With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of ${\mathrm{Ti}}_2\mathrm{C}$. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped ${\mathrm{Ti}}_2\mathrm{C}$ in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated ${\mathrm{Ti}}_2\mathrm{C}$ compounds which influences the mechanical behavior. The damage of ${\mathrm{Ti}}_2\mathrm{C}$ in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic ${\mathrm{Ti}}_2\mathrm{C}$.

Two hydrodynamic features of phonon transport, phonon drift and second sound, in a (20,20) single-wall carbon nanotube (SWCNT) are discussed using lattice dynamics calculations employing an optimized Tersoff potential for atomic interactions. We formally derive a formula for the contribution of drift motion of phonons to total heat flux at steady state. It is found that the drift motion of phonons carries more than $70\%$ and $90\%$ of heat at 300 and 100 K, respectively, indicating that phonon flow can be reasonably approximated as hydrodynamic if the SWCNT is long enough to avoid ballistic phonon transport. The dispersion relation of second sound is derived from the Peierls-Boltzmann transport equation with Callaway's scattering model and quantifies the speed of second sound and its relaxation. The speed of second sound is around 4000 m/s in a (20,20) SWCNT and the second sound can propagate more than 10 µm in an isotopically pure (20,20) SWCNT for frequency around 1 GHz at 100 K.

Rare-earth chromites ($R\mathrm{Cr}{\mathrm{O}}_3$) are an important subclass of functional materials with interesting magnetic, electrical, and catalytic properties that make them promising for various applications. Here, we report comparisons on the structural and magnetic properties of $\mathrm{HoCr}{\mathrm{O}}_3$ with those of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ powder samples, which were doped by $\mathrm{T}{\mathrm{m}}^{3+}$ (or $\mathrm{G}{\mathrm{d}}^{3+}$) ions at the $A$-site with ionic radius smaller (or larger) than that of $\mathrm{H}{\mathrm{o}}^{3+}$ ion. The structural properties of the samples were characterized by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy carried out at ambient. The band gaps of the three samples were determined by analyzing their ultraviolet Vis spectra. Magnetic studies carried out from 5 K to 300 K show that the Néel temperature ($T_{\mathrm{N}}^{\mathrm{Cr}}$, where $\mathrm{C}{\mathrm{r}}^{3+}$ ions order) increases with an increase in the average $A$-site ($R$-site) ionic radius, tolerance factor, and Cr1-O1-Cr1 bond angle but decreases with an increase in the orthorhombic strain factor. In addition, the application of external hydrostatic pressure was found to enhance $T_{\mathrm{N}}^{\mathrm{Cr}}$ of $\mathrm{HoCr}{\mathrm{O}}_3$, similar to the effect observed by doping $\mathrm{HoCr}{\mathrm{O}}_3$ with $\mathrm{G}{\mathrm{d}}^{3+}$ ions. These changes in $T_{\mathrm{N}}^{\mathrm{Cr}}$ with $A$-site doping and hydrostatic pressure are related to changes in $\mathrm{C}{\mathrm{r}}^{3+}-\mathrm{C}{\mathrm{r}}^{3+}$ exchange coupling resulting from changes in the Cr1-O1-Cr1 bond angle and Cr1-O1 bond lengths, respectively. Temperature dependent paramagnetic susceptibility data of the samples was fitted to the modified Curie-Weiss law that included the Dzyloshinskii-Moriya interaction. Isothermal magnetization data showed that the magnetic behavior of the samples changes from canted antiferromagnetic at low temperatures to paramagnetic at higher temperatures. The large magnetocaloric entropy change was observed in $\mathrm{HoCr}{\mathrm{O}}_3$, which was enhanced by Gd doping but lowered by Tm doping, showing its tunability by $A$-site doping. Correlation between magnetoelectric properties and magnetization is discussed.

