We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting ${\text{BaFe}}_{1.9}{\text{Ni}}_{0.1}{\text{As}}_2$ $\left(T_c=20\text{K}\right)$ is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7 meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm 1$ components of the triplet are involved.

Electrical resistivity, specific-heat, and NMR measurements classify noncentrosymmetric ${\text{Mo}}_3{\text{Al}}_2\text{C}$ ($\beta \text{-Mn}$ type, space group $P{4}_{1}32$) as a strong-coupled superconductor with $T_c=9\text{K}$ deviating notably from BCS-type behavior. The absence of a Hebbel-Slichter peak, a power-law behavior of the spin-lattice relaxation rate (from ${}^{27}\text{A}\text{l}$ NMR), an electronic specific heat strongly deviating from BCS model and a pressure enhanced $T_c$ suggest unconventional superconductivity with possibly a nodal structure of the superconducting gap. Relativistic density-functional theory calculations reveal a splitting of degenerate electronic bands due to the asymmetric spin-orbit coupling, favoring a mix of spin-singlet and spin-triplet components in the superconducting condensate, in absence of strong correlations among electrons.

We have synthesized polycrystalline samples of the noncentrosymmetric superconductor ${\text{Mo}}_3{\text{Al}}_2\text{C}$ by arc and RF melting, measured its transport, magnetic and thermodynamic properties, and computed its band structure. Experimental results indicate a bulk superconducting transition at $T_c\sim 9.2\text{K}$ while the density of states at the Fermi surface is found to be dominated by $\text{Mo}d$ orbitals. Using the measured values for the lower critical field $H_{c1}$, upper critical field $H_{c2}$, and the specific heat $C$, we estimated the thermodynamic critical field $H_c\left(0\right)$, coherence length $\xi \left(0\right)$, penetration depth $\lambda \left(0\right)$, and the Ginzburg-Landau parameter $\kappa \left(0\right)$. The specific-heat jump at $T_c$, $\Delta C/\gamma T_c=2.14$, suggests that ${\text{Mo}}_3{\text{Al}}_2\text{C}$ is moderately to strongly coupled, consistent with the fast opening of the gap, as evidenced by the rapid release of entropy below $T_c$ from our electronic specific-heat measurements. Above 2 K the electronic specific heat exhibits the power-law behavior, suggesting that synthesis of single crystals and measurements at lower temperature are needed to establish whether the gap is anisotropic. The estimated value of the upper critical field $H_{c2}\left(0\right)$ is close to the calculated Pauli limit, therefore further studies are needed to determine whether the absence of an inversion center results in a significant admixture of the triplet component of the order parameter.

We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride ($h$-BN) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, x-ray photoemission, and absorption techniques. It has been demonstrated that the transition of the $h$-BN layer from the “rigid” into the “quasifreestanding” state is accompanied by a change in its lattice constant. Using chemical vapor deposition, graphene has been successfully synthesized on the insulating, quasifreestanding $h$-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/$h$-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene.

We employ a dual-gated geometry to control the band gap $\Delta$ in bilayer graphene and study the temperature dependence of the resistance at the charge neutrality point, $R_{\text{NP}}\left(T\right)$, from 220 to 1.5 K. Above 5 K, $R_{\text{NP}}\left(T\right)$ is dominated by two thermally activated processes in different temperature regimes and exhibits $\text{exp}{\left(T_3/T\right)}^{1/3}$ below 5 K. We develop a simple model to account for the experimental observations, which highlights the crucial role of localized states produced by potential fluctuations. The high-temperature conduction is attributed to thermal activation to the mobility edge. The activation energy approaches $\Delta /2$ at large band gap. At intermediate and low temperatures, the dominant conduction mechanisms are nearest-neighbor hopping and variable-range hopping through localized states. Our systematic study provides a coherent understanding of transport in gapped bilayer graphene.

We present a renormalization-group theory for the onset of Ising-nematic order in a Fermi liquid in two spatial dimensions. This is a quantum phase transition, driven by electron interactions, which spontaneously reduces the point-group symmetry from square to rectangular. The critical point is described by an infinite set of $2+1$-dimensional local field theories, labeled by points on the Fermi surface. Each field theory contains a real scalar field representing the Ising order parameter and fermionic fields representing a time-reversed pair of patches on the Fermi surface. We demonstrate that the field theories obey compatibility constraints required by our redundant representation of the underlying degrees of freedom. Scaling forms for the response functions are proposed and supported by computations up to three loops. Extensions of our results to other transitions of two-dimensional Fermi liquids with broken point-group and/or time-reversal symmetry are noted. Our results extend also to the problem of a Fermi surface coupled to a U(1) gauge field.

