We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated as $M$-boron) by using ab initio evolutionary methodology. $M$-boron is entirely composed of ${\mathrm{B}}_{20}$ clusters in a hexagonal arrangement. Most strikingly, the highest valence band of $M$-boron is isolated, strongly localized, and quite flat, which induces spin polarization on either cap of the ${\mathrm{B}}_{20}$ cluster. This flat band originates from the unpaired electrons of the capping atoms and is responsible for magnetism. $M$-boron is thermodynamically metastable and is the first magnetic 2D form of elemental boron.

Manganese fluoride $\left({\mathrm{MnF}}_2\right)$ with the tetragonal rutile-type structure has been studied using a synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell up to 60 GPa at room temperature combined with first-principles density functional calculations. The experimental data reveal two pressure-induced structural phase transitions with the following sequence: rutile $\to {\mathrm{SrI}}_2$ type (3 GPa)$\to \alpha -{\mathrm{PbCl}}_2$ type (13 GPa). Complete structural information, including interatomic distances, has been determined in the case of ${\mathrm{MnF}}_2$ including the exact structure of the debated first high-pressure phase. First-principles density functional calculations confirm this phase transition sequence, and the two calculated transition pressures are in excellent agreement with the experiment. Lattice dynamics calculations also reproduce the experimental Raman spectra measured for the ambient and high-pressure phases. The results are discussed in line with the possible practical use of rutile-type fluorides in general and specifically ${\mathrm{MnF}}_2$ as a model compound to reveal the HP structural behavior of rutile-type ${\mathrm{SiO}}_2$ (Stishovite).

Compression of arsenolite has been studied from a joint experimental and theoretical point of view. Experiments on this molecular solid at high pressures with different pressure-transmitting media have been interpreted thanks to state-of-the-art ab initio calculations. Our results confirm arsenolite as one of the most compressible minerals and provide evidence for ordered helium trapping above 3 GPa between adamantane-type $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6$ cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$. All properties of $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ are different from those of parent $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6$. In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$, thus resulting in a mechanical stability of $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6·2\mathrm{He}$ beyond 30 GPa. Our work paves the way to explore the formation of alternative compounds by pressure-induced trapping and reaction of gases, small atomic or molecular species, in the voids of molecular solids containing active lone electron pairs.

By decoupling thermal equilibrium conditions in shock loaded porous materials, two regimes resulting in two-zone Hugoniot are revealed, specifically, a stable low-pressure regime characterized by achieving interphase temperature equilibrium and a metastable high-pressure regime. Porous materials, represented as two-phase mixtures, are analyzed with an analytically constructed Hugoniot employing a constant Grüneisen parameter of condensed phase. Results of the analysis agree well with experiments for various porous materials over a wide range of porosities, which confirms the consistency of this interpretation. The two-zone consideration discovers a limitation on the high-temperature states of solid materials accessible via the shock loading of porous materials and suggests that the two-phase nature of porous materials needs to be reflected in the use of Hugoniot data of powders when constructing equations of state for the corresponding condensed materials.

In this paper we analyze the predictions of the forward approximation in some models which exhibit an Anderson (single-body) or many-body localized phase. This approximation, which consists of summing over the amplitudes of only the shortest paths in the locator expansion, is known to overestimate the critical value of the disorder which determines the onset of the localized phase. Nevertheless, the results provided by the approximation become more and more accurate as the local coordination (dimensionality) of the graph, defined by the hopping matrix, is made larger. In this sense, the forward approximation can be regarded as a mean-field theory for the Anderson transition in infinite dimensions. The sum can be efficiently computed using transfer matrix techniques, and the results are compared with the most precise exact diagonalization results available. For the Anderson problem, we find a critical value of the disorder which is $0.9\%$ off the most precise available numerical value already in 5 spatial dimensions, while for the many-body localized phase of the Heisenberg model with random fields the critical disorder $h_c=4.0\pm 0.3$ is strikingly close to the most recent results obtained by exact diagonalization. In both cases we obtain a critical exponent $\nu =1$. In the Anderson case, the latter does not show dependence on the dimensionality, as it is common within mean-field approximations. We discuss the relevance of the correlations between the shortest paths for both the single- and many-body problems, and comment on the connections of our results with the problem of directed polymers in random medium.

