Motivated by the evidence in ${\text{Pr}}_{2-x}{\text{Ce}}_x{\text{CuO}}_{4-y}$ and ${\text{Nd}}_{2-x}{\text{Ce}}_x{\text{CuO}}_{4-y}$ of a magnetic quantum critical point at which Néel order is destroyed, we study the evolution with doping of the $T=0$ quantum phases of the electron-doped cuprates. At low doping, there is a metallic Néel state with small electron Fermi pockets, and this yields a fully gapped $d_{x^2-y^2}$ superconductor with coexisting Néel order at low temperatures. We analyze the routes by which the spin-rotation symmetry can be restored in these metallic and superconducting states. In the metal, the loss of Néel order leads to a topologically ordered “doublon metal” across a deconfined critical point with global O(4) symmetry. In the superconductor, in addition to the conventional spin-density wave transition, we find a variety of unconventional possibilities, including transitions to a nematic superconductor and to valence-bond supersolids. Measurements of the spin-correlation length and of the anomalous dimension of the Néel order by neutron scattering or NMR should discriminate these unconventional transitions from spin-density wave theory.

The electronic and vibronic processes at the interface between a pentacene monolayer and a graphite surface were characterized using a combination of density-functional theory (DFT) and dynamic vibronic coupling simulations. The electronic interactions were evaluated at the DFT level first between the highest occupied states of pentacene and the graphite surface, as well as among the pentacene molecules within a monolayer. The former are found to be ca. four times stronger than the latter for a parallel molecule/surface geometry. A dynamic vibronic model was used to analyze the interplay between the electronic and electron-vibration couplings and their effects on spectroscopic characteristics. The agreement between the simulated and experimental photoelectron spectra underlines the importance of weak electronic interactions on the vibronic coupling at the interface.

An unusual enhancement of resistance (i.e., a “superresistivity”) below a certain characteristic temperature $T_{\text{sr}}$ was observed in granular Bi nanowires. This “superresistive” state was found to be dependent on the applied magnetic field as well as the excitation current. The suppression of $T_{\text{sr}}$ by magnetic field resembles that of a superconductor. The observed superresistivity appears to be related to the nucleation of local superconductivity inside the granular nanowire without long-range phase coherence. The phenomenon is reminiscent of the “Cooper-pair insulator” observed previously in ultrathin two-dimensional superconducting films and three-dimensional percolative superconducting films.

The anisotropy of the electronic structure of ternary nanolaminate ${\text{V}}_2\text{GeC}$ is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of $\text{V}{L}_{2,3}$, $\text{C}K$, $\text{Ge}{M}_1$, and $\text{Ge}{M}_{2,3}$ in ${\text{V}}_2\text{GeC}$ are compared with those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed: $\text{V}3d\text{-C}2p$ bonding at $-3.8\text{eV}$, $\text{Ge}4p\text{-C}2p$ bonding at $-6\text{eV}$, and $\text{Ge}4p\text{-C}2s$ interaction mediated via the $\text{V}3d$ orbitals at $-11\text{eV}$ below the Fermi level. We find that the anisotropic effects are high for the $4p$ valence states and the shallow $3d$ core levels of Ge, while relatively small anisotropy is detected for the $\text{V}3d$ states. The macroscopic properties of the ${\text{V}}_2\text{GeC}$ nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.

Subpicosecond ac spin-current pulses are generated optically in GaAs bulk and quantum wells at room temperature and at 90 K through quantum interference between one-photon and two-photon absorptions driven by two phase-locked ultrafast laser pulses that are both circularly polarized. The dynamics of the current pulses are detected optically by monitoring, in real time and in real space, the nanoscale motion of electrons with high-resolution pump probe techniques. The spin polarization of the currents is $0.6\pm 0.1$, with peak current densities on the order of ${10}^2\text{A}/{\text{m}}^2$.

