Some unexpected features of the phase diagram of the monoaxial helimagnet in presence of an applied magnetic field perpendicular to the chiral axis are theoretically predicted. A rather general Hamiltonian with long-range Heisenberg exchange and Dzyaloshinskii–Moriya interactions is considered. The continuum limit simplifies the free energy, which contains only a few parameters which in principle are determined by the many parameters of the Hamiltonian, although in practice they may be tuned to fit the experiments. The phase diagram contains a chiral soliton lattice phase and a forced ferromagnetic phase separated by a line of phase transitions, which are of second order at low $T$ and of first order in the vicinity of the zero-field ordering temperature, and are separated by a tricritical point. A highly nonlinear chiral soliton lattice, in which many harmonics contribute appreciably to the spatial modulation of the local magnetic moment, develops only below the tricritical temperature, and in this case, the scaling shows a logarithmic behavior similar to that at $T=0$, which is a universal feature of the chiral soliton lattice. Below the tricritical temperature, the normalized soliton density curves are found to be independent of $T$, in agreement with the experimental results of magnetorresistance curves, while above the tricritical temperature they show a noticeable temperature dependence. The implications in the interpretation of experimental results of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ are discussed.

Using simple four-terminal resistance measurements and two-coil superfluid stiffness measurements, we observe an extended temperature range wherein both the superfluid density and the resistance are substantially suppressed, in some cases below experimental resolution, in severely underdoped ultrathin films of ${\mathrm{Ca}}_{0.3}{\mathrm{Y}}_{0.7}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (CaYBCO). This temperature range $\delta T_c$, deemed the “offset,” is in some films more than $\frac{1}{2}$ of the resistive transition temperature. $\delta T_c$ scales linearly with the characteristic two-dimensional (2D) vortex unbinding temperature $T_{\text{2D}}$, growing larger with underdoping upon approach to the superconductor-insulator transition. Absent in three-dimensional samples of CaYBCO, we discuss the offset in the context of a previously observed 2D quantum critical point in the vicinity of the superconductor-insulator transition, as well as in the context of intrinsic and extrinsic inhomogeneities in the superconducting state of the film.

In electromagnetism, acoustics, and quantum mechanics, scattering problems can routinely be solved numerically by virtue of perfectly matched layers (PMLs) at simulation domain boundaries. Unfortunately, the same has not been possible for general elastodynamic wave problems in continuum mechanics. In this Rapid Communication, we introduce a corresponding scattered-field formulation for the Navier equation. We derive PMLs based on complex-valued coordinate transformations leading to Cosserat elasticity-tensor distributions not obeying the minor symmetries. These layers are shown to work in two dimensions, for all polarizations, and all directions. By adaptative choice of the decay length, the deep subwavelength PMLs can be used all the way to the quasistatic regime. As demanding examples, we study the effectiveness of cylindrical elastodynamic cloaks of the Cosserat type and approximations thereof.

We calculate the spectrum of the Andreev boundary modes in a two-dimensional superconductor formed at an interface between two different nonsuperconducting materials, e.g., insulating oxides. Inversion symmetry is absent in this system, and both the electron band structure and the superconducting pairing are strongly affected by the spin-orbit coupling of the Rashba type. We consider isotropic $s$-wave pairing states, both with and without time-reversal symmetry breaking, as well as various $d$-wave states. In all cases, there exist subgap Andreev boundary states, whose properties, in particular, the number and location of the zero-energy modes, qualitatively depend on the gap symmetry and the spin-orbit coupling strength.

We present a study of modulation of spin-orbit interaction (SOI) at the interface of ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ by $\delta$ doping with an isostructural ferromagnetic perovskite ${\mathrm{LaCoO}}_3$. The sheet carrier density at the interface decreases exponentially with $\delta$-doping thickness. We have explored that the spin-orbit scattering time $\left({\tau }_{so}\right)$ can be decreased by nearly three orders of magnitude, whereas the inelastic scattering time $\left({\tau }_i\right)$ remains almost constant with $\delta$-doping thickness. We have also observed that the ${\tau }_i$ varies almost inversely proportional to temperature and ${\tau }_{so}$ remains insensitive to temperature, which suggest that the spin relaxation in these interfaces follows D'yakonov-Perel mechanism. The observed in-plane anisotropic magnetoresistance is attributed to the mixing of the spin-up and spin-down states of the $d$ band at the Fermi level due to SOI.