We have used inelastic neutron scattering to measure the magnetic excitation spectrum along the high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. The majority of our scattering data are consistent with the expectations of a recently proposed model for the magnetic excitations in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, and we report best-fit parameters for the dominant exchange interactions. Important differences exist, however, between our experimental findings and the model expectations. These include the identification of two energy scales that likely arise due to neglected anisotropic interactions. This feature of our work suggests that anisotropy should be considered in future theoretical work aimed at the full microscopic understanding of the emergence of the skyrmion state in this material.

The evolution of damping in Co/Pt, Co/Au, and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Pt bilayers was studied with increasing nonmagnetic (NM) heavy-metal layer thicknesses in the range $0.2\mathrm{nm}\le t_{\mathrm{NM}}\le 10\mathrm{nm}$, where $t_{\mathrm{NM}}$ is the NM layer thickness. Magnetization precession was measured in the time domain using time-resolved magneto-optical Kerr effect magnetometry. Fitting of the data with a damped sinusoidal function was undertaken in order to extract the phenomenological Gilbert damping coefficient $\alpha$. For Pt-capped Co and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layers a large and complex dependence of $\alpha$ on the Pt layer thickness was observed, while for Au capping no significant dependence was observed. It is suggested that this difference is related to the different localized spin-orbit interaction related to intermixing and to $d\text{−}d$ hybridization of Pt and Au at the interface with Co or ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$. Also it was shown that damping is affected by the crystal structure differences in FM thin films and at the interface, which can modify the spin-diffusion length and the effective spin-mixing conductance. In addition to the intrinsic damping an extrinsic contribution plays an important role in the enhancement of damping when the Pt capping layer is discontinuous. The dependence of damping on the nonmagnetic layer thickness is complex but shows qualitative agreement with recent theoretical predictions.

For a tetragonal material, order parameters of $p_x$ and $p_y$ symmetry are related by rotation and hence have the same $T_{\mathrm{c}}$ at a mean-field level. This degeneracy can be lifted by a symmetry-breaking field, such as (uniaxial) in-plane strain, such that at $T_{\mathrm{c}}$, the order parameter is only of $p_x$ or $p_y$ symmetry. Only at a lower temperature also the respective other order parameter condenses to form a chiral $p$-wave state. At the mean-field level, the derivative of $T_{\mathrm{c}}$ with strain is discontinuous at zero strain. We analyze the consequences of (thermal) fluctuations on the strain-temperature phase diagram within a Ginzburg-Landau approach. We find that the order-parameter fluctuations can drive the transition to be weakly first order, rounding off this discontinuity. We discuss the possibility of a second-order transition into a nonsuperconducting time-reversal-symmetry-breaking phase and consequences for the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We show that a two-dimensional topological superconductor (TSC) can be realized in a hybrid system with a conventional $s$-wave superconductor proximity coupled to a quantum anomalous Hall (QAH) state from the Rashba and exchange effects in single-layer graphene. With very low or even zero doping near the Dirac points, i.e., two inequivalent valleys, this TSC has a Chern number as large as 4, which supports four Majorana edge modes. More importantly, we show that this TSC has a robust topologically nontrivial bulk excitation gap, which can be larger or even 1 order of magnitude larger than the proximity-induced superconducting gap. This unique property paves a way for the application of QAH insulators as seed materials to realize robust TSCs and Majorana modes.

We report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in Fe(Se,Te) thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are, respectively, largely negligible or predominant.

