Correlated sampling has wide-ranging applications in Monte Carlo calculations. When branching random walks are involved, as commonly found in many algorithms in quantum physics and electronic structure, population control is typically not applied with correlated sampling due to technical difficulties. This hinders the stability and efficiency of correlated sampling. In this paper, we study schemes for allowing birth/death in correlated sampling and propose an algorithm for population control. The algorithm can be realized in several variants depending on the application. One variant is a static method that creates a reference run and allows other correlated calculations to be added a posteriori. Another optimizes the population control for a set of correlated, concurrent runs dynamically. These approaches are tested in different applications in quantum systems, including both the Hubbard model and electronic structure calculations in real materials.

A quantum kicked rotor model is one of the promising systems to realize various Floquet topological phases. We consider a double-kicked rotor model for a one-dimensional quasi-spin-1/2 Bose-Einstein condensate with spin-dependent and spin-independent kicks which are implementable for cold atomic experiments. We theoretically show that the model can realize all the Altland-Zirnbauer classes with nontrivial topology in one dimension. In the case of class CII, we show that a pair of winding numbers $(w_0,w_{\pi })\in 2\mathbb{Z}\times 2\mathbb{Z}$ featuring the edge states at zero and $\pi$ quasienergy, respectively, takes various values depending on the strengths of the kicks. We also find that the winding numbers change to $\mathbb{Z}$ when we break the time-reversal and particle-hole symmetries by changing the phase of a kicking lattice. We numerically confirm that the winding numbers can be obtained by measuring the mean chiral displacement in the long-time limit in the present case with four internal degrees of freedom. We further propose two feasible methods to experimentally realize the spin-dependent and spin-independent kicks required for various topological phases.

Raman spectra from molecular systems are typically regarded as comprising a set of sharp peaks. These arise from both single-molecule modes and extended $\mathrm{\Gamma }$ point phonons being, in theory, harmonic oscillators. Broadening is typically assigned to thermal, linewidth, and anharmonic effects. Here we measure significant broadening in the Raman signal from partially deuterated water ices VII and VIII, which is absent in the pure systems. This isotopic broadening is much greater than any anharmonic effects and is reproduced by lattice dynamics calculations assuming only harmonic bonding. Instead, it arises from mass disorder induced phonon localization. This localization leads to many overlapping modes with a range of frequencies dependant on local isotopic environment and Raman activity due to lack of distinct molecular or crystalline symmetry.

We derive two fundamental laws of chiral band crossings: (i) a local constraint relating the Chern number to phase jumps of rotation eigenvalues and (ii) a global constraint determining the number of chiral crossings on rotation axes. Together with the fermion doubling theorem, these laws describe all conditions that a network of chiral band crossing must satisfy. We apply the fundamental laws to prove the existence of enforced double Weyl points, nodal planes, and generic Weyl points, among others. In addition, we show that chiral space group symmetries can not stabilize nodal lines with finite Chern numbers. Combining the local constraint with explicit low-energy models, we determine the generic topological phase diagrams of all multifold crossings. Remarkably, we find a fourfold crossing with Chern number 5, which exceeds the previously conceived maximum Chern number of 4. We identify materials crystallizing in space group 198, such as B20 materials and BaAsPt, as suitable compounds with this Chern number 5 crossing.

We theoretically study the real-space distribution of the supercurrent that flows under a uniform vector potential in a two-dimensional quasiperiodic structure. This is done by considering the attractive Hubbard model on the quasiperiodic Ammann-Beenker structure and studying the superconducting phase within the Bogoliubov-de Gennes mean-field theory. Decomposing the local supercurrent into the paramagnetic and diamagnetic components, we numerically investigate their dependencies on average electron density, temperature, and the angle of the applied vector potential. We find that the diamagnetic current locally violates the current conservation law, necessitating compensation from the paramagnetic current, even at zero temperature. The paramagnetic current shows exotic behaviors in the quasiperiodic structure, such as local currents, which are oriented transversally or reversely to that of the applied vector potential.

We investigate the electron transport through a serially coupled quantum-dot molecule in which each dot is side-coupled to one Majorana zero mode (MZM) at an end of the topological-superconducting nanowire. Calculation results show that with the increase of dot number, the zero-bias conductance does not display the conventional odd-even effect but always keeps equal to $\frac{e^2}{h}$. On the other hand, if different superconducting phases are added to the dot-MZM couplings with the left-right asymmetry mode, the zero-bias conductances display apparent odd-even effect. Namely, in the odd-dot cases, the zero-bias conductances are less determined by the superconducting phase, whereas they are more dependent on the superconducting phase in the cases of even dots. Regarding the shot-noise Fano factors, they also exhibit clear odd-even effects, mainly manifested as the dependence of the relation between zero-bias Fano factors and superconducting phase difference on the parity of dot number. We believe that these results are helpful to construct mesoscopic cells based on quantum dots and MZMs.