We study the decay and oscillations of Majorana fermion wave functions and ground-state (GS) fermion parity in one-dimensional topological superconducting lattice systems. Using a Majorana transfer matrix method, we find that Majorana wave-function properties are encoded in the associated Lyapunov exponent, which in turn is the sum of two independent components: a “superconducting component,” which characterizes the gap induced decay, and the “normal component,” which determines the oscillations and response to chemical potential configurations. The topological phase transition separating phases with and without Majorana end modes is seen to be a cancellation of these two components. We show that Majorana wave-function oscillations are completely determined by an underlying nonsuperconducting tight-binding model and are solely responsible for GS fermion parity switches in finite-sized systems. These observations enable us to analytically chart out wave-function oscillations, the resultant GS parity configuration as a function of parameter space in uniform wires, and special parity switch points where degenerate zero energy Majorana modes are restored in spite of finite size effects. For disordered wires, we find that band oscillations are completely washed out leading to a second localization length for the Majorana mode and the remnant oscillations are randomized as per Anderson localization physics in normal systems. Our transfer matrix method further allows us to (i) reproduce known results on the scaling of midgap Majorana states and demonstrate the origin of its log-normal distribution, (ii) identify contrasting behavior of disorder-dependent GS parity switches for the cases of even versus odd number of lattice sites, and (iii) chart out the GS parity configuration and associated parity switch points as a function of disorder strength.

We determine the exact time-resolved photoemission spectroscopy for a nesting driven charge density wave (described by the spinless Falicov-Kimball model within dynamical mean-field theory). The pump-probe experiment involves two light pulses: the first is an ultrashort intense pump pulse that excites the system into nonequilibrium, and the second is a lower amplitude, higher frequency probe pulse that photoexcites electrons. We examine three different cases: the strongly correlated metal, the quantum-critical charge density wave, and the critical Mott insulator. Our results show that the quantum critical charge density wave has an ultraefficient relaxation channel that allows electrons to be de-excited during the pump pulse, resulting in little net excitation. In contrast, the metal and the Mott insulator show excitations that are closer to what one expects from these systems. In addition, the pump field produces spectral band narrowing, peak sharpening, and a spectral gap reduction, all of which rapidly return to their field free values after the pump is over.

We investigate the optical properties of type-${\mathrm{I}}_1$ basal-plane stacking faults (BSFs) in ZnO and ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ by cathodoluminescence spectroscopy supported by envelope function calculations. We report on a quantum-well-like band alignment of the ${\mathrm{I}}_1$ BSFs in ZnO taking into account the spontaneous polarization as well as an intrinsic self-screening effect on the polarization-related electric field. We present a systematic investigation of the luminescence properties associated with ${\mathrm{I}}_1$ BSFs in ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ for varying Mg content $\left(0\le x\le 0.1\right)$ using spatially and spectrally resolved cathodoluminescence spectroscopy. Both the near-band-edge emission and the luminescence line related to the ${\mathrm{I}}_1$ BSF exhibit the expected blueshift and line broadening with increasing Mg content. We propose a band diagram to describe the recombination mechanism of excitons in a ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ film containing ${\mathrm{I}}_1$ BSFs. Based on a statistical analysis, we compile the experimentally obtained ${\mathrm{I}}_1$ BSF emission energies of ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ samples and establish a linear dependence of the ${\mathrm{I}}_1$ BSF-related emission energy on the Mg content. This correlation provides an alternative way to identify the presence of ${\mathrm{I}}_1$ BSFs in ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ without the necessity of sophisticated transmission electron microscopy investigations.