The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by the geometry and exact atomic structure of the tip apex. However, the tip state is an experimentally unknown parameter, and the lack of insight into the tip apex often limits the possibilities of extracting precise quantitative and qualitative atomistic information on the surface under inspection. From an interplay between simultaneously recorded nc-AFM and scanning tunneling microscopy (STM) data, and atomistic STM simulations based on multiple scattering theory, we demonstrate how the state of the scanning probe microscopy (SPM) tip in the experiments may be determined. The analysis of a large number of experimental SPM images recorded with different tips reveals that no general correlation exists between the contrast observed in the nc-AFM and the tunneling current $\left(I_t\right)$ images on ${\text{TiO}}_2\left(110\right)$ surface. The exact state of the SPM tip must, therefore, be determined for each specific case, which is normally a very difficult endeavor. However, our analysis of the AFM contrast on ${\text{TiO}}_2\left(110\right)$ surface allows us to considerably reduce the number of tips to be considered in a full simulation. By carefully evaluating the contrast of a handpicked library of SPM tips, we manage to determine a very accurate model of the SPM tip used in an experiment for the first time. It is envisioned that the approach presented here may eventually be used in future studies to screen for and select a SPM tip with a special functionalization prior to imaging an unknown sample, and in that way facilitate precise modeling and chemical identification of surface species.

Thin films of $\alpha \text{-MoGe}$ show progressively reduced $T_c$’s as the thickness is decreased below 30 nm and the sheet resistance exceeds $100\Omega /◻$. We have performed far-infrared transmission and reflection measurements for a set of $\alpha \text{-MoGe}$ films to characterize this weakened superconducting state. Our results show the presence of an energy gap with ratio $2{\Delta }_0/k_B{T}_c=3.8\pm 0.1$ in all films studied, slightly higher than the BCS value, even though the transition temperatures decrease significantly as film thickness is reduced. The material properties follow BCS-Eliashberg theory with a large residual scattering rate but the expected coherence peak seen in the optical scattering rate is lost in the thinner samples. This result cannot be explained within conventional theories.

We measure the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.

We analyze magnetic-flux tubes at zero temperature in a superconductor that is coupled to a superfluid via both density and gradient (“entrainment”) interactions. The example we have in mind is high-density nuclear matter, which is a proton superconductor and a neutron superfluid, but our treatment is general and simple, modeling the interactions as a Ginzburg-Landau effective theory with four-fermion couplings, including only $s$-wave pairing. We numerically solve the field equations for flux tubes with an arbitrary number of flux quanta and compare their energies. This allows us to map the type-I/type-II transition in the superconductor, which occurs at the conventional $\kappa \equiv \lambda /\xi =1/\sqrt{2}$ if the condensates are uncoupled. We find that a density coupling between the condensates raises the critical $\kappa$ and, for a sufficiently high neutron density, resolves the type-I/type-II transition line into an infinite number of bands corresponding to “$\text{type-II}\left(n\right)$” phases, in which $n$, the number of quanta in the favored flux tube, steps from 1 to infinity. For lower neutron density, the coupling creates spinodal regions around the type-I/type-II boundary, in which metastable flux configurations are possible. We find that a gradient coupling between the condensates lowers the critical $\kappa$ and creates spinodal regions. These exotic phenomena may not occur in nuclear matter, which is thought to be deep in the type-II region but might be observed in condensed-matter systems.

The temperature dependence of electron-spin coherence in singly negatively charged (In,Ga)As/GaAs quantum dots is studied by time-resolved Faraday rotation. The decoherence time $T_2$ is constant on the microsecond scale for temperatures below 15 K; for higher temperatures it shows a surprisingly sharp drop into the nanosecond range. The decrease cannot be explained through inelastic scattering with phonons, and it may be related to elastic scattering due to phonon-mediated fluctuations of the hyperfine interaction.

Single crystals of ${\text{BaFe}}_2{\text{As}}_2$ and ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated, and can have dimensions as large as $3\times 3\times 0.2{\text{mm}}^3$. The ${\text{BaFe}}_2{\text{As}}_2$ single crystals manifest a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_2{\text{As}}_2$ single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near $T_c$ is small, but resolvable, giving $\Delta C_p/\gamma T_c\approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by only $\sim 4\text{K}$ when applied along the $c$ axis and by $\sim 2\text{K}$ when applied perpendicular to the $c$ axis. The ratio of the anisotropic upper critical fields, $\gamma =H_{c2}^{\perp c}/H_{c2}^{\parallel c}$, varies between 2.5 and 3.5 for temperatures down to $\sim 2\text{K}$ below $T_c$.

