Uniaxial pressure applied in the $$ crystallographic direction perpendicular to spontaneous magnetization in the heavy fermion ferromagnet URhGe strongly stimulates superconductivity in this compound. The phenomenological approach allows us to point out two mechanisms of the increase in the superconducting temperature. They originate from stimulation by the uniaxial stress of both the intraband and interband amplitudes of triplet Cooper pairing. The phenomenon of reentrant superconductivity under a magnetic field along the $$ axis is also strongly sensitive to uniaxial stress in the same direction. Uniaxial stress accelerates suppression of the Curie temperature by the transversal magnetic field. The emergence of a first-order transition to the paramagnetic state occurs at a much lower field than in the absence of uniaxial stress.

We study a general phase transition between spinless topological nodal-line semimetal and Weyl semimetal phases. We classify topological nodal lines into two types based on their positions and shapes, and their phase transitions depend on their types. We show that a topological nodal-line semimetal becomes a Weyl semimetal by breaking time-reversal symmetry when the nodal lines enclose time-reversal-invariant momenta (type-A nodal lines). We also discuss an effect of crystallographic symmetries determining the band structure of the topological nodal-line semimetals. Thanks to protection by the symmetries, the topological nodal-line semimetals can transition into spinless Weyl semimetals or maintain the nodal lines in many crystals after inversion symmetry is broken.

We have studied the electronic structure of the surface unoccupied band (SUB) of clean Ge(001)-$c(4\times 2)$, with high energy and momentum resolution, by means of time- and angle-resolved two-photon photoelectron spectroscopy. The time evolution of photoelectron intensity images, measured as functions of energy and emission angle after photoexcitation with laser pulses (1.5 eV, 200 fs), provides a momentum space view of the relaxation pathways of surface excited electrons toward the bottom of the SUB. Surface excited electrons relax in several picoseconds along the strongly dispersive directions ($\overline{\mathrm{\Gamma }}\overline{J^{\prime }}$ and $\overline{\mathrm{\Gamma }}\overline{{J_2}^{\prime }}$) and then accumulate near the band bottom. Taking into account the ultrafast change of surface potential, possibly due to the spatial redistribution of nonthermal carriers generated by photoexcitation, an energy width of 0.22 eV was determined as the surface band gap, as well as the surface dispersion properties along three high-symmetry directions.

Friction on few-layer graphene is known to exhibit unique layer dependence where friction measured via atomic force microscopy (AFM) on the nanometer scale is generally observed to decrease with increasing number of layers. However, this trend is not always observed for AFM probe tips with different sizes and for graphene on different substrates. Within this context, the precise role played by the interface, in particular, the size of the contact and substrate roughness, in the layer dependence of friction on graphene is not yet completely understood. Here, we probe the origins of the roughness dependence of layer-dependent friction on graphene by a combination of AFM measurements and molecular dynamics (MD) simulations. In the experiments, friction is observed to monotonically decrease with increasing number of graphene layers for tips with various apex radii, while the roughness of the sample surface is observed to decrease. In the simulations, two opposite layer-dependence trends for friction are observed on few-layer graphene on substrates with different roughness values. The underlying mechanisms are investigated using atomistic details obtained from the simulations, where the different friction trends are found to originate from an interplay between surface roughness, the trajectory of the tip, and the number of atoms in contact. Finally, the effect of topographical correlation length on the layer dependence of friction on graphene is discussed.

We investigate the anisotropic hybrid surface optical (SO) phonon-plasmon dispersion relations in monolayer and double-layer phosphorene systems located on the polar substrates, such as ${\mathrm{SiO}}_2$, $h$-BN, and ${\mathrm{Al}}_2{\mathrm{O}}_3$. We calculate these hybrid modes by using the dynamical dielectric function in the random phase approximation in which the electron-electron interaction and long-range electric field generated by the substrate SO phonons via Fröhlich interaction are taken into account. In the long-wavelength limit, we obtain some analytical expressions for the hybrid SO phonon-plasmon dispersion relations which agree with those obtained from the loss function. Our results indicate a strong anisotropy in SO phonon-plasmon modes, which are stronger along the light-mass direction in our heterostructures. Furthermore, we find that the type of substrate has a significant effect on the dispersion relations of the coupled modes. Importantly, the hybrid excitations are apparently sensitive to the misalignment and separation between layers in double-layer phosphorene.

