. We further propose a possible interface configuration that contain residual apical oxygen. These oxygen atoms lead to hybrids of $d_{z^2}$ and $d_{x^2-y^2}$ states at the Fermi level, which weaken the single-band feature and may be detrimental to superconductivity.

We investigate the band structure of twisted monolayer-bilayer graphene (tMBG) trilayers, or twisted graphene on bilayer graphene, as a function of twist angles and perpendicular electric fields in search of optimal conditions for achieving isolated nearly flat bands. Narrow bandwidths comparable to or smaller than the effective Coulomb energies satisfying $U_{\text{eff}}/W\gtrsim 1$ are expected for twist angles in the range of $0.3^{\circ }–1.5^{\circ }$, more specifically in islands around $\theta \sim 0.5^{\circ },0.{85}^{\circ },1.3^{\circ }$ for appropriate perpendicular electric field magnitudes and directions. The valley Chern numbers of the electron-hole asymmetric bands depend intrinsically on the details of the hopping terms in the bilayer graphene, and extrinsically on factors like electric fields or average staggered potentials in the graphene layer aligned with the contacting hexagonal boron nitride substrate. This tunability of the band isolation, bandwidth, and valley Chern numbers makes tMBG trilayers a more versatile system than twisted bilayer graphene for finding nearly flat bands prone to strong correlations.

Generally, stacking two quantum spin Hall insulators gives rise to a trivial insulator. Here, based on first-principles electronic structure calculations, we confirm that monolayer ${\mathrm{TaIrTe}}_4$ is a quantum spin Hall insulator and remarkably find that bilayer ${\mathrm{TaIrTe}}_4$ is still a quantum spin Hall insulator. Theoretical analysis indicates that the covalentlike interlayer interaction in combination with the small band gap at the time-reversal invariant $\mathrm{\Gamma }$ point results in new band inversion in bilayer ${\mathrm{TaIrTe}}_4$, namely, the emergence of quantum spin Hall phase. Meanwhile, a topological phase transition can be observed by increasing the interlayer distance in bilayer ${\mathrm{TaIrTe}}_4$. Considering that bulk ${\mathrm{TaIrTe}}_4$ is a type-II Weyl semimetal, layered ${\mathrm{TaIrTe}}_4$ thus provides an ideal platform to realize different topological phases at different dimensions.

Bardasis-Schrieffer modes in superconductors are fluctuations in subdominant pairing channels, e.g., $d$-wave fluctuations in an $s$-wave superconductor. This Rapid Communication shows that these modes also generically occur in excitonic insulators. In $s$-wave excitonic insulators, a $p$-wave Bardasis-Schrieffer mode exists below the gap energy, is optically active, and hybridizes strongly with photons to form Bardasis-Schrieffer polaritons, which are observable in both far-field and near-field optical experiments.

Quantum dot devices allow one to access the critical point of the two-channel Kondo model. The effective critical theory involves a free Majorana fermion quasiparticle localized on the dot. As a consequence, this critical point shows both the phenomenon of non-Fermi-liquid physics and fractionalization. Although a violation of the Wiedemann-Franz law is often considered to be a sign of non-Fermi-liquid systems, we show by exact calculations that it holds at the critical point, thereby providing a counterexample to this lore. Furthermore, we show that the fractionalized Majorana character of the critical point can be unambiguously detected from the heat conductance, opening the door to a direct experimental measurement of the elusive Majorana central charge $c=\frac{1}{2}$.