We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\parallel \left[100\right]$ in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q\to 0$ limit and, hence, sensitive to the tetragonal to orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.94}{\mathrm{Co}}_{0.06}\right)}_2{\mathrm{As}}_2$ [Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of $C_{66}$ deduced from our experiments and those derived from measurements of the Young modulus in ${\mathrm{Ba}}_{1-x}{\left(\mathrm{K},\mathrm{Na}\right)}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ at optimal doping. These results provide an upper limit of about 50 Å for the nematic correlation length for the investigated optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where $C_{66}$ is not accessible by volume probes, as well as the $C_4$ tetragonal magnetic phase.

Spinel compound, ${\mathrm{MgV}}_2{\mathrm{O}}_4$, known as a highly frustrated magnet has been extensively studied both experimentally and theoretically for its exotic quantum magnetic states. However, due to its intrinsic insulating nature in its antiferromagnetic (AFM) ground state, its realistic applications in spintronics are quite limited. Here, based on first-principles calculations, we examine the ferromagnetic (FM) phase of ${\mathrm{MgV}}_2{\mathrm{O}}_4$, which was found to host three-dimensional flat band (FB) right near the Fermi level, consequently yielding a large anomalous Hall effect (AHE, $\sigma \approx 670{\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$). Our calculations suggest that the half-metallicity feature of ${\mathrm{MgV}}_2{\mathrm{O}}_4$ is preserved even after interfacing with MgO due to the excellent lattice matching, which could be a promising spin filtering material for spintronics applications. Lastly, we explore experimental feasibility of stabilizing this FM phase through strain and doping engineering. Our study suggests that an experimentally accessible amount of hole doping might induce a AFM-FM phase transition.

A physical model for a mixed-valence impurity in metal must satisfy the Friedel screening theorem for both valences. Such a model is shown, following earlier work which showed low-energy singularities in it, to be supersymmetric, leading to a free Majorana and a phase-shifted Majorana excitation. The theory extended approximately to a lattice of mixed-valence ions at appropriate filling gives, without fine-tuning the parameters, a protected gapless Majorana fermion band across the chemical potential, besides the mixed-valence particle and hole bands separated by gaps. In this situation, the system is electrically neutral in linear response but has de Haas-van Alphen oscillations. This is used to explain the recently observed magneto-oscillations in mixed-valence insulators as well as their accompanying low-energy thermodynamic and relaxation rate anomalies. Some predictions to test the validity of the theoretical results are provided, the most striking of which are that there should be extensive ground-state entropy in such compounds and no Zeeman splitting of the magneto-oscillation frequencies.

Twisted double bilayer graphene has recently emerged as an interesting moiré material that exhibits strong correlation phenomena that are tunable by an applied electric field. Here we study the atomic and electronic properties of three different graphene double bilayers: double bilayers composed of two AB stacked bilayers (AB/AB), double bilayers composed of two AA stacked bilayers (AA/AA), as well as heterosystems composed of one AB and one AA bilayer (AB/AA). The atomic structure is determined using classical force fields. We find that the inner layers of the double bilayer exhibit significant in-plane and out-of-plane relaxations, similar to twisted bilayer graphene. The relaxations of the outer layers depend on the stacking: atoms in AB bilayers follow the relaxations of the inner layers, while atoms in AA bilayers attempt to avoid higher-energy AA stacking. For the relaxed structures, we calculate the electronic band structures using the tight-binding method. All double bilayers exhibit flat bands at small twist angles, but the shape of the bands depends sensitively on the stacking of the outer layers. To gain further insight, we study the evolution of the band structure as the outer layers are rigidly moved away from the inner layers, while preserving their atomic relaxations. This reveals that the hybridization with the outer layers results in an additional flattening of the inner-layer flat band manifold. Our results establish AA/AA and AB/AA twisted double bilayers as interesting moiré materials with different flat band physics compared to the widely studied AB/AB system.

Materials with strong electronic correlations host remarkable—and technologically relevant—phenomena such as magnetism, superconductivity, and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_3$ can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_3$ thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counterintuitive effect is due to a change of orientation of the oxygen octahedra. The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.

We calculate the interactions between the Wannier functions of the 8-orbital model for twisted bilayer graphene (TBG). In this model, two orbitals per valley centered at the AA regions, the AA-p orbitals, account for the most part of the spectral weight of the flats bands. Exchange and assisted-hopping terms between these orbitals are found to be small. Therefore, the low energy properties of TBG will be determined by the density-density interactions. These interactions decay with the distance much faster than in the 2-orbital model, following a $1/r$ law in the absence of gates. The magnitude of the largest interaction in the model, the onsite term between the flat band orbitals, is controlled by the size of the AA regions and is estimated to be $\sim 40$ meV. To screen this interaction, the metallic gates have to be placed at a distance $\lesssim 5$ nm. For larger distances only the long-range part of the interaction is substantially screened. The model reproduces the band deformation induced by doping found in other approaches within the Hartree approximation. Such deformation reveals the presence of other orbitals in the flat bands and is sensitive to the inclusion of the interactions involving them.