We study capacitance of the two-dimensional topological insulator (TI) edge states. The total capacitance is combined as a serial circuit of three capacitors presenting geometrical $C_G$, quantum $C_Q$, and correlation $C_{\mathrm{corr}}$ contributions to the electron energy. If the Coulomb interaction is weak, they obey an inequality $C_G<C_Q<C_{\mathrm{corr}}$. Quantities $C_G$ and $C_Q$ are found in the case of a round TI dot. The quantum capacitance at the finite temperature is determined taking into account the edge states quantization with and without the magnetic field. We have concluded that, in the accepted approximations, $C_{\mathrm{corr}}^{-1}=0$.

An incoming electron is reflected back as a hole at a normal-metal–superconductor interface, a process known as Andreev reflection. We predict that there exists a universal transverse shift in this process due to the effect of spin-orbit coupling in the normal metal. Particularly, using both the scattering approach and the argument of angular momentum conservation, we demonstrate that the shifts are pronounced for lightly doped Weyl semimetals, and are opposite for incoming electrons with different chirality, generating a chirality-dependent Hall effect for the reflected holes. The predicted shift is not limited to Weyl systems, but exists for a general three-dimensional spin-orbit-coupled metal interfaced with a superconductor.

In this Rapid Communication, we investigate the magneto-chiral dichroism (MChD) of right- and left-handed ${\mathrm{CsCuCl}}_3$ crystals in a strong magnetic field up to 14.5 T. We perform near-infrared optical measurements, and the results show a change in the absorption coefficient of an intra-atomic $d\text{−}d$ transition in a ${\mathrm{Cu}}^{2+}$ ion at approximately 1.5 eV. The magnitude of MChD was found to be proportional to the magnetization, which depends on the magnetic field and temperature. We verify this relation by developing a simple ${\mathrm{CuCl}}_6$ cluster model.

Symmetry protected Dirac semimetals can be transformed into Weyl semimetals by breaking the protecting symmetry, leading to many exotic quantum phenomena such as chiral anomaly and anomalous Hall effect. Here we show that, due to the large Zeeman $g$ factor and small bandwidth along the $b$ axis in Dirac semimetal $\mathrm{ZrT}{\mathrm{e}}_5$, a magnetic field of about 8 T along the $b$-axis direction may annihilate the Weyl points and open up a two-dimensional (2D) Dirac mass gap, when the Zeeman splitting exceeds the bandwidth along the $b$ axis. This is manifested by a sharp drop of magnetoresistance (MR) above 8 T, which is probably due to additional carriers induced by the orbital splitting of the zeroth Landau level associated with the 2D Dirac point, which is a descendant of the original Weyl points. Further evidence of the additional carriers is provided by the Hall effect and different anisotropic magnetoresistance in low and high field regions. Our experiment reveals a probable topological quantum phase transition of field-induced Weyl points annihilation in Dirac semimetal $\mathrm{ZrT}{\mathrm{e}}_5$ and gives an alternative explanation for the drop of MR at high field.

Due to the interaction between the topological defects of an order parameter and underlying fermions, the defects can possess induced fermion numbers, leading to several exotic phenomena of fundamental importance to both condensed matter and high-energy physics. One of the intriguing outcomes of induced fermion numbers is the presence of fluctuating competing orders inside the core of a topological defect. In this regard, the interaction between fermions and skyrmion excitations of an antiferromagnetic phase can have important consequences for understanding the global phase diagrams of many condensed matter systems where antiferromagnetism and several singlet orders compete. We critically investigate the relation between fluctuating competing orders and skyrmion excitations of the antiferromagnetic insulating phase of a half-filled Kondo-Heisenberg model on a honeycomb lattice. By combining analytical and numerical methods, we obtain the exact eigenstates of underlying Dirac fermions in the presence of a single skyrmion configuration, which are used for computing the induced chiral charge. Additionally, by employing this nonperturbative eigenbasis, we calculate the susceptibilities of different translational symmetry breaking charges, bond and current density wave orders, and translational symmetry preserving Kondo singlet formations. Based on the computed susceptibilities, we establish spin Peierls and Kondo singlets as dominant competing orders of antiferromagnetism. We show favorable agreement between our findings and field theoretic predictions based on the perturbative gradient expansion scheme, which crucially relies on the adiabatic principle and plane-wave eigenstates for Dirac fermions. The methodology developed here can be applied to many other correlated systems supporting competition between spin-triplet and spin-singlet orders in both lower and higher spatial dimensions.

