Nematic order in the iron-based superconductors is closely tied to a lattice distortion and a structural transition from tetragonal to orthorhombic symmetry. External stress of the appropriate symmetry acts as a conjugate field of the nematic order parameter and can be utilized to detwin nematic domains but also smears an otherwise sharp nematic transition. On the other hand, applying stress in proper symmetry channels allows one to tune the nematic phase transition. Recent experiments analyzed the stress-induced changes of the nematic and magnetic phase transition temperature. Here we show that the observed trends can be understood in terms of spin-induced nematicity. The strain sensitivity is shown to be a fluctuation effect. The strong sensitivity to antisymmetric strain is a consequence of the anisotropic nature of the magnetic excitation spectrum. The formalism presented here can be naturally generalized to determine the strain dependence of vestigial phases related to other magnetic states that have been observed in the iron-based systems, such as the spin-charge density wave and the spin-vortex crystals.

Chemical doping is an effective method for tailoring physical properties of semiconductors. It has been long known that trace-level doping can obviously influence vibrational properties of bulk semiconductors. Nevertheless, less is known about the effect of chemical doping on the lattice dynamics of two-dimensional (2D) semiconductors although electron-phonon interaction can significantly influence the electronic mobility of the 2D semiconductors. In this study, we carried out Raman-scattering measurements in combine with atom-resolved scanning transmission electron microscope characterization to unveil the effects of Re doping on the phonon dispersion of single-layer (1L) $\mathrm{Mo}{\mathrm{S}}_2$. Second-order Raman modes in a wide spectral range reveal the distinct softening of the longitudinal acoustic phonon branch of 1L $\mathrm{Mo}{\mathrm{S}}_2$ but with an imperceptible shift of other branches. Such zone-edge phonon renormalization mainly results from electron doping via strong electron-phonon coupling along with a minor contribution from the increased atomic weight from heavier Re dopants. This study enriches the understanding of dopant-host interactions in 2D materials in the aspect of phonons, which is important for tailoring physical properties of 2D materials, such as thermal and electrical transport.

A formal description of collective electronic states in condensed-matter systems lacking long-range periodicity remains a theoretical challenge. To experimentally explore the differences in electronic and magnetic behavior between metallic quasicrystals (QCs) and their conventional crystalline analogs [quasicrystal approximants (QCAs)], we have grown single crystals of ${\mathrm{Y}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Cd}}_6$ (QCA) together with their QC counterparts $i\text{−}{\mathrm{Y}}_{1-x}{\mathrm{Gd}}_x\text{−}\mathrm{Cd}$ for $x=0.006$, 0.01, 0.1, and 1.00, and we carried out comparative $T$-dependent electron spin resonance (ESR) measurements. On the high Gd concentration side, $x=1.00$, we confirm that ${\mathrm{GdCd}}_6$ adopts an antiferromagnetic ground state below $T_N\sim 22\mathrm{K}$, whereas $i$-Gd-Cd presents spin-glass-like behavior showing similar local and dynamical properties from the point of view of ESR. For the diluted samples, our ESR experimental results show similar local conduction electron polarization behavior at the ${\mathrm{Gd}}^{3+}$ site in all QC/QCA pairs investigated, supporting the validity of using QCAs as periodic representations of QCs in terms of short-range electronic interactions. However, there is a measurable difference in the Korringa relaxation rate (spin-flip relaxation process between the localized ${\mathrm{Gd}}^{3+}4f$ electron and the delocalized $s$-type conduction electrons at the Fermi surface) between the QC/QCA pairs probably associated with the lack of periodicity. We expect that our comparative ESR study may provide support and motivation for the development of new theoretical approaches toward a generalized band-structure theory, contemplating condensed-matter systems beyond the scope of traditional periodicity.

The high-entropy alloys ${\mathrm{Al}}_x\mathrm{CrFeCoNi}$ exist over a broad range of Al concentrations ($0<x<2$). With increasing Al content their structure is changed from the fcc to bcc phase. We investigate the effect of such structural changes on transport properties including the residual resistivity and the anomalous Hall resistivity. We have performed a detailed comparison of the first-principles simulations with available experimental data. We show that the calculated residual resistivities for all studied alloy compositions are in a fair agreement with available experimental data as concerns both the resistivity values and concentration trends. We emphasize that a good agreement with experiment was obtained also for the anomalous Hall resistivity. We have completed study by estimation of the anisotropic magnetoresistance, spin-disorder resistivity, and Gilbert damping. The obtained results prove that the main scattering mechanism is due to the intrinsic chemical disorder whereas the effect of spin polarization on the residual resistivity is appreciably weaker.