We provide an extensive look at Bott periodicity in the context of complex gapped topological phases of free fermions. In doing so, we remark on the existence of a product structure in the set of inequivalent phases induced by the external tensor product of vector bundles—a structure which has not yet been explored in condensed-matter literature. Bott periodicity appears in the form of a generalized Dirac monopole built out of a given phase, which is equivalent to the product of a Dirac monopole phase with that same given phase. The complex $K$-theory cohomology ring is presented as a natural way to store the information of these phases, with a grading corresponding to the number of Clifford symmetries modulo 2. The Künneth formula allows us to derive the result that, for band insulators, the Su-Schrieffer-Heeger (SSH) chain in one dimension allows one to generate the $K$-cohomology of the $d$-dimensional Brillouin zone. In particular, we find that the product of two SSH chains in independent momentum directions yields a two-dimensional Chern insulator. The results obtained relate the associated topological phases of charge-conserving band insulators and their topological invariants in all spatial dimensions in a unified way.

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, $A{B}^{\prime }$, or ABC stacking when directly converting graphite into $s{p}^3$ bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and $A{B}^{\prime }$ stacking configurations, we report the ultrafast formation of a transient two-dimensional diamondlike structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photoinduced phase transition is evidenced by the appearance of bond lengths of 1.94 and 3.14 Å in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first-principles calculation indicate that $s{p}^3$ bonds nucleate at AA and $A{B}^{\prime }$ stacked areas in a moiré pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

Enhancing the light-matter interaction in two-dimensional (2D) materials with high $Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode, two-port system composed of graphene, coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude, from 0.9 nm to 94 nm, by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level, and the layer number of graphene. This work reveals the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.

A proper control of nanoscale friction is mandatory for the fabrication and operation of optimal nanoengineered devices. In this respect, the use of electric fields looks to be promising, since they are able to alter the frictional response without imprinting permanent deformations into the structure. To this aim, we perform ab initio simulations to study the microscopic mechanisms governing friction in low-dimensional materials in the presence of electrostatic fields. We consider ${\mathrm{MX}}_2$ transition metal dichalcogenides as a case study. By applying an electric field along an axis orthogonal to the atom layers, we induce a transfer of charge along the same axis; this transfer modifies the interatomic forces, leading, in general, to easier relative layer motion. The reported outcomes constitute a starting point to study the effect of the field direction on the intrinsic friction in future investigations. Finally, the present results can be used to predict the preferential electronic redistribution in nanostructured devices where metal-to-insulator transitions may occur in working conditions.

We propose that there is a universal picture for different constructions of holographic toy models, and its continuous limit can be described by a gravitylike field theory, namely, the rank-2 U(1) gauge theory. First, we show that two different toy models for holography—the perfect tensor networks and the hyperbolic fracton models—are both equivalent to a picture of evenly distributed bit threads on geodesics in the anti–de Sitter space. We name this picture “geodesic string condensation.” It is actually a natural leading-order approximation to the holographic entanglement structure. Then, we reason that the rank-2 U(1) gauge theory with linearized diffeomorphism as its gauge symmetry, also known as a case of Lifshitz gravity, is the bulk field theory that gives rise to this picture. The Gauss's laws and spatial curvature require the geodesic gauge field lines, instead of the local loops (magnetic fields), to be the fundamental dynamical variables, which lead to geodesic string condensation. These results provide an intuitive way to understand the entanglement structure of gravity in anti–de Sitter/conformal field theory.

Through a high-precision soft x-ray magnetic circular dichroism (XMCD) study of the intermediate-valence compound $\mathrm{Sm}{\mathrm{Os}}_4{\mathrm{Sb}}_{12}$, we show our successful approach of revealing unprecedented details of Kondo screening below a characteristic temperature of $T_0\sim 20$ K in the paramagnetic phase. The multiplet XMCD structure at the Sm $M_5$ edge sensitive to the $4f$ configuration enabled us to observe a clear difference between the temperature evolution above and below $T_0$ with one- and two-component behavior, respectively. Our findings are in strong contrast to the conventional Kondo crossover that coincides with the valence transition, but are qualitatively accounted for by theoretical XMCD predictions combined with the two-fluid phenomenology recently proposed. This work contributes to the large context of Kondo physics that is closely related to quantum criticality in heavy fermion systems.