While a monolayer ${\mathrm{PtSe}}_2$ film is a semiconductor with interesting spin structure, bulk ${\mathrm{PtSe}}_2$ crystal has been predicted to be a topological Dirac semimetal that can host a new type of Lorentz-violating Dirac fermions. Despite the intriguing predictions, experimental progress on the electronic structure of bulk ${\mathrm{PtSe}}_2$ has been hindered due to the lack of large, high-quality single-crystal samples. Here we report the growth and characterization of high-quality ${\mathrm{PtSe}}_2$ single crystals and reveal the electronic structure to provide direct evidence for the existence of three-dimensional type-II Dirac fermions. A comparison of the crystal, vibrational, and electronic structure to a related compound, ${\mathrm{PtTe}}_2$, is also discussed. Our work provides an important platform for exploring the novel quantum phenomena associated with type-II Dirac fermions in the $1T-{\mathrm{PtSe}}_2$ class of transition-metal dichalcogenides.

Light-matter interactions inside a turbid medium can be controlled by tailoring the spatial distribution of energy density throughout the system. Wavefront shaping allows selective coupling of incident light to different transmission eigenchannels, producing dramatically different spatial intensity profiles. In contrast to the density of transmission eigenvalues that is dictated by the universal bimodal distribution, the spatial structures of the eigenchannels are not universal and depend on the confinement geometry of the system. Here, we develop and verify a model for the transmission eigenchannel with the corresponding eigenvalue close to unity. By projecting the original problem of two-dimensional diffusion in a homogeneous scattering medium onto a one-dimensional inhomogeneous diffusion, we obtain an analytical expression relating the intensity profile to the shape of the confining waveguide. Inverting this relationship enables the inverse design of the waveguide shape to achieve the desired energy distribution for the perfectly transmitting eigenchannel. Our approach also allows to predict the intensity profile of such a channel in a disordered slab with open boundaries, pointing to the possibility of controllable delivery of light to different depths with local illumination.

Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge state of a local vacancy defect. These two centers were found to exhibit a mutual metastability, such that each center can be excited in one configuration and emit as the other. This metastability was found to be dependent on temperature and the wavelength of the excitation laser. Furthermore, one of these centers was found to have an effective magnetic $g$ factor that is substantially larger than what is expected for an isolated $\mathrm{E}{\mathrm{u}}^{3+}$ ion and is explained by a change in the charge state of the defect environment around the Eu. This prediction could also offer a new explanation for the saturation magnetization previously observed in GaN : Eu and other GaN:RE systems.

A first-order-like resistivity hysteresis is induced by a subtle structural transition under hydrostatic pressure in the topological nodal-line superconductor ${\mathrm{PbTaSe}}_2$. This structure transition is quickly suppressed to zero at pressure $\sim 0.25$ GPa. As a result, superconductivity shows a marked suppression, accompanied with pronounced changes in the magnetoresistance and Hall resistivity. The first-principles calculations show that the spin-orbit interactions partially gap out the Dirac nodal line around $K$ point in the bulk Brillouin zone upon applying a small pressure, whereas the Dirac states around $H$ point are completely destroyed. The calculations further reveal a second structural phase transition under a pressure as high as $\sim 30$ GPa, through which a transition from a topologically nontrivial bulk phase to a trivial phase is uncovered, with a superconducting dome emerging under this high-pressure phase. Our calculations also reveal how the bulk Fermi surfaces and the surface bands evolve with pressure. This theoretical study shall inspire in-depth experimental investigations on the electronic structure of this novel topological superconductor under higher pressures.

