We report the ${}^{11}\mathrm{B}$ nuclear magnetic resonance measurements on noncentrosymmetric superconductors $\mathrm{Nb}{\mathrm{Rh}}_2{\mathrm{B}}_2$ (superconducting transition temperature $T_c=7.8 \mathrm{K}$) and $\mathrm{Ta}{\mathrm{Rh}}_2{\mathrm{B}}_2$ $(T_c=5.9 \mathrm{K})$ with a chiral crystal structure. The nuclear spin-lattice relaxation rate $1/T_1$ shows no coherence peak below $T_{\mathrm{c}}$, which suggests the unconventional nature of the superconductivity. In the normal state, $1/T_{1}T$ increases with decreasing temperature $T$ at low temperatures below $T=200 \mathrm{K}$ for $\mathrm{Ta}{\mathrm{Rh}}_2{\mathrm{B}}_2$ and $T=15 \mathrm{K}$ for $\mathrm{Nb}{\mathrm{Rh}}_2{\mathrm{B}}_2$, while the Knight shift remains constant. These results suggest the presence of antiferromagnetic spin fluctuations in both compounds. The stronger spin fluctuations in $\mathrm{Ta}{\mathrm{Rh}}_2{\mathrm{B}}_2$ compared to $\mathrm{Nb}{\mathrm{Rh}}_2{\mathrm{B}}_2$ is discussed in the context of spin-orbit coupling.

Two spins located at the edge of a quantum spin Hall insulator may interact with each other via indirect spin-exchange interaction mediated by the helical edge states, namely, the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which can be measured by the magnetic correlation between the two spins. By means of the newly developed natural orbitals renormalization group (NORG) method, we investigated the magnetic correlation between two Kondo impurities interacting with the helical edge states, based on the Kane-Mele model defined in a finite zigzag graphene nanoribbon (ZGNR) with spin-orbital coupling (SOC). We find that the SOC effect breaks the symmetry in spatial distribution of the magnetic correlation, leading to anisotropy in the RKKY interaction. Specifically, the total correlation is always ferromagnetic (FM) when the two impurities are located at the same sublattice, while it is always antiferromagnetic (AFM) when at the different sublattices. Meanwhile, the behavior of the in-plane correlation is consistent with that of the total correlation. However, the out-of-plane correlation can be tuned from FM to AFM by manipulating either the Kondo coupling or the interimpurity distance. Furthermore, the magnetic correlation is tunable by the SOC, especially that the out-of-plane correlation can be adjusted from FM to AFM by increasing the strength of SOC. Dynamic properties of the system, represented by the spin-staggered excitation spectrum and the spin-staggered susceptibility at the two impurity sites, are finally explored. It is shown that the spin-staggered susceptibility is larger when the two impurities are located at the different sublattices than at the same sublattice, which is consistent with the behavior of the out-of-plane correlation. On the other hand, our study further demonstrates that the NORG is an effective numerical method for studying the quantum impurity systems.

The competition between spin-orbit coupling (SOC) $\lambda$ and electron-electron interaction $U$ leads to a plethora of novel states of matter, extensively studied in the context of $t_{2g}^4$ and $t_{2g}^5$ materials, such as ruthenates and iridates. Excitonic magnets—the antiferromagnetic state of bounded electron-hole pairs-are prominent examples of phenomena driven by those competing energy scales. Interestingly, recent theoretical studies predicted that excitonic magnets can be found in the ground state of SOC $t_{2g}^4$ Hubbard models. Here we present a detailed computational study of the magnetic excitations in that excitonic magnet, employing one-dimensional chains (via density matrix renormalization group) and small two-dimensional clusters (via Lanczos). Specifically, first we show that the low-energy spectrum is dominated by a dispersive (acoustic) magnonic mode, with extra features arising from the $\lambda =0$ state in the phase diagram. Second, and more importantly, we found a novel magnetic excitation forming a high-energy optical mode with the highest intensity at wave-vector $q\to 0$. In the excitonic condensation regime at large $U$, we also have found a novel high-energy $\pi$ mode composed solely of orbital excitations. These features do not appear all together in any of the neighboring states in the phase diagram and thus constitute unique fingerprints of the $t_{2g}^4$ excitonic magnet, of importance in the analysis of neutron and resonant inelastic x-ray scattering experiments.