We have performed synchrotron radiation x-ray and neutron diffraction measurements on magnetoelectric multiferroic ${\text{CuFe}}_{1-x}{\text{Al}}_x{\text{O}}_2$ $\left(x=0.0155\right)$, which has a proper helical magnetic structure with incommensurate propagation wave vector in the ferroelectric phase. The present measurements revealed that the ferroelectric phase is accompanied by lattice modulation with a wave number $2q$, where $q$ is the magnetic modulation wave number. We have calculated the Fourier spectrum of the spatial modulations in the local electric polarization using a microscopic model proposed by T. Arima [J. Phys. Soc. Jpn. 76, 073702 (2007)]. Comparing the experimental results with the calculation results, we found that the origin of the $2q$-lattice modulation is not the conventional magnetostriction but the variation in the metal-ligand hybridization between the magnetic ${\text{Fe}}^{3+}$ ions and ligand ${\text{O}}^{2-}$ ions. Combining the present results with the results of a previous polarized neutron diffraction study [Nakajima et al., Phys. Rev. B 77, 052401 (2008)], we conclude that the microscopic origin of the ferroelectricity in ${\text{CuFe}}_{1-x}{\text{Al}}_x{\text{O}}_2$ is the variation in the metal-ligand hybridization with spin-orbit coupling.

Torquato and Stillinger have constructed a new family of frustrated lattices by an unusually high dilution of close-packed structures while preserving structural stability. We show that an infinite subclass of these structures has an underlying topology that greatly simplifies the determination of their magnetic phase structure for nearest-neighbor antiferromagnetic interactions and $O\left(N\right)$ spins.

The ternary iron arsenide ${\text{BaFe}}_2{\text{As}}_2$, with the tetragonal ${\text{ThCr}}_2{\text{Si}}_2$-type structure, exhibits a spin-density-wave (SDW) anomaly at 140 K, very similar to LaFeAsO, which is the parent compound of the iron arsenide superconductors. ${\text{BaFe}}_2{\text{As}}_2$ is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance, and magnetic susceptibility. In the course of this transition, the space-group symmetry changes from tetragonal $\left(I4/mmm\right)$ to orthorhombic $\left(Fmmm\right)$. ${}^{57}\text{F}\text{e}$ Mössbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase-transition temperature (5.2 T at 77 K). Our results suggest that ${\text{BaFe}}_2{\text{As}}_2$ can serve as a parent compound for oxygen-free iron arsenide superconductors.

We study transport properties of graphene-based $p\text{−}n$ junctions irradiated by electromagnetic field (EF). The resonant interaction of propagating quasiparticles with an external monochromatic radiation opens dynamical gaps in their spectrum, resulting in a strong modification of current-voltage characteristics of the junctions. The values of the gaps are proportional to the amplitude of EF. We find that the transmission of the quasiparticles in the junctions is determined by the tunneling through the gaps and can be fully suppressed when applying a sufficiently large radiation power. However, EF can not only suppress the current but also generate it. We demonstrate that if the height of the potential barrier exceeds a half of the photon energy, the directed current (photocurrent) flows through the junction without any dc bias voltage applied. Such a photocurrent arises as a result of inelastic quasiparticle tunneling assisted by one- or two-photon absorption. We calculate current-voltage characteristics of diverse graphene-based junctions and estimate their parameters necessary for the experimental observation of the photocurrent and transmission suppression.

