Heavy-fermion systems and other strongly correlated electron materials often exhibit a competition between antiferromagnetic (AF) and singlet ground states. Using exact quantum Monte Carlo simulations, we examine the effect of impurities in the vicinity of such an AF-singlet quantum critical point (QCP), through an appropriately defined “impurity susceptibility” ${\chi }_{\mathrm{imp}}$. Our key finding is a connection within a single calculational framework between AF domains induced on the singlet side of the transition and the behavior of the nuclear magnetic resonance (NMR) relaxation rate $1/T_1$. We show that local NMR measurements provide a diagnostic for the location of the QCP, which agrees remarkably well with the vanishing of the AF order parameter and large values of ${\chi }_{\mathrm{imp}}$.

In multiferroic ${\mathrm{BiFeO}}_3$ a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that substituting samarium for 10% of the bismuth ions increases the periodicity of the room-temperature cycloid, and upon cooling to below $\sim 15$ K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from $f\text{−}d$ exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order—a result of general significance regarding the stability of noncollinear magnetic structures in the presence of multiple magnetic sublattices.

In two-dimensional Dirac semimetals, Cooper pairing instability occurs only when the attractive interaction strength $\left|u\right|$ is larger than some critical value $|u_c|$ because the density of states vanishes at Dirac points. Disorders enhance the low-energy density of states but meanwhile shorten the lifetime of fermions, which tend to promote and suppress superconductivity, respectively. To determine which of the two competing effects wins, we study the interplay of Cooper pairing interaction and disorder scattering by means of renormalization group method. We consider three types of disorders, including random mass, random gauge potential, and random chemical potential, and show that the first two suppress superconductivity. In particular, the critical BCS coupling $|u_c|$ is increased to certain larger value if the system contains only random mass or random gauge potential, which makes the onset of superconductivity more difficult. In the case of random chemical potential, the effective disorder parameter flows to the strong coupling regime, where the perturbation expansion breaks down and cannot provide a clear answer concerning the fate of superconductivity. When different types of disorder coexist in one system, their strength parameters all flow to strong couplings. In the strong coupling regime, the perturbative renormalization group method becomes invalid, and one needs to employ other methods to treat the disorder effects. We perform a simple gap equation analysis of the impact of random chemical potential on superconductivity by using the Abrikosov-Gorkov diagrammatic approach, and also briefly discuss the possible generalization of this approach.

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved (“wiped-out”) signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates.

Hydrogenation provides a novel way to tune the electronic properties of graphene. Recent scanning tunneling microscopy experiments have demonstrated that local graphene magnetism can be selectively switched on and off by hydrogen (H) dimers. Employing first-principles calculations in conjunction with the constrained random-phase approximation we determine the strength of the effective Coulomb interaction $U$ in hydrogenated graphene. We find that the calculated $U$ parameters are smaller than the ones in graphene and depend on the H concentration. Moreover, the $U$ parameters are very sensitive to the position of H atoms adsorbed on the graphene lattice. We discuss the instability of the paramagnetic state of the hydrogenated graphene towards the ferromagnetic one on the basis of calculated $U$ parameters within the Stoner model. Spin-polarized calculations reveal that the itinerant ferromagnetism in hydrogenated graphene can be well accounted for by the Stoner model.

We demonstrate the linear frequency conversion of ferromagnetic resonance (FMR) frequency by optically excited elastic waves in a thin metallic film on dielectric substrates. Time-resolved probing of the magnetization directly witnesses magnetoelastically driven parametric second-harmonic generation, sum- and difference-frequency mixing from two distinct frequencies, as well as excitation of parametric resonances. Starting from the Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an elastically driven (nonlinear) parametric oscillator and show that frequency mixing is dominated by the parametric modulation of the linear FMR oscillator.

The Slater-Pauling rule states that $L{2}_1$ Heusler compounds with 24 valence electrons never exhibit a total spin magnetic moment. In the case of strongly localized magnetic moments at one of the atoms (here Mn) they will exhibit a fully compensated half-metallic ferrimagnetic state instead, in particular, when symmetry does not allow for antiferromagnetic order. With the aid of magnetic and anomalous Hall effect measurements, it is experimentally demonstrated that ${\mathrm{Mn}}_{1.5}{\mathrm{V}}_{0.5}\mathrm{FeAl}$ follows such a scenario. The ferrimagnetic state is tuned by the composition. A small residual magnetization, which arises due to a slight mismatch of the magnetic moments in the different sublattices, results in a pronounced change of the temperature dependence of the ferrimagnet. A compensation point is confirmed by observation of magnetic reversal and sign change of the anomalous Hall effect. Theoretical models are presented that correlate the electronic structure and the compensation mechanisms of the different half-metallic ferrimagnetic states in the Mn-V-Fe-Al Heusler system.