We present a field-theoretic renormalization group analysis of Abanov and Chubukov’s model of the spin density wave transition in two dimensional metals. We identify the independent field scale and coupling constant renormalizations in a local field theory and argue that the damping constant of spin density wave fluctuations tracks the renormalization of the local couplings. The divergences at two-loop order overdetermine the renormalization constants and are shown to be consistent with our renormalization scheme. We describe the physical consequences of our renormalization-group equations, including the breakdown of Fermi liquid behavior near the “hot spots” on the Fermi surface. In particular, we find that the dynamical critical exponent $z$ receives corrections to its mean-field value $z=2$. At higher orders in the loop expansion, we find infrared singularities similar to those found by Lee [Phys. Rev. B 80, 165102 (2009)] for the problem of a Fermi surface coupled to a gauge field. A treatment of these singularities implies that an expansion in $1∕N$ (where $N$ is the number of fermion flavors) fails for the present problem. We also discuss the renormalization of the pairing vertex and find an enhancement which scales as logarithm squared of the energy scale. A similar enhancement is also found for a modulated bond order which is locally an Ising-nematic order.

The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition-metal oxides. The most direct coupling is obtained in so-called magnetically induced multiferroics where ferroelectricity arises directly from magnetic order that breaks spatial inversion symmetry. However, it has been difficult to find nonoxide-based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organometallic quantum magnet containing $S=1/2$ Cu spins, in which electric polarization arises from noncollinear magnetic order. We show that the electric polarization can be switched in a stunning hysteretic fashion. Because the magnetic order in CDC is mediated by large organic molecules, our study shows that magnetoelectric interactions can exist in this important class of materials, opening the road to designing magnetoelectrics and multiferroics using large molecules as building blocks. Further, we demonstrate that CDC undergoes a magnetoelectric quantum phase transition where both ferroelectric and magnetic order emerge simultaneously as a function of magnetic field at very low temperatures.

We study the magnetic properties of graphene edges and graphene/graphane interfaces under the influence of electrostatic gates. For this an effective one-dimensional low-energy theory for the edge states, which is derived from the Hubbard model of the honeycomb lattice, is used. We first study the edge-state model in a mean-field approximation for the Hubbard Hamiltonian and show that it reproduces the results of the two-dimensional lattice theory. Quantum fluctuations around the mean-field theory of the effective one-dimensional model are treated by means of the bosonization technique in order to check the stability of the mean-field solution. We find that edge magnetism at graphene/graphane interfaces can be switched on and off by means of electrostatic gates. We describe a quantum phase transition between an ordinary and a ferromagnetic Luttinger liquid—a realization of itinerant one-dimensional ferromagnetism. This effect may provide means to experimentally discriminate between edge magnetism or disorder as the reason for a transport gap in very clean graphene nanoribbons.

We have studied the effect of dynamical correlations on the electronic structure of single Co adatoms on graphene monolayers with a recently developed method for nanoscopic materials that combines density-functional calculations with a fully dynamical treatment of the strongly interacting $3d$ electrons. The coupling of the $\text{Co}3d$ shell to the graphene substrate and hence the dynamic correlations are strongly dependent on the orbital symmetry and the system parameters (temperature, distance of the adatom from the graphene sheet, and gate voltage). When the Kondo effect takes place, we find that the dynamical correlations give rise to strongly temperature-dependent peaks in the $\text{Co}3d$ spectra near the Fermi level. Moreover, we find that the Kondo effect can be tuned by the application of a gate voltage. It turns out that the position of the Kondo peaks is pinned to the Dirac points of graphene rather than to the chemical potential.

The fractional quantum Hall (FQH) effect is reported in a high mobility CdTe quantum well at millikelvin temperatures. Fully developed FQH states are observed at filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state for which the lowest energy excitation is not a spin flip. This can be accounted for by the relatively high intrinsic Zeeman energy in this single valley two-dimensional electron gas. FQH minima are also observed in the first excited $\left(N=1\right)$ Landau level at filling factor 7/3 and 8/3 for intermediate temperatures. In contrast, the 5/2 FQH state remains absent down to $T\sim 10\text{mK}$.

Nuclear-magnetic resonances were detected optically in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The resonances were found in the magnetic field dependence of photoluminescence polarization (Hanle curve) when applying a radio frequency (RF) field. The frequency dependences of the resonances allow us to ascribe them to transitions between Zeeman states of the ${}^{71}\text{G}\text{a}$ and ${}^{75}\text{A}\text{s}$ nuclei perturbed by quadrupole interaction. Applying an RF-field sweep over a wide frequency range results in a notable narrowing of the Hanle curve, which can be explained by suppression of dynamic nuclear polarization that otherwise is stabilized by the quadrupole splitting.