We report on our investigation on the magnetism of the iridate double perovskite ${\mathrm{Sr}}_2{\mathrm{CoIrO}}_6$, a nominally ${\mathrm{Ir}}^{5+}$ Van Vleck $J_{\mathrm{eff}}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{\mathrm{eff}}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-$L_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% ${\mathrm{Ir}}^{5+}$ ($J_{\mathrm{eff}}=0$) and ${\mathrm{Co}}^{3+}$ ($S=2$) and 10% ${\mathrm{Ir}}^{6+}$ ($S=3/2$) and ${\mathrm{Co}}^{2+}$ ($S=3/2$). The magnetic signal of this minority ${\mathrm{Ir}}^{6+}$ component is almost equally as strong as that of the main ${\mathrm{Ir}}^{5+}$ component. We infer that there is a competition between the ${\mathrm{Ir}}^{5+}-{\mathrm{Co}}^{3+}$ and the ${\mathrm{Ir}}^{6+}-{\mathrm{Co}}^{2+}$ configurations in this stoichiometric compound.

Atomistic first-principles-based simulations are used to investigate polarization reversal in ferroelectrics in both the intrinsic and extrinsic regimes in order to determine the origin of nearly an order of magnitude difference in the coercive field predicted theoretically and observed in experiments. We find that the residual depolarizing field that is routinely ignored from considerations is responsible for the drastic reduction of the coercive field. The depolarizing field stabilizes a polydomain phase which allows for counterintuitive cooperation with the applied field to achieve polarization reversal in an energy-efficient way. Contrary to the common belief that low coercive field necessitates polarization reversal via domains formation and propagation, we predict that the same fields could be achieved if the residual depolarizing field is taken into account. An efficient way to incorporate such depolarizing field in any type of atomistic simulations is proposed, which is expected to resolve the longstanding issue of overestimation of fields in simulations of ferroelectrics.

We investigate the magnetic, thermal, and electrical transport properties on the single-crystalline compound ${\mathrm{EuBiTe}}_3$. The magnetization measurements reveal the antiferromagnetic transition at 7 K along the $ab$ plane with a stable ${\mathrm{Eu}}^{2+}$ ion. A short-range ferromagnetic interaction along the $c$ axis is inferred from magnetic susceptibility and heat capacity measurements, which we associate with the presence of magnetic polarons. The magnetic polaron gives rise to unconventional negative magnetoresistance such as the weak antilocalization (WAL) and weak localization crossover. The WAL of the angle-resolved magnetoconductivity is well described by the Al'tshuler and Aronov model along the longitudinal magnetoconductivity. The magnetic polaron is widely observed above Néel temperature up to 50 K, which is supported by the Majumdar-Littlewood model in a scaled magnetoresistance. Even though ${\mathrm{EuBiTe}}_3$ should have one hole per formula unit, the electrical conductivity $\sigma \left(T\right)$ exhibits strong localized insulating behavior. The transport mechanism of $\sigma \left(T\right)$ is distinguished by two different mechanism: Efros-Shklovskii-type variable range hopping (VRH) transport in the high-temperature range ($T\ge 52$  K) and three-dimensional VRH transport in the low-temperature regime ($T\le$ 50 K). The VRH transport suggests that the insulating behavior of $\sigma \left(T\right)$ on the compound can originate from a Mott-type insulator with strong Coulomb interaction.

Nontrivial spin configurations tend to bring about novel transport properties in magnetic materials. We report the magnetic anisotropy and topological Hall effect in trigonal ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ single crystals. Below the magnetic transition temperature, the spins are canted ferromagnetic aligned along $c$ axis with antiferromagnetic components in $ab$ plane. This peculiar magnetic structure gives rise to the anisotropic anomalous Hall effect (AHE) in ${\mathrm{Cr}}_5{\mathrm{Te}}_8$. When the current flows along the $ab$ plane, ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ exhibits a conventional AHE depicted by skew scattering mechanism. On the other hand, the topological Hall effect appears with current along $c$ axis, which stems from the noncoplanar spin configuration with scalar spin chirality during the spin flop process.

The way doping affects the spin wave stiffness and the exchange stiffness of permalloy (Py) is investigated via ab initio calculations, using the Korringa-Kohn-Rostoker (KKR) Green function formalism. By considering various types of dopants of different nature (V, Gd, and Pt), we are able to draw general conclusions. To describe the trends of the stiffness with doping it is sufficient to account for the exchange coupling between nearest neighbors. The polarizability of the impurities is not important for the spin wave stiffness. Rather, the decisive factor is the hybridization between the impurity and the host states as reflected by changes in the Bloch spectral function. Our theoretical results agree well with earlier experiments.