The report on square ice sandwiched between two graphene layers by Algara-Siller et al. [Nature (London) 519, 443 (2015)] has generated a large interest in this system. By applying high lateral pressure on nanoconfined water, we found that monolayer ice is transformed to bilayer ice when the two graphene layers are separated by $\mathrm{H}=6,7$ Å. It was also found that three layers of a denser phase of ice with smaller lattice constant are formed if we start from bilayer ice and apply a lateral pressure of about 0.7 GPa with $\mathrm{H}=8,9$ Å. The lattice constant (2.5–2.6 Å) in both transitions is found to be smaller than those typical for the known phases of ice and water, i.e., 2.8 Å. We validate these results using ab initio calculations and find good agreement between ab initio O-O distance and those obtained from classical molecular dynamics simulations. The reversibility of the mentioned transitions is confirmed by decompressing the systems.

The capillary forces in mesoporous materials, when imbibed with liquid, are large enough to induce mechanical deformations. Using anisotropic porous silicon, we show that systematic measurements of strain as a function of the pore pressure can yield most of the elastic constants characterizing the porous matrix. The results of this poroelastic approach are in agreement with independent standard stress-strain measurements. The porosity dependence of Young's moduli as well as the values of Poisson's ratios are qualitatively consistent with porous silicon having a honeycomb structure. For a quantitative comparison, we performed finite element modeling of realistic pore geometries. The calculated elastic moduli are found to be much smaller than the measured ones. This is presumably due to both (i) finite-size effects, the Young's modulus of the 5-nm thick walls of the honeycomb could be notably smaller than the Young's modulus of bulk Si, and (ii) defects of the honeycomb structure along the pore axis.

We study the electron transport through the graphene $pnp$ junction under a magnetic field and show that mode mixing plays an essential role. By using the nonequilibrium Green's function method, the space distribution of the scattering state for a specific incident mode as well the elements of the transmission and reflection coefficient matrices are investigated. All elements of the transmission (reflection) coefficient matrices are very different for a perfect $pnp$ junction, but they are the same at a disordered junction due to the mode mixing. The space distribution of the scattering state for the different incident modes also exhibits similar behaviors, i.e., that they distinctly differ from each other in the perfect junction but are almost the same in the disordered junction. For a unipolar junction, when the mode number in the center region is less than that in the left and right regions, the fluctuations of the total transmission and reflection coefficients are zero, although each element has a large fluctuation. These results clearly indicate the occurrence of perfect mode mixing and play an essential role in a graphene $pnp$ junction transport.

The magnetization dynamics of exchange-biased IrMn/CoFe bilayers have been investigated using broadband and in-plane angle-dependent ferromagnetic resonance spectroscopy. The interface energy of the exchange bias effect in these bilayers exceeds values previously reported for metallic antiferromagnets. A strong perpendicular magnetic anisotropy and a small in-plane uniaxial anisotropy are also observed in these films. The magnetization relaxation of the bilayers has a strong unidirectional contribution, which is in part caused by two-magnon scattering. However, a detailed analysis of in-plane angle– and thickness-dependent linewidth data strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism.

Since its discovery in 2014, the magnetic tetragonal $C_4$ phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the $C_4$ phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the $C_4$ instability. Here, we report the discovery of the $C_4$ phase in a fourth member of the hole-doped 122 materials ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}(0.20\le x\le 0.50)$ as determined from neutron and x-ray powder diffraction studies. The maximum of the $C_4$ dome is observed at $x=0.44$ with a reentrant temperature $T_r=52$ K and an extent of $\mathrm{\Delta }x\sim 0.07$ in composition. It is observed that for a range of compositions within the $C_4$ dome $(0.40\le x\le 0.42$), there is a second reentrance $(T_{r_2}<T_r)$ where the antiferromagnetic $C_2$ phase is recovered—a feature previously only seen in ${\mathrm{Ba}}_{\text{1-x}}{\mathrm{K}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$. A phase diagram is presented for ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ and compared to the other $\mathrm{Na}$-doped 122's—$A_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ with $A$ = Ba, Sr, and Ca. The structural parameters for these three systems are compared and the importance of the “chemical pressure” due to changing the $A$-site ion ($A$ = Ba, Sr, Ca) is discussed.

We present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$: monoclinic $M_2$ and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the $M_2$ and triclinic phases. Moreover, the optical interband features in the $M_2$ and triclinic phases are remarkably similar to those observed in the well-studied monoclinic $M_1$ insulating phase of $\mathrm{V}{\mathrm{O}}_2$. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.