We propose the linear and nonlinear anomalous planar Hall effect (APHE) in tilted Weyl semimetals in the presence of an in-plane magnetic and electric field, where the field-induced Berry connection plays a key role. The conductivity of linear APHE is ascribed to the quantum metric and is antisymmetric in nature, distinct from the well-known chiral anomaly induced PHE arising from the Berry curvature. Using a tilting vector to describe the model, we demonstrate the constrains on the linear and nonlinear APHE by the tilting directions. The linear APHE is intrinsic that is determined by the topological properties of energy bands, whereas the nonlinear APHE is extrinsic. The predicted linear and nonlinear APHE are inherently different from others and may shed light on a deeper understanding on transport nature of the tilted Weyl semimetals.

The acceleration theorem for wave packet propagation in periodic potentials disentangles the $k$-space dynamics and real space dynamics. This is well known and understood for Bloch oscillations and super Bloch oscillations in the presence of position-independent forces. Here, we analyze the dynamics of a model system in which the $k$-space dynamics and the real space dynamics are inextricably intertwined due to a position-dependent force provided by a parabolic trap. We demonstrate that this coupling gives rise to significantly modified and rich dynamics when the lattice is shaken by a modulated parabolic potential. The dynamics range from chirped Bloch-harmonic oscillations to asymmetric spreading oscillations. We analyze these findings by tracing the spatiotemporal dynamics in real space and by visualizing the relative phase in the $k$-space dynamics which leads to an accurate explanation of the obtained phenomena. We also compare our numerical results to a local acceleration model and obtain very good agreement for the case of coherent oscillations, however, deviations for oscillations with spreading dynamics which altogether supports the interpretations of our findings.

The optical properties of metallic nanoparticles are most often considered in terms of plasmons, the coupled states of light and quasifree electrons. Confinement of electrons inside the nanostructure leads to another, very different type of resonances. We demonstrate that these confinement-induced resonances typically join into a single composite “super-resonance,” located at significantly lower frequencies than the plasmonic resonance. This super-resonance influences the optical properties in the low-frequency range, in particular, producing giant nonlinearities. We show that such nonlinearities can be used for efficient down-conversion from optical to terahertz and midinfrared frequencies on the submicrometer propagation distances in nanocomposites. We discuss the interaction of the quantum-confinement-induced super-resonance with the conventional plasmonic ones, as well as the unusual quantum level statistics, adapting here the paradigms of the quantum billiard theory and showing the possibility to control the resonance position and width using the geometry of the nanostructures.

Neuromorphic computing mimics the brain's architecture to create energy-efficient devices. Reconfigurable synapses are crucial for neuromorphic computing, which can be achieved through memory-resistive (memristive) switching. Graphene-based memristors have shown nonvolatile multibit resistive switching with desirable endurance. Through first-principles calculations, we study the structural and electronic properties of graphene in contact with an ultra-thin alumina overlayer and demonstrate how one can use charge doping to exert direct control over its interfacial covalency, reversibly switching between states of conductivity and resistivity in the graphene layer. We further show that this proposed mechanism can be stabilized through the $p$-type doping of graphene, e.g., by naturally occurring defects, the passivation of dangling bonds or defect engineering.