The antiferromagnetic (AFM) insulator-superconductor transition has always been a center of interest in the underlying physics of unconventional superconductors. However, in the family of iron-based high-$T_c$ superconductors, no intrinsic superconductor-insulator transition has been confirmed so far. Here, we report a first-order transition from superconductor to AFM insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal (Li,Fe)OHFeSe. The superconducting transition temperature $\left(T_c\right)$ is continuously enhanced with electron doping by ionic gating up to a maximum $T_c$ of 43 K, and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest $T_c$. A phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out, indicating that the superconductor-insulator transition is a common feature for unconventional superconductivity. These results help to uncover the underlying physics of iron-based superconductivity as well as the universal mechanism of high-$T_c$ superconductivity. Our finding also suggests that the gate-controlled strong charge doping makes it possible to explore novel states of matter in a way beyond traditional methods.

In copper-oxide and iron-based high-temperature (high-$T_{\mathrm{c}})$ superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\mathrm{s}}$ coincides with the magnetic transition temperature $T_{\mathrm{N}}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\mathrm{s}}=54$ K is well separated from $T_{\mathrm{N}}=42$ K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ $\left(0\le x\le 0.042\right)$ that revealed orbital and spin nematicity occurring at a temperature $T^{*}$ far above $T_{\mathrm{s}}$ in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below $T^{*}$ and becomes commensurate below $T_{\mathrm{s}}$, which brings about the observed spin nematicity.

We use neutron diffraction to study the structure and magnetic phase diagram of the newly discovered pressure-induced superconductor CrAs. Unlike most magnetic unconventional superconductors where the magnetic moment direction barely changes upon doping, here we show that CrAs exhibits a spin reorientation from the $ab$ plane to the $ac$ plane, along with an abrupt drop of the magnetic propagation vector at a critical pressure ($P_c\approx 0.6$ GPa). This magnetic phase transition, accompanied by a lattice anomaly, coincides with the emergence of bulk superconductivity. With further increasing pressure, the magnetic order completely disappears near the optimal $T_c$ regime ($P\approx 0.94$ GPa). Moreover, the Cr magnetic moments tend to be aligned antiparallel between nearest neighbors with increasing pressure toward the optimal superconductivity regime. Our findings suggest that the noncollinear helimagnetic order is strongly coupled to structural and electronic degrees of freedom, and that the antiferromagnetic correlations between nearest neighbors might be essential for superconductivity.

Using path integral Monte Carlo (PIMC) and density functional molecular dynamics (DFT-MD) simulation methods, we compute a coherent equation of state (EOS) of nitrogen that spans the liquid, warm dense matter (WDM), and plasma regimes. Simulations cover a wide range of density-temperature space, $1.5–13.9\mathrm{g}{\mathrm{cm}}^{-3}$ and ${10}^3–{10}^9$ K. In the molecular dissociation regime, we extend the pressure-temperature phase diagram beyond previous studies, providing dissociation and Hugoniot curves in good agreement with experiments and previous DFT-MD work. Analysis of pair-correlation functions and the electronic density of states in the WDM regime reveals an evolving plasma structure and ionization process that is driven by temperature and pressure. Our Hugoniot curves display a sharp change in slope in the dissociation regime and feature two compression maxima as the K and L shells are ionized in the WDM regime, which have some significant differences from the predictions of plasma models.

Hand-on routes to reduce lattice thermal conductivity (LTC) in bismuth have been explored by employing a combination of Boltzmann's transport equation and ab initio calculations of phonon-phonon interaction within the density functional perturbation theory. We have first obtained the temperature dependence of the bulk LTC in excellent agreement with available experiments. A very accurate microscopic description of heat transport has been achieved and the electronic contribution to thermal conductivity has been determined. By controlling the interplay between phonon-phonon interaction and phonon scattering by sample boundaries, we predict the effect of size reduction for various temperatures and nanostructure shapes. The largest heat transport reduction is obtained in polycrystals with grain sizes smaller than 100 nm.

Laser-induced ultrafast demagnetization in TbFeCo has been studied with a dual-pumping system. Five different laser fluence combinations were applied at three different time intervals between two pump pulses. The experimental results are also compared with computational simulations using the atomistic model. Importantly, this demagnetization can be controllably manipulated in both its magnitude and temporal response.