In this paper, we study how, with the aid of impurity engineering, two-dimensional $p$-wave superconductors can be employed as a platform for one-dimensional topological phases. We discover that, while chiral and helical parent states themselves are topologically nontrivial, a chain of scalar impurities on both systems supports multiple topological phases and Majorana end states. We develop an approach which allows us to extract the topological invariants and subgap spectrum, even away from the center of the gap, for the representative cases of spinless, chiral, and helical superconductors. We find that the magnitude of the topological gaps protecting the nontrivial phases may be a significant fraction of the gap of the underlying superconductor.

We study topological properties of one-triplon bands in an extended Shastry-Sutherland model relevant for the frustrated quantum magnet ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$. To this end perturbative continuous unitary transformations are applied about the isolated dimer limit allowing us to calculate the one-triplon dispersion up to high order in various couplings including intra- and interdimer Dzyaloshinskii-Moriya interactions and a general uniform magnetic field. We determine the Berry curvature and the Chern number of the different one-triplon bands. We demonstrate the occurrence of Chern numbers $\pm 1$ and $\pm 2$ for the case that two components of the magnetic field are finite. Finally, we also calculate the triplon Hall effect arising at finite temperatures.

We use temperature- and field-dependent resistivity measurements (Shubnikov–de Haas quantum oscillations) and ultrahigh-resolution, tunable, vacuum ultraviolet laser-based angle-resolved photoemission spectroscopy (ARPES) to study the three-dimensionality (3D) of the bulk electronic structure in $\mathrm{WTe}{}_2$, a type II Weyl semimetal. The bulk Fermi surface (FS) consists of two pairs of electron pockets and two pairs of hole pockets along the $X\text{−}\mathrm{\Gamma }\text{−}X$ direction as detected by using an incident photon energy of 6.7 eV, which is consistent with the previously reported data. However, if using an incident photon energy of 6.36 eV, another pair of tiny electron pockets is detected on both sides of the $\mathrm{\Gamma }$ point, which is in agreement with the small quantum oscillation frequency peak observed in the magnetoresistance. Therefore, the bulk, 3D FS consists of three pairs of electron pockets and two pairs of hole pockets in total. With the ability of fine tuning the incident photon energy, we demonstrate the strong three-dimensionality of the bulk electronic structure in $\mathrm{WTe}{}_2$. The combination of resistivity and ARPES measurements reveals the complete, and consistent, picture of the bulk electronic structure of this material.

Measuring the magnetoresistance (MR) of ultraclean GaAs two-dimensional holes for a large $r_s$ range of 20–50, two striking behaviors in relation to the spin-orbit coupling (SOC) emerge in response to strong electron-electron interaction. First, in exact correspondence to the zero-field metal-to-insulator transition (MIT), the sign of the MR switches from being positive in the metallic regime to being negative in the insulating regime when the carrier density crosses the critical density $p_c$ of MIT ($r_s\sim 39$). Second, as the SOC-driven correction $\mathrm{\Delta }\rho$ to the MR decreases with reducing carrier density (or the in-plane wave vector), it exhibits an upturn in the close proximity just above $p_c$ where $r_s$ is beyond 30, indicating a substantially enhanced SOC effect. This peculiar behavior echoes with a trend of delocalization long suspected for the SOC-interaction interplay. Meanwhile, for $p<p_c$ or $r_s>40$, in contrast to the common belief that a magnet field enhances Wigner crystallization, the negative MR is likely linked to enhanced interaction.

We study the Kitaev chain under generalized twisted boundary conditions, for which both the amplitudes and the phases of the boundary couplings can be tuned at will. We explicitly show the presence of exact zero modes for large chains belonging to the topological phase in the most general case, in spite of the absence of “edges” in the system. For specific values of the phase parameters, we rigorously obtain the condition for the presence of the exact zero modes in finite chains, and show that the zero modes obtained are indeed localized. The full spectrum of the twisted chains with zero chemical potential is analytically presented. Finally, we demonstrate the persistence of zero modes (level crossing) even in the presence of disorder or interactions.