In this paper, we theoretically study the topological phase transition from nodal-line semimetal to Weyl semimetal. The nodal-line structure is protected by mirror symmetry and located on the $k_x\text{−}{k}_y$ mirror reflection plane, and the Hamiltonian of nodal-line semimetal has an emergent chiral symmetry on this plane. When the mirror symmetry is broken, the topological nodal line opens the gap and the nodal-line semimetal transition to Weyl semimetal with Weyl points on the $k_x$ axis or the $k_y$ axis. In addition, we break the chiral symmetry and realize the Weyl semimetal with the Weyl points on the $k_z$ axis. Destruction of the chiral symmetry leads to the gradual bending of the energy bands. With the evolution of the energy bands, the type-II nodal-line semimetal, the type-II Weyl semimetal and the type-I Weyl semimetal are successively realized. Furthermore, we also study the surface states of the nodal-line semimetal and the corresponding Weyl semimetals after the phase transition. Our work provides more ways to study the phase transition between nodal-line semimetal and Weyl semimetal and helps realize possible applications in topological electronic devices in the future.

We investigate electron-phonon coupling (EPC) in the charge density wave (CDW) phase of $\mathrm{V}{\mathrm{Se}}_2$ by Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and ab initio calculations. Zone folding induced by the $4\times 4$ in-plane CDW phase promotes the appearance of a Raman peak at $\sim 170{\mathrm{cm}}^{-1}$. The suppression of ARPES intensity in parts of the Fermi surface is also a result of CDW-induced zone folding and anticrossing of the electron energy bands. The appearance of the new Raman peak is in line with the ARPES observation of a kink feature in the spectral function at the same energy. A self-energy analysis yields an EPC constant of $\lambda =0.3$. Our calculations of the EPC are in excellent agreement and reveal that the kink is caused by several optical phonon branches close in energy. Our paper highlights the CDW phase as a means of inducing EPC pathways to optical phonons that directly affect its Raman spectrum.

It has recently been conjectured that the transport relaxation rate in metals is bounded above by the temperature of the system. In this paper, we discuss the transport phenomenology of overdoped electron-doped cuprates, which we show constitute an unambiguous counterexample to this putative “Planckian” bound, raising serious questions about the efficacy of the bound.

Strong, direct Rashba spin-orbit coupling in Si, Ge, and the Ge/Si core/shell nanowire quantum dot (QD) allows for all electrical manipulation of the hole spin qubit. Motivated by this fact, we analyze different fabrication-dependent properties of nanowires, such as orientation, cross section, and the presence of strain, with the goal being to find the material and geometry that enables the fastest qubit manipulation, whose speed can be identified using the Rabi frequency. We show that QD in nanowires with a circular cross section (cNWs) enables much weaker driving of the hole spin qubit than QDs embedded in square profile nanowires (sNWs). Assuming the orientation of the Si nanowire that maximizes the spin-orbit effects, our calculations predict that the Rabi frequencies of the hole spin qubits inside Ge and Si sNW QD have comparable strengths for weak electric fields. The global maximum of the Rabi frequency is found in Si sNW QD for strong electric fields, putting this setup ahead of others in creating the hole spin qubit. Finally, we demonstrate that strain in the Si/Ge core/shell nanowire QD decreases the Rabi frequency. In cNW QD, this effect is weak; in sNW QD, it is possible to optimize the impact of strain with the appropriate tuning of the electric field strength.

External electric fields are highly attractive for dynamical manipulation of excitons in two-dimensional materials. Here, we theoretically study the ionization of excitons in monolayer transition metal dichalcogenides (TMDs) by intense pulsed electric fields in the terahertz (THz) regime. We find that THz pulses with realistic field strengths are capable of ionizing a significant fraction of photogenerated excitons in TMDs into free charge carriers. Short THz pulses are therefore an efficient, noninvasive method of dynamically controlling the free carrier concentration in monolayer TMDs, which is useful for applications such as THz modulators. We further demonstrate that exciton ionization probabilities should be experimentally measurable by comparing free carrier absorption before and after the THz pulse. Detailed results are provided for different TMDs in various dielectric environments.