We present muon spin rotation $\left(\mu \mathrm{SR}\right)$ and susceptibility measurements on single crystals of isoelectronically doped ${\mathrm{URu}}_{2-x}{\mathrm{T}}_x{\mathrm{Si}}_2$ (T = Fe, Os) for doping levels up to 50%. Zero field (ZF) $\mu \mathrm{SR}$ measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment-hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping.

Crystals of superconducting ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ $\left(T_c=9.8\mathrm{K}\right)$ and ${\mathrm{Mo}}_{8-x}{\mathrm{V}}_x{\mathrm{Ga}}_{41}$ limited solid solution $[x_{\max }=1.9\left(2\right)]$ have been grown from the high-temperature Ga flux and confirmed to have the ${\mathrm{V}}_8{\mathrm{Ga}}_{41}$ type of crystal structure at room temperature. Thermodynamic and transport measurements as well as electronic structure calculations were performed to investigate both normal- and superconducting-state properties of ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$. The discontinuity in the heat capacity at $T_c, \mathrm{\Delta }{c}_p/{\gamma }_N{T}_c=2.83$ in zero magnetic field, indicates a much stronger electron-phonon coupling than in the standard BCS limit. From the heat-capacity data, we estimated the electron-phonon coupling constant ${\lambda }_{ep}=0.9$. The upper critical field is ${\mu }_0{H}_{c2}\left(0\right)=8.3\mathrm{T}$, while the lower one is only ${\mu }_0{H}_{c1}\left(0\right)=131\mathrm{Oe}$. The upper critical field of ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ exhibits a clear enhancement with respect to the Werthamer-Helfand-Honenberg prediction, consistent with the strong electron-phonon coupling. The ${\mathrm{Mo}}_{8-x}{\mathrm{V}}_x{\mathrm{Ga}}_{41}$ limited solid solution also exhibits superconducting properties, and the critical temperature $T_c$ is reduced only slightly with increasing $x$.

In the heavy-fermion superconductor ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{CoIn}}_5$, Yb doping was reported to cause a possible change from nodal $d$-wave superconductivity to a fully gapped $d$-wave molecular superfluid of composite pairs near $x\approx 0.07$ (nominal value $x_{\mathrm{nom}}=0.2$). Here we present systematic thermal conductivity measurements on ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{CoIn}}_5$ ($x=0.013$, 0.084, and 0.163) single crystals. The observed finite residual linear term ${\kappa }_0/T$ is insensitive to Yb doping, verifying the universal heat conduction of the nodal $d$-wave superconducting gap in ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{CoIn}}_5$. Similar universal heat conduction is also observed in the $\mathrm{CeCo}({\mathrm{In}}_{1-y}{\mathrm{Cd}}_y$)${}_5$ system. These results reveal a robust nodal $d$-wave gap in ${\mathrm{CeCoIn}}_5$ upon Yb or Cd doping.

The pressure dependence of the upper critical field, $H_{\mathrm{c}2,\mathrm{c}}$, of single crystalline FeSe was studied using measurements of the interplane resistivity, ${\rho }_{\text{c}}$, in magnetic fields parallel to tetragonal $c$ axis. $H_{\mathrm{c}2,\mathrm{c}}\left(T\right)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_{\mathrm{c}}$, of FeSe exhibits a nonmonotonic dependence with local maximum at $p_1\approx 0.8$ GPa and local minimum at $p_2\approx 1.2$ GPa. The slope of the upper critical field at $T_{\mathrm{c}},{\left(d{H}_{\text{c2,c}}/dT\right)}_{T_{\mathrm{c}}}$, also exhibits a nonmonotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p<p_1$ the slope can be described within a multiband orbital model. For both $p_1<p<p_2$ and $p>p_2$ the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov–de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_{\mathrm{c}}\left(p\right)$ at $p_2\approx 1.2$ GPa.