A solid solution between ${\mathrm{CaCu}}_3{\mathrm{Ti}}_{4-x}{\mathrm{Ru}}_x{\mathrm{O}}_{12}$ end members was first synthesized using a high-pressure technique. The residual magnetization at 2 K sharply changes at $2\le x\le 3$, and a spin-glass-like phase suddenly disappears at $x=2.5$, suggesting a first-order electronic phase transition. The magnetic susceptibility shows a strong dependence on the temperature for $x\le 1$ and a weak dependence for $x\ge 3$. At an intermediate value of $x$, the weak dependence at high temperatures changes to a strong dependence at low temperatures. The electrical resistivity shows variable-range-hopping-type behavior for $x\le 1$ and metallic behavior for $x\ge 3.5$, changing from the latter to the former at $T_{\rho }\sim 150$, 8.5, and 3.5 K for $x=2.0$, 2.5, and 3.0, respectively. An electronic phase diagram based on these results is similar to that for an organic system showing a Mott transition and to a theoretical diagram for the Mott transition. Considering electronic band structure calculations, the local magnetic moments at the ${\mathrm{CuO}}_4$ squares of the Ti compound become itinerant with increasing Ru content by means of a first-order phase transition, suggesting that the transition can be regarded as a Mott transition. The Mott transition is unique in that the atomic orbitals of the Ru cations play an important role in the increased hopping amplitude of the electrons/holes of the ${\mathrm{CuO}}_4$ molecularlike orbitals.

Combining nonresonant inelastic x-ray scattering experiments with state-of-the-art ab initio many-body calculations, we investigate the electronic screening mechanisms in strongly correlated CuO in a large range of energy and momentum transfers. The excellent agreement between theory and experiment, including the low-energy charge excitations, allows us to use the calculated dynamical screening as a safe building block for many-body perturbation theory and to elucidate the crucial role played by $d\text{−}d$ excitations in renormalizing the band gap of CuO. In this way we can dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.

There is considerable interest in collective effects in hybrid systems formed by molecular or atomic ensembles strongly coupled by an electromagnetic resonance. For analyzing such collective effects, we develop an efficient and general theoretical formalism based on the natural modes of the resonator. The main strength of our approach is its generality and the high level of analyticity enabled by modal analysis, which allows one to model complex hybrid systems without any restriction on the resonator shapes or material properties, and to perform statistical computations to predict general properties that are robust to spatial and polarization disorders. Most notably, we establish that super-radiant modes remain even after ensemble averaging and act as an “invisibility cloak” with a spectral bandwidth that scales with the number of oscillators and the spatially averaged Purcell factor.

We investigate the supercurrent in a silicene-based Josephson junction under external-field modulations spatially. Employing the qualitative analysis and solving the Dirac–Bogoliubov–de Gennes equation, it is found that, for the bulk states, a $\pi$ junction is generated from the valley polarization by combining an antiferromagnetic exchange magnetization and spin-orbit coupling. In contrast, for the topologically protected edge states, a $\pi$ as well as a ${\phi }_0$ junction can be obtained by adjusting ferromagnetic exchange field or antiferromagnetic exchange magnetization to shift the edge states in wave vector space; or alternatively by modulating electric and light fields to modify the Fermi velocity of the edge states. It is proposed that a direct current superconducting quantum interference devices can be used to observe these $\pi$ and ${\phi }_0$ junctions in experiment.

Despite numerous studies of transition metal dichalcogenides, the diversity of their chemical bonding characteristics and charge transfer is not well understood. Based on density functional theory we investigate their static and dynamic charges. The dynamic charge of the transition metal dichalcogenides with trigonal symmetry are anomalously large, while in their hexagonally symmetric counterparts, we even observe a counterintuitive sign, i.e., the transition metal takes a negative charge, opposite to its static charge. This phenomenon, so far never remarked on or analyzed, is understood by investigating the perturbative response of the system and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, a link is established between the sign of the Born effective charge and the process of $\pi$ backbonding from organic chemistry. Experiments are proposed to verify the calculated sign of the dynamical charge in these materials. Employing a high-throughput search we also identify other materials that present counterintuitive dynamic charges.