We investigate an all-optical microscale planar lensing technique based on coherent fluids of semiconductor cavity exciton-polariton condensates. Our theoretical analysis underpins the potential in using state-of-the-art spatial light modulation of nonresonant excitation beams to guide and focus polariton condensates away from their pumping region. The nonresonant excitation profile generates an excitonic reservoir that blueshifts the polariton mode and provides gain, which can be spatially tailored into lens shapes at the microscale to refract condensate waves. We propose several different avenues in controlling the condensate fluid, and demonstrate formation of highly enhanced and localized condensates away from the pumped reservoirs. This opens new perspectives in guiding quantum fluids of light and generating polariton condensates that are removed from detrimental reservoir dephasing effects.

We present a detailed first-principles study on phonon-limited electronic transport in germanane and hexagonal boron nitride (h-BN). We find a high electron mobility of $2380\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and a low hole mobility of $60\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for germanane. For h-BN, we also find a respectable electron mobility of $118\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ together with a high hole mobility of $444\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. H-BN does not suffer from scattering associated with out-of-plane (flexural) acoustic (ZA) phonons due to its ${\sigma }_h$ symmetry, but germanane does. Different cutoff wavelengths (${\lambda }_{\mathrm{cutoff}}$), ranging from 0.4 to 17.6 nm, are considered to simulate the possible effect of a supporting substrate on the ZA phonons for germanane. Increasing ${\lambda }_{\mathrm{cutoff}}$ results in a degradation of the mobility down to $1640\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for electrons and $1\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for holes assuming ${\lambda }_{\mathrm{cutoff}}=17.6\mathrm{nm}$. We also study transport in the presence of a homogeneous electric field $E$. A negative electron differential mobility is observed in germanane for $E>{10}^4\mathrm{V}\mathrm{c}{\mathrm{m}}^{-1}$ due to transfer to the higher effective-mass $M$ valleys. Finally, the calculated mobilities are compared with those obtained for other two-dimensional (2D) materials, showing that germanane has the highest electron mobility, whereas holes in h-BN have the third highest mobility in our list of materials, outperformed only by ${\mathrm{WS}}_2$ and ${\mathrm{WSe}}_2$. This shows that germanane and h-BN can be considered for future nanoscaled electronic devices. Finally, the high germanane mobility shows that 2D materials can outperform bulk materials if the right material is found and if scattering with acoustic flexural phonons can be suppressed.

We calculate the form of quasiparticle interference patterns in bilayer graphene within a low-energy description, taking into account perturbatively the trigonal warping terms. We introduce four different types of impurities localized on the A and B sublattices of the first and the second layer, and we obtain closed-form analytical expressions both in real and Fourier spaces for the oscillatory corrections to the local density of states generated by the impurities. Finally, we compare our findings with recent experimental and semianalytical $T$-matrix results from Joucken et al. [Nano Lett. 21, 7100 (2021)], and we show that there is a very good agreement between our findings and the previous results, as well as with the experimental data.

It is well known that symmetry-protected topological (SPT) phases host nontrivial boundaries that cannot be mimicked in a lower-dimensional system with a conventional realization of symmetry. However, for SPT phases of bosons (fermions) within the cohomology (supercohomology) classification the boundary can be recreated without the bulk at the cost of a non-onsite-symmetry action. This raises the following question: Can one also mimic the boundaries of SPT phases which lie outside the (super)cohomology classification? In this paper, we study this question in the context of $(2+1)$-dimensional $\left[\right(2+1\left)\mathrm{D}\right]$ fermion SPTs. We focus on the root SPT phase for the symmetry group $G=Z_2\times Z_2^f$. Starting with an exactly solvable model for the bulk of this phase constructed by Tarantino and Fidkowski, we derive an effective one-dimensional (1D) lattice model for the boundary. Crucially, the Hilbert space of this 1D model does not have a local tensor product structure, but rather is obtained by placing a local constraint on a local tensor product Hilbert space. We derive the action of the $Z_2$ symmetry on this Hilbert space and find a simple three-site Hamiltonian that respects this symmetry. We study this Hamiltonian numerically using exact diagonalization and DMRG and find strong evidence that it realizes an Ising conformal field theory where the $Z_2$ symmetry acts as the Kramers-Wannier duality; this is the expected stable gapless boundary state of the present SPT. A simple modification of our construction realizes the boundary of the $(2+1)\mathrm{D}$ topological superconductor protected by time-reversal symmetry $\mathcal{T}$ with ${\mathcal{T}}^2={(-1)}^{\mathcal{F}}$.