We present the three-layer model to calculate the surface second-harmonic generation (SSHG) yield. This model considers that the surface is represented by three regions or layers. The first layer is the vacuum region with a dielectric function ${\epsilon }_v\left(\omega \right)=1$ from where the fundamental electric field impinges on the material. The second layer is a thin layer $\left(\ell \right)$ of thickness $d$ characterized by a dielectric function ${\epsilon }_{\ell }\left(\omega \right)$, and it is in this layer where the SSHG takes place. The third layer is the bulk region denoted by $b$ and characterized by ${\epsilon }_b\left(\omega \right)$. Both the vacuum and bulk layers are semi-infinite. The model includes the multiple reflections of both the fundamental and the second-harmonic (SH) fields that take place at the thin layer $\ell$. We obtain explicit expressions for the SSHG yield for the commonly used $s$ and $p$ polarizations of the incoming $1\omega$ and outgoing $2\omega$ electric fields, where no assumptions for the symmetry of the surface are made. These symmetry assumptions ultimately determine which components of the surface nonlinear second-order susceptibility tensor $\mathbf{\chi }(-2\omega ;\omega ,\omega )$ are different from zero, and thus contribute to the SSHG yield. Then, we particularize the results for the most commonly investigated surfaces, the (001), (110), and (111) crystallographic faces, taking their symmetries into account. We use the three-layer model and compare it against the experimental results of a Si(111)$\left(1\times 1\right)$:H surface, as a test case, and use it to predict the SSHG yield of a Si(001)$\left(2\times 1\right)$ surface.

It is generally accepted that the exposed surfaces of silicon crystals are highly reactive due to the dangling bonds which protrude into the vacuum. However, surface reconstruction not only modifies the reactivity of bulk silicon crystals, but also plays a key role in determining the properties of silicon nanocrystals. In this study we probe the reactivity of silicon clusters at the end of a scanning probe tip by examining their interaction with closed-shell fullerene molecules. Counter to intuitive expectations, many silicon clusters do not react strongly with the fullerene cage, and we find that only specific highly oriented clusters have sufficient reactivity to break open the existing carbon-carbon bonds.

Ultraclean graphene sheets encapsulated between hexagonal boron nitride crystals host two-dimensional electron systems in which low-temperature transport is solely limited by the sample size. We revisit the theoretical problem of carrying out microscopic calculations of nonlocal ballistic transport in such micron-scale devices. By employing the Landauer-Büttiker scattering theory, we propose a scaling approach to tight-binding nonlocal transport in realistic graphene devices. We test our numerical method against experimental data on transverse magnetic focusing (TMF), a textbook example of nonlocal ballistic transport in the presence of a transverse magnetic field. This comparison enables a clear physical interpretation of all the observed features of the TMF signal, including its oscillating sign.

We study the topological nature of single layers of correlated honeycomb materials $\alpha \text{−}{\mathrm{RuCl}}_3$ and $A_2{\mathrm{IrO}}_3$ $(A=\text{Li},\text{Na})$ with strong spin-orbit coupling. An effective tight-binding model based on first principles band structure calculations including Hubbard and spin-orbit coupling is derived. Two pairs of propagating edge modes centered at the zone center and zone boundary are found when their one-dimensional boundary forms a zigzag shape, while the bulk has a gap with trivial time-reversal $Z_2$ invariants. The effects of strong electronic interactions and doping on the edge modes in these Mott insulators are discussed. We further suggest a heterostructure of $\alpha \text{−}{\mathrm{RuCl}}_3/{\mathrm{IrCl}}_3$ to search for the proposed topological Mott phase.

Motivated by recent experiments on the generation of coherent light in engineered hybrid quantum systems, we investigate gain in a microwave photonic cavity coupled to quantum dot structures and develop concrete directions for achieving a giant amplification in photon transmission. We propose two architectures for scaling up the electronic gain medium: (i) $N$-double quantum dot systems and (ii) $M$-quantum dots arranged in series akin to a quantum cascade laser setup. In both setups, the fermionic reservoirs are voltage biased, and the quantum dots are coupled to a single-mode cavity. Optical amplification is explained based on a sum rule for the transmission function, and it is determined by an intricate competition between two different processes: charge-density response in the gain medium and cavity losses to input and output ports. The same design principle is also responsible for the corresponding giant amplification in other photonic observables, mean photon number, and emission spectrum, thereby realizing a quantum device that behaves as a giant microwave amplifier.