Spin-phonon coupling is investigated in epitaxially strained $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the $\mathrm{S}{\mathrm{r}}_{0.6}\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$ composition grown on ${\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_{0.3}{\left(\mathrm{S}{\mathrm{r}}_2\mathrm{AlTa}{\mathrm{O}}_6\right)}_{0.7}$ substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a $17\%$ hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with Néel temperature ($T_N$) around 150 K. Thus, our results confirm that epitaxial $\mathrm{S}{\mathrm{r}}_{0.6}\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$ thin films display strong spin-phonon coupling below $T_N$, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ system.

In an isotropic strongly interacting quantum liquid without quasiparticles, general scaling arguments imply that the dimensionless ratio $(k_B/\hslash )\eta /s$, where $\eta$ is the shear viscosity and $s$ is the entropy density, is a universal number. We compute the shear viscosity of the Ising-nematic critical point of metals in spatial dimension $d=2$ by an expansion below $d=5/2$. The anisotropy associated with directions parallel and normal to the Fermi surface leads to a violation of the scaling expectations: $\eta$ scales in the same manner as a chiral conductivity, and the ratio $\eta /s$ diverges at low temperature ($T$) as $T^{-2/z}$, where $z$ is the dynamic critical exponent for fermionic excitations dispersing normal to the Fermi surface.

We have carried out a comprehensive study of the intrinsic anomalous Hall effect and spin Hall effect of several chiral antiferromagnetic compounds ${\mathrm{Mn}}_{3}X$ ($X$ = Ge, Sn, Ga, Ir, Rh and Pt) by ab initio band structure and Berry phase calculations. These studies reveal large and anisotropic values of both the intrinsic anomalous Hall effect and spin Hall effect. The ${\mathrm{Mn}}_{3}X$ materials exhibit a noncollinear antiferromagnetic order which, to avoid geometrical frustration, forms planes of Mn moments that are arranged in a Kagome-type lattice. With respect to these Kagome planes, we find that both the anomalous Hall conductivity (AHC) and the spin Hall conductivity (SHC) are quite anisotropic for any of these materials. Based on our calculations, we propose how to maximize AHC and SHC for different materials. The band structures and corresponding electron filling, that we show are essential to determine the AHC and SHC, are compared for these different compounds. We point out that ${\mathrm{Mn}}_3\mathrm{Ga}$ shows a large SHC of about 600 $(\hslash /e){\left(\mathrm{\Omega }\text{cm}\right)}^{-1}$. Our work provides insights into the realization of strong anomalous Hall effects and spin Hall effects in chiral antiferromagnetic materials.

A sufficiently strong long-range Coulomb interaction can induce excitonic pairing in gapless Dirac semimetals, which generates a finite gap and drives the semimetal-insulator quantum phase transition. This phenomenon is in close analogy to dynamical chiral symmetry breaking in high-energy physics. In most realistic Dirac semimetals, including suspended graphene, the Coulomb interaction is too weak to open an excitonic gap. The Coulomb interaction plays a more important role at low energies in a two-dimensional semi-Dirac semimetal, in which the fermion spectrum is linear in one component of momenta and quadratic in the other, than a Dirac semimetal, and indeed leads to breakdown of Fermi liquid theory. We study dynamical excitonic gap generation in a two-dimensional semi-Dirac semimetal by solving the Dyson-Schwinger equation, and show that a moderately strong Coulomb interaction suffices to induce excitonic pairing. Additional short-range four-fermion coupling tends to promote excitonic pairing. Among the available semi-Dirac semimetals, we find that the ${\mathrm{TiO}}_2$/${\mathrm{VO}}_2$ nanostructure provides a promising candidate for the realization of an excitonic insulator. We also apply the renormalization group method to analyze the strong coupling between the massless semi-Dirac fermions and the quantum critical fluctuation of the excitonic order parameter at the semimetal-insulator quantum critical point, and reveal non-Fermi liquid behaviors of semi-Dirac fermions.

Recent experiments demonstrating proximity induced ferromagnetism in graphene motivate this study of commensurate graphene/EuO heterostructures. Due to the commensurability of graphene with the (111)-EuO layer, graphene's Dirac points are mapped to the $\mathbf{\Gamma }$ point of the commensurate Brillouin zone. The Eu atoms not only induce proximity exchange on the graphene layer, but they also introduce intervalley interactions resulting in a nonlinear dispersion at $\mathbf{\Gamma }$. We develop a model Hamiltonian, consistent with the lattice symmetries, that includes proximity induced exchange splitting, spin-orbit coupling, and intervalley interactions with parameters fitted to ab initio calculations. The intervalley interaction opens up a trivial gap preventing the system from crossing into a nontrivial state. The model Hamiltonian is analyzed to determine the conditions under which the heterostructures can exhibit topologically nontrivial bands.