Superlenses are imaging components that can overcome the diffraction limit associated with conventional dielectric lenses. An ideal superlens is a flat infinite slab of homogeneous double-negative material with $\varepsilon =\mu =-1$ embedded in air, which achieves perfect imaging by restoring all the spatial frequency components of an object to the image plane. It has been shown that any deviation from these homogeneous material parameter values limits the resolution of the lens and introduces a surface mode resonance with fields that dominate the image. While material loss can suppress the resonant mode, loss also reduces spatial resolution. In this paper, we investigate resonant modes arising in metamaterial superlenses of infinite and finite extent and show how they cause imaging artifacts and reduce imaging fidelity. We choose a well-studied periodic structure consisting of an array of magnetodielectric cylinders for the lens medium under test. We demonstrate that the presence of these artifacts can lead to erroneous interpretation of the standard two-source resolution test for lenses. We show that artifacts can be mitigated by introducing point defects into the array, which move the resonant modes to higher spatial frequencies and, in the case of finite lenses, suppress their amplitudes through radiation losses. This strategy enables more robust and reliable subwavelength imaging performance, improves the spatial resolution of the metamaterial lens, and reduces the deleterious effects of material losses.

When the interface of a superconductor (SC) with a Weyl semimetal (WSM) supports Fermi arcs, the chirality blockade eliminates the induction of superconductivity into the bulk of time-reversal symmetry (TRS) breaking WSM. This leaves the Fermi arc states as the only low-energy degrees of freedom in the proximity problem. Therefore the $\mathrm{SC}|\mathrm{WSM}$ system will be a platform to probe transport properties which involve only the Fermi arcs. With a boundary condition that flips the spin at the boundary, we find a $Z_2$ protected Bogoliubov Fermi contour (BFC) around which the Bogoliubov quasiparticles disperse linearly. The resulting BFC and excitations around it leave a distinct $T^2$ temperature dependence in their contribution to specific heat. Furthermore, the topologically protected BFC being a Majorana Fermi surface gives rise to a zero-bias peak, the strength of which characteristically depends on the length of Fermi arc and tunneling strength. For the other BC that flips the chirality at the interface, instead of BFCs, we have Bogoliubov-Weyl nodes whose location depends on the tunneling strength.

We develop the tensor renormalization group (TRG) algorithm for statistical systems with open boundaries, which allows us to investigate not only the bulk but also the boundary property, such as the surface magnetization. We demonstrate that the tensors representing the boundary in our algorithm exhibit the fixed-point structures, just as bulk tensors in previous TRG algorithms. At criticality, the scale-invariant boundary fixed-point tensors have the information of the conformal tower, which is described by the underlying boundary conformal field theory.

A Fermi-Hubbard system with a periodically-modulated interaction has been recently shown to resonantly absorb energy at a series of drive frequencies. In the present work, with the help of static perturbation theory, we argue that driving couples to Hubbard bands in a similar fashion as a classical field couples to a two-level atom. The latter, significantly simpler setup, described by the Rabi model, is known to support resonant rapid energy absorption at virtually the same series of driving frequencies. Our interpretation is also supported by the equivalency between dynamics of the two-site periodically driven Hubbard and Rabi models. Relying on this insight, we establish that the same Rabi-resonant behavior is displayed also by fermions confined in a harmonic trap with periodically driven contact interactions.

A monolayer of jacutingaite (${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$) has recently been identified as a novel quantum spin Hall insulator. By first-principles calculations, we study its Fermiology in the doped regime and unveil a type-I and type-II van Hove singularity for hole and electron doping, respectively. We find that the common link between the propensity for a topological band gap at pristine filling and unconventional superconductivity at finite doping is rooted in the longer-ranged hybridization integrals on the honeycomb lattice. In a combined effort of random phase approximation and functional renormalization group, we find chiral $d$-wave order for the type-I and odd-parity $f$-wave order for the type-II regime. When longer-ranged Coulomb interaction is included, a propensity of the type-II regime towards a topological $p_x+i{p}_y$-wave order emerges.

Despite a rapidly expanding inventory of possible crystalline Weyl semimetals, all of them are constrained by the Nielsen-Ninomiya no-go theorem, namely, that left- and right-handed Weyl points appear in pairs. With time-reversal ($\mathcal{T}$) symmetry, an even stronger version holds for the semimetals, i.e., all eight time-reversal-invariant points in the Brillouin zone (BZ) simultaneously host Weyl points or not. Accompanying the no-go theorem, the surface of the system features Fermi arc states, which connect pairs of surface projected Weyl points. Here, we explicitly construct a topological phase of a $\mathcal{T}$-invariant crystalline metal, which features a single Weyl point residing at the center of the BZ, surrounded by nodal walls spreading over the entire BZ boundary. In other words, a single Weyl point is realized with the no-go theorem being circumvented. Moreover, the surface Fermi arcs, considered as a hallmark of Weyl semimetals, do not appear for this composite topological phase. We show that this phase can be realized for space groups No. 19 and No. 92, with and without spin-orbit coupling, respectively.

We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities ($2.0-11\times {10}^{11}{\mathrm{cm}}^{-2}$) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov–de Haas oscillations, we measure a light mass of $0.061m_e$ at a density of $2.2\times {10}^{11}{\mathrm{cm}}^{-2}$. We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of $\sim 0.05m_e$ at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.