We have studied the crystal and magnetic structures of Fe-doped hexagonal manganites $\mathrm{LuM}{\mathrm{n}}_{1-x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3(x=0,0.1,0.2$, and 0.3) by using bulk magnetization and neutron powder diffraction methods. The samples crystalize consistently in a hexagonal structure and maintain the space group $P{6}_{3}cm$ from 2 to 300 K. The Néel temperature $T_{\mathrm{N}}$ increases continuously with increasing Fe doping. In contrast to a single ${\mathrm{\Gamma }}_4$ representation in $\mathrm{LuMn}{\mathrm{O}}_3$, the magnetic ground state of the Fe-doped samples can only be described with a combination of ${\mathrm{\Gamma }}_3\left(P{6_3}^{\prime }c{m}^{\prime }\right)$ and ${\mathrm{\Gamma }}_4\left(P{6_3}^{\prime }{c}^{\prime }m\right)$ irreducible representations, whose contributions have been quantitatively estimated. The drastic effect of Fe doping is highlighted by composition-dependent spin reorientations. A phase diagram of the entire composition series is proposed based on the present result and those reported in literature. Our result demonstrates the importance of tailoring compositions in increasing magnetic transition temperatures of multiferroic systems.

The family of two-dimensional transition metal carbides, so called MXenes, has recently found new members with ordered double transition metals $M_2^{\prime }{M}^{″}{\mathrm{C}}_2$, where $M^{\prime }$ and $M^{″}$ stand for transition metals. Here, using a set of first-principles calculations, we demonstrate that some of the newly added members, oxide $M_2^{\prime }{M}^{″}{\mathrm{C}}_2$ ($M^{\prime }=\mathrm{Mo}$, W; $M^{″}=\mathrm{Ti}$, Zr, Hf) MXenes, are topological insulators. The nontrivial topological states of the predicted MXenes are revealed by the $Z_2$ index, which is evaluated from the parities of the occupied bands below the Fermi energy at time reversal invariant momenta, and also by the presence of the edge states. The predicted $M_2^{\prime }{M}^{″}{\mathrm{C}}_2{\mathrm{O}}_2$ MXenes show nontrivial gaps in the range of 0.041–0.285 eV within the generalized gradient approximation and 0.119–0.409 eV within the hybrid functional. The band gaps are induced by the spin-orbit coupling within the degenerate states with $d_{x^2-y^2}$ and $d_{xy}$ characters of $M^{\prime }$ and $M^{″}$, while the band inversion occurs at the $\mathrm{\Gamma }$ point among the degenerate $d_{x^2-y^2}/d_{xy}$ orbitals and a nondegenerate $d_{3z^2-r^2}$ orbital, which is driven by the hybridization of the neighboring orbitals. The phonon dispersion calculations find that the predicted topological insulators are structurally stable. The predicted W-based MXenes with large band gaps might be suitable candidates for many topological applications at room temperature. In addition, we study the electronic structures of thicker ordered double transition metals $M_2^{\prime }{M}_2^{″}{\mathrm{C}}_3{\mathrm{O}}_2$ ($M^{\prime }=\mathrm{Mo}$, W; $M^{″}=\mathrm{Ti}$, Zr, Hf) and find that they are nontrivial topological semimetals. Among the predicted topological insulators and topological semimetals, ${\mathrm{Mo}}_2{\mathrm{TiC}}_2$ and ${\mathrm{Mo}}_2{\mathrm{Ti}}_2{\mathrm{C}}_3$ functionalized with a mixture of F, O, and OH have already been synthesized, and therefore some of the topological materials proposed here can be experimentally accessed.