We study the interplay of spin and charge coherence in a single-level quantum dot. A tunnel coupling to a superconducting lead induces superconducting correlations in the dot. With full spin symmetry retained, only even-singlet superconducting correlations are generated. An applied magnetic field or attached ferromagnetic leads partially or fully reduce the spin symmetry, and odd-triplet superconducting correlations are generated as well. For single-level quantum dots, no other superconducting correlations are possible. We analyze, with the help of a diagrammatic real-time technique, the interplay of spin symmetry and superconductivity and its signatures in electronic transport, in particular current and shot noise.

The collective magnetic excitations in the spin-orbit Mott insulator ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_2{\mathrm{IrO}}_4$ ($x=0,0.01,0.04,0.1$) were investigated by means of resonant inelastic x-ray scattering. We report significant magnon energy gaps at both the crystallographic and antiferromagnetic zone centers at all doping levels, along with a remarkably pronounced momentum-dependent lifetime broadening. The spin-wave gap is accounted for by a significant anisotropy in the interactions between $J_{\text{eff}}=1/2$ isospins, thus marking the departure of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.

We report on a combined study of the de Haas-van Alphen effect and angle-resolved photoemission spectroscopy on single crystals of the metallic delafossite ${\mathrm{PdRhO}}_2$ rounded off by ab initio band structure calculations. A high-sensitivity torque magnetometry setup with superconducting quantum interference device readout and synchrotron-based photoemission with a light spot size of $50\mu \mathrm{m}$ enabled high-resolution data to be obtained from samples as small as $150\times 100\times 20{\left(\mu \mathrm{m}\right)}^3$. The Fermi surface shape is nearly cylindrical with a rounded hexagonal cross section enclosing a Luttinger volume of 1.00(1) electrons per formula unit.

The combined effects of defect-defect interaction and strains of up to 10% on the onset of magnetic states in the quasi-one-dimensional electronic states generated by the so-called 558 linear defect in graphene monolayers are investigated by means of ab initio calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) as well as by uniaxial compressive perpendicular strains, at both limits of isolated and interacting 558 defects. Parallel tensile strains and perpendicular compressive strains are shown to give rise to two cooperative effects that favor the emergence of itinerant magnetism on the 558 defect in graphene: enhancement of the density of states (DOS) of the resonant defect states in the region of the Fermi level and tuning of the Fermi level to the maximum of the related DOS peak. On the other hand, parallel compressive strains and perpendicular tensile strains are shown to be detrimental to the development of magnetic states in the 558 defect, because in these cases the Fermi level is found to shift away from the maximum of the DOS of the defect states. Effects of isotropic and unisotropic biaxial strains are also analyzed in terms of the conditions encoded in the Stoner criterion.

Memristive systems are generalizations of memristors, which are resistors with memory. In this paper, we present a quantum description of quantum dot memristive systems. Using this model we propose and experimentally demonstrate a simple and practical scheme for realizing memristive systems with quantum dots. The approach harnesses a phenomenon that is commonly seen as a bane of nanoelectronics, i.e., switching of a trapped charge in the vicinity of the device. We show that quantum dot memristive systems have hysteresis current-voltage characteristics and quantum jump-induced stochastic behavior. While our experiment requires low temperatures, the same setup could, in principle, be realized with a suitable single-molecule transistor and operated at or near room temperature.

We study the ground-state properties of a double-layer graphene system with the Coulomb interlayer electron-electron interaction modeled within the random-phase approximation. We first obtain an expression of the quantum capacitance of a two-layer system. In addition, we calculate the many-body exchange-correlation energy and quantum capacitance of the hybrid double-layer graphene system at zero temperature. We show an enhancement of the majority density layer thermodynamic density of states owing to an increasing interlayer interaction between two layers near the Dirac point. The quantum capacitance near the neutrality point behaves like a square root of the total density $\alpha \sqrt{n}$ where the coefficient $\alpha$ decreases by increasing the charge-density imbalance between two layers. Furthermore, we show that the quantum capacitance changes linearly by the gate voltage. Our results can be verified by current experiments.