We have succeeded in realizing a single ferroelectric phase in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ by substituting nonmagnetic ${\text{Ga}}^{3+}$ for ${\text{Fe}}^{3+}$ sites in ${\text{CuFeO}}_2$. Ferroelectric polarization $P$ in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ is observed below 7.5 K, and has the relatively large value of $\sim 250\mu \text{C}/{\text{m}}^2$, which is comparable to $P=300\sim 400\mu \text{C}/{\text{m}}^2$ in the magnetic-field-induced ferroelectric phase of ${\text{CuFeO}}_2$. In neutron-diffraction measurements, a single magnetic diffraction peak with an incommensurate wave number was observed below 7.5 K in ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$, indicating that the ferroelectric-incommensurate (FEIC) phase is realized as a single phase. Therefore, ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$ with a single FEIC phase is strongly expected to provide the best opportunity to investigate unresolved problems regarding the ferroelectric mechanism in ${\text{CuFeO}}_2$. In this paper, we report measurements of magnetic susceptibility, specific heat, pyroelectric, dielectric constant, and neutron diffraction of a single crystal of ${\text{CuFe}}_{0.963}{\text{Ga}}_{0.037}{\text{O}}_2$.

At certain quantum critical points in metals, an entire Fermi surface may disappear. A crucial question is the nature of the electronic excitations at the critical point. Here we provide arguments showing that at such quantum critical points, the Fermi surface remains sharply defined even though the Landau quasiparticle is absent. The presence of such a critical Fermi surface has a number of consequences for the universal phenomena near the quantum critical point which are discussed. In particular, the structure of scaling of the universal critical singularities can be significantly modified from more familiar criticality. Scaling hypotheses appropriate to a critical Fermi surface are proposed. Implications for experiments on heavy fermion critical points are discussed. Various phenomena in the normal state of the cuprates are also examined from this perspective. We suggest that a phase transition that involves a dramatic reconstruction of the Fermi surface might underlie a number of strange observations in the metallic states above the superconducting dome.

The complete lack of theoretical understanding of the quantum critical states found in the heavy-fermion metals and the normal states of the high-$T_c$ superconductors is rooted in a deep fundamental problem of condensed-matter physics: the infamous minus signs associated with Fermi-Dirac statistics render the path integral nonprobabilistic and do not allow the establishment of a connection to critical phenomena in classical systems. Using Ceperley’s constrained path-integral formalism, we demonstrate that the workings of scale invariance and Fermi-Dirac statistics can be reconciled. The latter is self-consistently translated into a geometrical constraint structure. We show that this “nodal hypersurface” encodes the scales of the Fermi liquid, and we conjecture that it turns fractal when the system becomes quantum critical. To substantiate this, we analyze the nodal structures of fermionic Feynman backflow wave functions to find that the nodal surface indeed turns into a scale-invariant fractal when the backflow becomes hydrodynamical. Moreover, by following the evaluation of the quasiparticle momentum distribution, we demonstrate that the emergence of scale invariance in the nodal structure is accompanied by a divergence in the effective quasiparticle mass. Such a collapse of a Fermi liquid at a critical point has been observed in the heavy-fermion intermetallics in a spectacular fashion.

Motivated by recent neutron scattering experiments, we study the ordering of spins in the iron-based superconductors $\mathrm{LaFeAs}\left({\mathrm{O}}_{1-x}{\mathrm{F}}_x\right)$, assuming them in proximity to a Mott insulator in the phase diagram. The ground state of the parent system with $x=0$ is a spin-density wave with ordering wave vector $\vec{Q}=(0,\pi )$ or $(\pi ,0)$. Upon raising the temperature, we find that the system restores SU(2) symmetry, while an Ising symmetry remains broken, explaining the experimentally observed lattice distortion to a monoclinic crystal structure. Upon further temperature increase, the spins finally disorder at a second transition. The phase transition driven by doping with charge carriers similarly splits into an $\mathrm{O}\left(3\right)$ transition and an Ising transition with $z=3$ at larger doping.

Strongly correlated electron insulators are one of the most important classes of materials in modern solid-state physics. We demonstrate that certain classes of Mott insulators, that usually are geometrically frustrated, exhibit unexpected charge effects: for certain spin textures, spontaneous circular electric currents or nonuniform charge distribution exist in the ground state of Mott insulators. In addition, low-energy “magnetic” states contribute comparably to the dielectric and magnetic response functions, ${ϵ}_{ik}\left(\omega \right)$ and ${\mu }_{ik}\left(\omega \right)$, leading to interesting phenomena such as dipole-active ESR transitions, rotation of the electric-field polarization, and resonances which may be common to both functions producing a negative refraction index in a window of frequencies.