Gold and platinum both have the face-centered cubic crystal structure and are considered noble metals, yet they differ by one in atomic number. Near-edge $M_{4,5}$ x-ray absorption and resonant $M_{4,5}{N}_{6,7}{N}_{6,7}$ Auger spectra reveal “dark” edges and a photon-energy dependence of Auger electron ($4f^{12}$ final-state) energy distributions. These findings are a direct consequence of the delayed onset of $d\to f$ transitions and space quantization of the $3d$ core hole prepared by photoexcitation that is predominantly $d\to p$ vs $d\to f$ near the edge, in agreement with theoretical predictions. This study showcases the centrifugal barrier for $f$-wave electrons and spectral evolution arising from the existence of multiple intermediate-state channels and final states for both photoexcitation and Auger decay.

Rydberg excitons are, with their ultrastrong mutual interactions, giant optical nonlinearities, and very high sensitivity to external fields, promising for applications in quantum sensing and nonlinear optics at the single-photon level. To design quantum applications it is necessary to know how Rydberg excitons and other excited states relax to lower-lying exciton states. Here, we present photoluminescence excitation spectroscopy as a method to probe transition probabilities from various excitonic states in cuprous oxide. We show giant Rydberg excitons at $T=38$ mK with principal quantum numbers up to $n=30$, corresponding to a calculated diameter of $3\mu \mathrm{m}$.

The mixing thermodynamics of Se and Te atoms in $2H\text{−}\mathrm{W}{\left({\mathrm{Se}}_{1-x}{\mathrm{Te}}_x\right)}_2$ solid solutions as well as their electronic properties under high pressure up to 45 GPa is studied using first-principles methods. The results suggest that, regardless of pressure, $2H\text{−}\mathrm{W}{\left({\mathrm{Se}}_{1-x}{\mathrm{Te}}_x\right)}_2$ exhibits a tendency toward local phase segregation into $2H\text{−}{\mathrm{WSe}}_2$ and $2H\text{−}{\mathrm{WTe}}_2$ (or $1T_d\text{−}{\mathrm{WTe}}_2$) as the temperature approaches 0 K. The critical temperature, at which $2H\text{−}\mathrm{W}{\left({\mathrm{Se}}_{1-x}{\mathrm{Te}}_x\right)}_2$ is stable as a single-phase random solid solution over the entire composition range is, on the other hand, predicted to increase with the applied pressure. Upon increasing the pressure, the electronic band gap of $2H\text{−}\mathrm{W}{\left({\mathrm{Se}}_{1-x}{\mathrm{Te}}_x\right)}_2$ shrinks, and it finally undergoes the semiconductor-to-semimetal transition. The magnitude and the shrinkage rate of the band gap are found to be affected both by the degree of compression and by the alloy composition. These findings offer a possibility to engineer the electronic properties of $2H\text{−}\mathrm{W}{\left({\mathrm{Se}}_{1-x}{\mathrm{Te}}_x\right)}_2$ by varying its composition and also the pressure applied on it for future utilization of the alloy in electronic devices operating in a high-pressure environment.

We study edge states in AB-stacked bilayer graphene (BLG) ribbon where the Chern number of the corresponding two-dimensional bulk Hamiltonian is zero. The existence and topological features of edge states when two layers ended with the same or different edge terminations (zigzag, bearded, armchair) are discussed. The edge states (nondispersive bands near the Fermi level) are states localized at the edge of graphene nanoribbon that only exists in a certain range of momentum $k_y$. Their existence near the Fermi level are protected by the chiral symmetry with topology well described by the coupled Su-Schrieffer-Heeger (SSH) chains model, i.e., the SSH ladder, based on the bulk-edge correspondence of one-dimensional systems. These zero-energy edge states can exist in the whole $k_y$ region when two layers have zigzag and bearded edges, respectively. A winding number calculation shows a topological phase transition between two distinct nontrivial topological phases when crossing the Dirac points. Interestingly, we find the stacking configuration of the BLG ribbon is important since it can lead to unexpected edge states without protection from the chiral symmetry both near the Fermi level in armchair-armchair case and in the gap within bulk bands that are away from Fermi level in the general case. The influence of interlayer next-nearest-neighbor interactions and interlayer bias are also discussed to fit the realistic graphene materials, which suggest the robust topological features of edge states in BLG systems.