We consider a Laughlin droplet in a confining potential which is very steep but also weak compared to the ultra-short-ranged interparticle interactions. We find that the eigenstates have a Jack polynomial structure, and have an energy spectrum which is extremely different from the well-known Luttinger liquid edge.

The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the $s$- or $p$-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation.

Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two holelike ($\alpha$ and $\beta$) and two electronlike ($\eta$ and $\delta$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third holelike band ($\gamma$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering—and yet superconductivity can exist without coherent quasiparticles in the $\gamma$ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the $\beta$ bandwidths of various iron chalcogenides, including FeS, ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$, FeSe, and ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$, we find that the $\beta$ bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$.

Many model quantum spin systems have been proposed to realize critical points or phases described by 2+1 dimensional conformal gauge theories. On a torus of size $L$ and modular parameter $\tau$, the energy levels of such gauge theories equal $(1/L)$ times universal functions of $\tau$. We compute the universal spectrum of ${\mathrm{QED}}_3$, a U(1) gauge theory with $N_f$ two-component massless Dirac fermions, in the large-$N_f$ limit. We also allow for a Chern-Simons term at level $k$, and show how the topological $k$-fold ground state degeneracy in the absence of fermions transforms into the universal spectrum in the presence of fermions; these computations are performed at fixed $N_f/k$ in the large-$N_f$ limit.

We derive the Bloch equations for the spin dynamics of a two-dimensional electron gas in the presence of spin-orbit coupling. For the latter we consider both the intrinsic mechanisms of structure inversion asymmetry (Rashba) and bulk inversion asymmetry (Dresselhaus), and the extrinsic ones arising from the scattering from impurities. The derivation is based on the SU(2) gauge-field formulation of the Rashba-Dresselhaus spin-orbit coupling. Our main result is the identification of a spin-generation torque arising from Elliot-Yafet scattering, which opposes a similar term arising from Dyakonov-Perel relaxation. Such a torque, which to the best of our knowledge has gone unnoticed so far, is of basic nature, i.e., should be effective whenever Elliott-Yafet processes are present in a system with intrinsic spin-orbit coupling, irrespective of further specific details. The spin-generation torque contributes to the current-induced spin polarization (CISP), also known as inverse spin-galvanic or Edelstein effect. As a result, the behavior of the CISP turns out to be more complex than one would surmise from consideration of the internal Rashba-Dresselhaus fields alone. In particular, the symmetry of the current-induced spin polarization does not necessarily coincide with that of the internal Rashba-Dresselhaus field, and an out-of-plane component of the CISP is generally predicted, as observed in recent experiments. We also discuss the extension to the three-dimensional electron gas, which may be relevant for the interpretation of experiments in thin films.

We develop an accurate analytical model for the stray field energy of parallel stripe domains in multilayer films with perpendicular magnetic anisotropy, taking into account the effects of finite domain wall width and variable domain wall angle. By minimizing the total energy, we predict the domain width, the domain wall width, and the domain wall angle for given material parameters. We show how the domain wall width depends on the film thickness and the domain size. We explore the domain wall angle as a function of Dzyaloshinskii-Moriya interaction (DMI) and derive a threshold value $D_{\text{thr}}$ beyond which the system is in a Néel state. We find that thicker films require larger values of DMI to stabilize the Néel state. Finally, we test the effective medium theory, which allows treating multilayers as effective single layer films, and provide criteria for the applicability of the model in the presence of both surface and volume stray fields. Our results are supported by micromagnetic simulations, which indicate that the predictions are still precise even if the system is in a labyrinthine domain state. Using our model, otherwise inaccessible magnetic parameters, such as the DMI constant or the exchange constant, can now be obtained straightforwardly from static measurements of the stripe domain width in such films.