${\mathrm{MgV}}_2{\mathrm{O}}_4$ is a spinel based on magnetic ${\mathrm{V}}^{3+}$ ions, which host both spin $\left(S=1\right)$ and orbital $\left(l_{\mathrm{eff}}=1\right)$ moments. Owing to the underlying pyrochlore coordination of the magnetic sites, the spins in ${\mathrm{MgV}}_2{\mathrm{O}}_4$ only antiferromagnetically order once the frustrating interactions imposed by the $Fd\overline{3}m$ lattice are broken through an orbitally-driven structural distortion at ${\mathrm{T}}_S\simeq 60$ K. Consequently, a Néel transition occurs at ${\mathrm{T}}_N\simeq 40$ K. Low-temperature spatial ordering of the electronic orbitals is fundamental to both the structural and magnetic properties; however, considerable discussion on whether it can be described by complex or real orbital ordering is ambiguous. We apply neutron spectroscopy to resolve the nature of the orbital ground state and characterize hysteretic spin-orbital correlations using x-ray and neutron diffraction. Neutron spectroscopy finds multiple excitation bands and we parametrize these in terms of a multilevel (or excitonic) theory based on the orbitally degenerate ground state. Meaningful for the orbital ground state, we report an “optical-like” mode at high energies that we attribute to a crystal-field-like excitation from the spin-orbital $j_{\mathrm{eff}}=2$ ground-state manifold to an excited $j_{\mathrm{eff}}=1$ energy level. We parametrize the magnetic excitations in terms of a Hamiltonian with spin-orbit coupling and local crystalline electric field distortions resulting from deviations from perfect octahedra surrounding the ${\mathrm{V}}^{3+}$ ions. We suggest that this provides compelling evidence for complex orbital order in ${\mathrm{MgV}}_2{\mathrm{O}}_4$. We then apply the consequences of this model to understand hysteretic effects in the magnetic diffuse scattering where we propose that ${\mathrm{MgV}}_2{\mathrm{O}}_4$ displays a high-temperature orbital memory of the low-temperature spin order.

We extend the theory of shot noise in coherent metals to shot noise in strange metals without quasiparticle excitations. This requires a generalization of the Boltzmann equation with a noise source to distribution functions which depend independently on the excitation momentum and energy. We apply this theory to a model of a strange metal with linear in temperature ($T$) resistivity, describing a Fermi surface with a spatially random Yukawa coupling to a critical boson. We find a suppression of the Fano factor in the strange metal, and describe the dependence of the shot noise on temperature and applied voltage. At low temperatures, we obtain a Fano factor equal to $1/6$, in contrast to the $1/3$ Fano factor in diffusive metals with quasiparticles. Our results are in general agreement with recent observations by Chen et al. (arXiv:2206.00673). We further compare the random Yukawa model to quasi-elastic electron-phonon scattering that also generates $T$-linear resistivity, and argue that shot noise observations offer a useful diagnostic to distinguish between them.

We investigate ultrafast harmonic generation (HG) in Si:B, driven by intense pump pulses with fields reaching $\sim 100\mathrm{kV}{\mathrm{cm}}^{-1}$ and a carrier frequency of 300 GHz, at 4 K and 300 K, both experimentally and theoretically. We report several findings concerning the nonlinear charge carrier dynamics in intense sub-THz fields: (i) Harmonics of order up to $n=9$ are observed at room temperature, while at low temperature we can resolve harmonics reaching at least $n=11$. The susceptibility per charge carrier at moderate field strength is as high as for charge carriers in graphene, considered to be one of the materials with the strongest sub-THz nonlinear response. (ii) For $T=300$ K, where the charge carriers bound to acceptors are fully thermally ionized into the valence subbands, the susceptibility values decrease with increasing field strength. Simulations incorporating multi-valence-band Monte Carlo and finite-difference-time-domain (FDTD) propagation show that here, the HG process becomes increasingly dominated by energy-dependent scattering rates over the contribution from band nonparabolicity, due to the onset of optical-phonon emission, which ultimately leads to the saturation at high fields. (iii) At $T=4\mathrm{K}$, where the majority of charges are bound to acceptors, we observe a drastic rise of the HG yields for internal pump fields of $\sim 30\mathrm{kV}{\mathrm{cm}}^{-1}$, as one reaches the threshold for tunnel ionization. We disentangle the HG nonlinear response into contributions associated with the initial photoionization and subsequent motion in the bands, and show that intracycle scattering seriously degrades any contribution to HG emission from coherent recollision of the holes with their parent ions.

Measurements of physical properties, including transport and magnetic properties and thermal conductivity, have been performed on single crystal MnRuP with helical magnetism structure. A field-induced first-order metamagnetism transition with a magnetization jump is discovered when the magnetic field is applied perpendicular to the $c$ axis. The arising transition is a result of the competition between magnetic energy and elastic energy with a complex noncollinear character in the vicinity of the metastable state in MnRuP, which is due to the transition from the incommensurate helical phase to the commensurate Yafet-Kittel ferrimagnetic phase or from the helical phase to the conical phase. It is worthwhile to note that both negative and positive magnetoresistance (MR) are observed when the field is parallel or perpendicular to the electric current, respectively, which is rarely found in the other system simultaneously. Such positive MR is unexpected on the basis that the application of magnetic field should favor a low-resistive state due to alignment of spins. The ability to manipulate magnetic properties with current directions may pave the way to a new method of realizing magnetic memories based on the spin internal degrees of freedom.