We consider the classical ballistic dynamics of massless electrons on the conducting surface of a three-dimensional topological insulator, influenced by random variations in the surface height. By solving the geodesic equation and the Boltzmann equation in the limit of shallow deformations, we obtain the scattering cross section and the conductivity $\sigma$, for arbitrary anisotropic dispersion relation. At large surface electron densities $n$ this geodesic scattering mechanism (with $\sigma \propto \sqrt{n}$) is more effective at limiting the surface conductivity than electrostatic potential scattering.

The full counting statistics of the charge transport through an undoped graphene sheet in the presence of smooth disorder is studied. At the Dirac point both in clean and diffusive limits, transport properties of a graphene sample are described by the universal Dorokhov distribution of transmission probabilities. In the crossover regime, deviations from universality occur which can be studied analytically both on ballistic and diffusive sides. In the ballistic regime, we use a diagrammatic technique with matrix Green’s functions. For a diffusive system, the sigma model is applied. Our results are in good agreement with recent numerical simulations of electron transport in disordered graphene.

We report on the coexistence of magnetic and superconducting states in ${\text{CeFeAsO}}_{1-x}{\text{F}}_x$ for $x=0.06\left(2\right)$, characterized by transition temperatures $T_m=30\text{K}$ and $T_c=18\text{K}$, respectively. Zero-field and transverse-field muon-spin-relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the ${\text{CeFeAsO}}_{1-x}{\text{F}}_x$ phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies.

Two recent quantum oscillation studies find contradictory results concerning the existence of spin zeros—zeros of the oscillatory signal induced by Zeeman splitting of the Landau levels. We discuss these experiments in light of calculations of the oscillations assuming a spin-density wave state. We find that the lack of spin zeros reported in one of the experiments is consistent with either hole or electron pockets in such a state, if the staggered moment is perpendicular to the external field. An analysis for field directions near the planes might be able to differentiate between the two. On the other hand, if spin zeros exist as reported in the other experiment, then the staggered moment would have to have a substantial longitudinal component. We suggest several experiments to test whether this is indeed the case.

We report on the transition between two regimes from several-atom clusters to much larger nanoparticles in Ar magnetron sputter deposition of ${\text{WSi}}_2$, and the effect of nanoparticles on the properties of amorphous thin films and multilayers. Sputter deposition of thin films is monitored by in situ x-ray scattering, including x-ray reflectivity and grazing incidence small-angle x-ray scattering. The results show an abrupt transition at an Ar background pressure $P_c$; the transition is associated with the threshold for energetic particle thermalization, which is known to scale as the product of the Ar pressure and the working distance between the magnetron source and the substrate surface. Below $P_c$ smooth films are produced while above $P_c$ roughness increases abruptly, consistent with a model in which particles aggregate in the deposition flux before reaching the growth surface. The results from ${\text{WSi}}_2$ films are correlated with in situ measurement of stress in ${\text{WSi}}_2/\text{Si}$ multilayers, which exhibits a corresponding transition from compressive to tensile stress at $P_c$. The tensile stress is attributed to coalescence of nanoparticles and the elimination of nanovoids.

We have studied Josephson junctions with barriers prepared from the Heusler compound ${\text{Cu}}_2\text{MnAl}$. In the as-prepared state the ${\text{Cu}}_2\text{MnAl}$ layers are nonferromagnetic and the critical Josephson current density $j_c$ decreases exponentially with the thickness of the Heusler layers $d_F$. On annealing the junctions at $240°\text{C}$ the Heusler layers develop ferromagnetic order and we observe a dependence $j_c\left(d_F\right)$ with $j_c$ strongly enhanced and weakly thickness dependent in the thickness range $7.0<d_F<10.6\text{nm}$. We interpret this feature as an indication of a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin ${\text{Cu}}_2\text{MnAl}$ layers.

We investigate Dirac fermions on the surface of the topological insulator ${\text{Bi}}_2{\text{Se}}_3$ using scanning tunneling spectroscopy. Landau levels (LLs) are observed in the tunneling spectra in a magnetic field. In contrast to LLs of conventional electrons, a field-independent LL appears at the Dirac point, which is a hallmark of Dirac fermions. A scaling analysis of LLs based on the Bohr-Sommerfeld quantization condition allowed us to determine the dispersion of the surface band. Near the Fermi energy, fine peaks mixed with LLs appear in the spectra, which may be responsible for the anomalous magnetofingerprint effect [J. G. Checkelsky et al., Phys. Rev. Lett. 103, 246601 (2009)].

We show that a combination of the field-susceptibility technique with the optical Bloch equations gives access to the temporal evolution of the populations and coherences of any quantum system placed in the optical near field of a nanostructure. In particular, we show that the near-field evanescent states, confined around dielectric or plasmonic particles, can be used to modify and control the photon statistics of the quantum system. This theoretical scheme leads to second-order autocorrelation functions in good agreement with recent experimental measurements performed with nitrogen-vacancy center in diamond nanocrystals placed in interaction with gold nanoparticles.