A model is constructed to describe the thermal-field emission of electrons from a three-dimensional (3D) Dirac and Weyl semimetal hosting Dirac/Weyl node(s). The traditional thermal-field electron emission model is generalized to accommodate the 3D nonparabolic energy band structures in the Dirac/Weyl semimetals, such as cadmium arsenide $\left({\mathrm{Cd}}_3{\mathrm{As}}_2\right)$, sodium bismuthide $\left({\mathrm{Na}}_3\mathrm{Bi}\right)$, tantalum arsenide (TaAs), and tantalum phosphide (TaP). Due to the nontrivial energy decomposition of the energy dispersion and the vanishing transverse density of states, an unusual dual-peak feature is observed in the total energy distribution spectrum. This nontrivial dual-peak feature, absent from traditional materials, plays a critical role in manipulating the magnitude of the emission current through the variation of an applied field, temperature, and Fermi level. This feature suggests that a higher Fermi level will achieve a larger current density (apart from low work function). At zero temperature limit, a $F^3$ scaling law for pure field emission is derived and it is different from the well-known Fowler-Nordheim $F^2$ scaling law. Furthermore, these new behaviors have shown to exist beyond the Dirac cone approximated model. This model expands the recent understandings of electron emission studied for the Dirac two-dimensional (2D) materials into the 3D regime, and thus offers a theoretical foundation for the exploration in using Dirac semimetallic materials as novel electrodes.

We present a theoretical study of the thermodynamical factors that determine epitaxial formation of single layer 2D pnictogen (P, As, Sb, Bi) allotropes on substrates of any type. The interplay of substrate-adlayer interaction and strain induced by epitaxial matching is analyzed in terms of the phase diagram describing growth during the gaseous deposition stage. The necessary conditions to favor particular allotrope growth (in particular, the $\alpha$ and $\beta$ phases) are determined. We show that buckled Sb and Bi layers can overcome large tensile strain and form flat honeycomb layers even on common metal surfaces. An alternative strategy for controlled allotrope formation via thermally induced phase transformations between $\alpha$ and $\beta$ phases is examined in detail, including important methodological analysis. All four elements follow reconstructive transition pathways, whose activation barrier correlates with the bond dissociation energy. If nucleation is considered, the barrier can further reduce by about 13% and thus becomes quite accessible under typical annealing conditions. The role of van der Waals and spin-orbit corrections in the stability of several allotropes is carefully addressed. The theoretical insight gained is evaluated in light of experimental reports and strategies for controlling growth are outlined.

In spin-polarized itinerant electron systems, collective spin-wave modes arise from dynamical exchange and correlation (xc) effects. We consider here spin waves in doped paramagnetic graphene with adjustable Zeeman-type band splitting. The spin waves are described using time-dependent spin density functional response theory, treating dynamical xc effects within the Slater and Singwi-Tosi-Land-Sjölander approximations. We obtain spin-wave dispersions and spin stiffnesses as a function of doping and spin polarization, and we discuss the prospects for their experimental observation.

We develop a theory of superconducting pairing in low-density strontium titanate due to the quadratic coupling of electron density to soft transverse optical phonons first proposed by Ngai [Phys. Rev. Lett. 32, 215 (1974)]. This leads to a static attractive potential between electrons with decay length $l_{\mathrm{eff}}$ that scales inversely with soft optical gap ${\omega }_T$. For low electron densities $n\le {10}^{18}$ ${\mathrm{cm}}^{-3}$, attraction between electrons is static and local in space; thus the transition temperature $T_c$ was found using known results for low-density electron gas. The $T_c\left(n\right)$ dependence for low doping was calculated and found to be in agreement with experimental data. In addition, we show that suppression of $T_c$ by hydrostatic pressure and strong increase in $T_c$ due to isotope substitution ${}^{16}\mathrm{O}{\to }^{18}\mathrm{O}$ observed experimentally also can be explained within our theory.