Higher-order topological insulators (HOTIs) have enticed enormous research interests owing to their novelty in supporting gapless states along the hinges of a crystal. Despite several theoretical predictions, enough experimental confirmation of the HOTI state in crystalline solids is still lacking. It is shown that interplay between topology and magnetism can give rise to various magnetic topological states, including HOTI and axion insulator states. Here, using high-resolution angle-resolved photoemission spectroscopy combined with the first-principles calculations, we report a systematic study of the electronic structure and its evolution across the magnetic phase transition in ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ which possesses an antiferromagnetic ground state below 16 K. Antiferromagnetic ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ has been predicted to host both the axion insulator and the HOTI states. We directly observe the linearly dispersing holelike bands crossing the Fermi level and the change in their dispersion across the magnetic phase transition. Our paper points to ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ as being a promising material for the exploration of interplay between topology and magnetism.

We have performed ${}^{59}\mathrm{Co}$ NMR measurements of ${\mathrm{CeCoIn}}_5$ down to ultralow temperatures. We find that the temperature dependence of the spin-echo intensity provides a good measure of the sample temperature, enabling us to determine a pulse condition not heating up the sample by the NMR pulses down to ultralow temperatures. From the longitudinal relaxation time ($T_1$) measurements at 5 T applied along the $c$ axis, a pronounced peak in $1/T_{1}T$ is observed at 20 mK, implying an appearance of magnetic order as suggested by the recent quantum oscillation measurements [H. Shishido et al., Phys. Rev. Lett. 120, 177201 (2018)]. On the other hand, the NMR spectrum shows no change below 20 mK. Moreover, the peak in $1/T_{1}T$ disappears at 6 and 8 T in contrast to the results of the quantum oscillation. We discuss that an antiferromagnetic state with a moment lying in the $a-b$ plane can be a possible origin for the peak in $1/T_{1}T$ at 5 T.

We study the magnetic-field dependence of the longitudinal spin relaxation time $T_1$ of donor-bound electrons placed in the middle of an 8-nm CdTe quantum well with different doping concentrations in the range from $1\times {10}^9$ to $2.9\times {10}^{11}$ $\mathrm{c}{\mathrm{m}}^{-2}$ and at low temperature. We use an extended photoinduced Faraday rotation technique, which expands the usual domain of the measured decays from tens of ns to $\mu \mathrm{s}$. As in high-purity bulk semiconductors, a maximum relaxation time of around $T_1\sim 10\mu \mathrm{s}$ is observed for a residually doped sample at low magnetic field of $B=0.08\mathrm{T}$. For higher doping concentrations, the magnetic-field dependence of $T_1$ shows a nonmonotonic behavior: first a rapid increase, followed by a plateau or a decrease of $T_1$. The fast increase of $T_1$ at low magnetic fields is explained by the inhibition of the mechanisms identified at zero field—hyperfine and anisotropic exchange interactions—while the behavior at high magnetic field can be succesfully explained by a mechanism proposed by Lyubinskiy and associated to electron hops [I. S. Lyubinskiy, JETP Lett. 88, 814 (2008)]. A good agreement between experiment and theory is found for samples below the metal-insulator transition, when Dresselhaus terms of spin-orbit coupling are considered to be the dominant ones in the Hamiltonian describing the system.

We report the experimental study of the temperature dependence of the longitudinal spin relaxation time $T_1$ of single nitrogen-vacancy (NV) centers hosted in nanodiamonds. To determine the relaxation mechanisms at stake, measurements of the $T_1$ relaxation time are performed for a set of individual NV centers both at room and cryogenic temperatures. The results are consistant with a temperature-dependent relaxation process, which is attributed to a thermally activated magnetic noise produced by paramagnetic impurities lying on the nanodiamond surface. These results confirm the existence of surface-induced spin relaxation processes occurring in nanodiamonds, which are relevant for future developments of sensitive nanoscale NV-based quantum sensors.

In quantum spin systems, a phase transition is studied from the perspective of magnetization curve and a magnetic susceptibility. We propose a new method for studying the anomaly of magnetic susceptibility $\chi$ that indicates a phase transition. In addition, we introduce the fourth derivative $A$ of the lowest-energy eigenvalue per site with respect to magnetization, i.e., the second derivative of ${\chi }^{-1}$. To verify the validity of this method, we apply it to an $S=1/2XXZ$ antiferromagnetic chain. The lowest energy of the chain is calculated by numerical diagonalization. As a result, the anomalies of $\chi$ and $A$ exist at zero magnetization. The anomaly of $A$ is easier to observe than that of $\chi$, indicating that the observation of $A$ is a more efficient method of evaluating an anomaly than that of $\chi$. The observation of $A$ reveals an anomaly that is different from the Kosterlitz-Thouless (KT) transition. Our method is useful in analyzing critical phenomena.

Spin-phonon coupling, the interaction of spins with a surrounding lattice, is a key parameter to understand the underlying physics of multiferroics and to engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal $\mathrm{BaMn}{\mathrm{O}}_3$ are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$ at 230, 280, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$ at 230 and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ∼ 280 K, possibly arising from a local lattice distortion due to the displacement of Mn ions, that exhibits an anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidence from temperature- and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific-heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in $15R\text{−}\mathrm{BaMn}{\mathrm{O}}_3$, thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.