The B phase of superfluid ${}^3$He can support regions of extremely long-lived coherent spin precession at ultralow temperatures, known as persistent precessing domains (PPD). The domains have been described in terms of a Bose-Einstein condensate of magnons and in terms of Q balls in field theory. The domains form in a magnetic field minimum along the vertical axis of a cylindrical cell. When far from the ends of the cell, the PPD lifetime grows exponentially on decreasing temperature. When the PPD is close to the horizontal end wall of the cell, an extra surface dissipation mechanism dominates at low temperatures. We present measurements of the PPD generated at various locations in the cell over a broad range of temperatures below 0.3 $T_C$. We compare the measured properties with theoretical expectations for spin-wave modes. We present model calculations of different dissipation mechanisms and we compare these to the measured lifetimes.

We report on superconductivity in CeFeAs${}_{1-x}$P${}_x$O and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around $x=30\%$ with ordering temperatures of $T_{\text{SC}}\cong T_{\text{C}}\cong 4$ K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to $T_{\mathrm{N}}^{\mathrm{Fe}}\approx 40$ K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantum-critical-point scenario with $T_{\mathrm{N}}^{\mathrm{Fe}}\to 0$ K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering.

We report measurements of the elastic moduli as a function of temperature (5–300) K and magnetic field (0–2 T) for the Zintl ferromagnet Yb${}_{14}$MnSb${}_{11}$, which is believed to be a rare example of an underscreened Kondo lattice. The elastic moduli measured below the Curie temperature in this complex ferromagnet exhibit unusual lattice stiffening that is independent of the magnetic field and can be adequately modeled using the Landau theory.

We investigate layered acoustic metamaterials capable of exhibiting a wide variety of wave propagation phenomena, including backward and forward waves with and without negative refraction. The metamaterials are formed by periodically perforated hard plates, which we describe analytically in the limit of small holes compared to both the period and the separation between plates. In particular, we derive expressions for the index of refraction and the transmission and reflection coefficients of finite slabs. We provide illustrative examples of near fields for all four combinations of backward and forward waves with and without negative refraction, along with dispersion diagrams that explain the observed behavior. A comprehensive study of the range of geometrical parameters in which these distinct phenomena are observed is presented as well. These metamaterials hold great promise for achieving full control of sound using simple structures, with potential application to acoustic technologies relying on subwavelength control and imaging of sound.

We study the effects of Cu substitution in Fe${}_{1.1}$Te, the nonsuperconducting parent compound of the iron-based superconductor, Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at $\sim$60 K without Cu, are monotonically depressed with increasing Cu content. By 10$\%$ Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of ($0.5-\delta$, 0, 0.5) with $\delta$ being the incommensurability of 0.02, and correlation length of 12 Å along the $a$ axis and 9 Å along the $c$ axis. With 4$\%$ Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42, 0, 0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first-order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has a large effect on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins.

Topologically ordered phases are gapped states, defined by the properties of excitations when taken around one another. Here we demonstrate a method to extract the statistics and braiding of excitations, given just the set of ground-state wave functions on a torus. This is achieved by studying the topological entanglement entropy (TEE) upon partitioning the torus into two cylinders. In this setting, general considerations dictate that the TEE generally differs from that in trivial partitions and depends on the chosen ground state. Central to our scheme is the identification of ground states with minimum entanglement entropy, which reflect the quasiparticle excitations of the topological phase. The transformation of these states allows for the determination of the modular $\mathcal{S}$ and $\mathcal{U}$ matrices which encode quasiparticle properties. We demonstrate our method by extracting the modular $\mathcal{S}$ matrix of a chiral spin liquid phase using a Monte Carlo scheme to calculate the TEE and prove that the quasiparticles obey semionic statistics. This method offers a route to nearly complete determination of the topological order in certain cases.

We analyze the phase diagram of bilayer graphene (BLG) at zero temperature and zero doping. Assuming that at high energies the electronic system of BLG can be described within a weak-coupling theory (consistent with the experimental evidence), we systematically study the evolution of the couplings with going from high to low energies. The divergences of the couplings at some energies indicate the tendency towards certain symmetry breakings. Carrying out this program, we found that the phase diagram is determined by microscopic couplings defined on the short distances (initial conditions). We explored all plausible space of these initial conditions and found that the three states have the largest phase volume of the initial couplings: nematic, antiferromagnetic, and spin flux (a.k.a. quantum spin Hall). In addition, ferroelectric and two superconducting phases appear only near the very limits of the applicability of the weak-coupling approach. The paper also contains the derivation and analysis of the renormalization group equations and the group theory classification of all the possible phases which might arise from the symmetry breakings of the lattice, spin rotation, and gauge symmetries of graphene.

We present measurements of the dissipative turbulent drag on a vibrating grid in superfluid ${}^4$He over a wide range of (low) frequencies. At high velocities, the dissipative drag is independent of frequency and is approximately the same as that measured in normal liquid ${}^4$He. We present measurements on a similar grid in superfluid ${}^3$He-B at low temperatures which shows an almost identical turbulent drag coefficient at low frequencies. However, the turbulent drag in ${}^3$He-B is substantially higher at higher frequencies. We also present measurements of the inertial drag coefficient for grid turbulence in ${}^4$He. The inertial drag coefficient is significantly reduced by turbulence in both superfluid and normal liquid ${}^4$He.

Numerical calculations indicate that by suitably controlling the individual gate voltages of a capacitively coupled parallel double quantum dot, with each quantum dot coupled to one of two independent nonmagnetic channels, this system can be set into a spin-orbital Kondo state by applying a magnetic field. This Kondo regime, closely related to the SU(4) Kondo, flips spin from 1 to 0 through cotunneling processes that generate almost totally spin-polarized currents with opposite spin orientation along the two channels. Moreover, by appropriately changing the gate voltages of both quantum dots, one can simultaneously flip the spin polarization of the currents in each channel. As a similar zero magnetic field Kondo effect has been recently observed by Okazaki et al. [Phys. Rev. B 84, 161305(R) (2011)], we analyze a range of magnetic field values where this polarization effect seems robust, suggesting that the setup may be used as an efficient bipolar spin filter, which can generate electrostatically reversible spatially separated spin currents with opposite polarizations.

We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement $\overline{D}$. As a band gap is induced with increasing $\overline{D}$, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a one-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For $\overline{D}>0$, the additional peak is observed only for electrons, while for $\overline{D}<0$ the peak appears only for holes. This asymmetry can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.

We report on a comprehensive experimental and theoretical study of the quasi-one-dimensional quantum magnet CuNCN. Based on magnetization measurements above room temperature, as well as muon spin rotation and electron spin resonance measurements, we unequivocally establish the localized Cu${}^{+2}$-based magnetism and the magnetic transition around 70 K, both controversially discussed in the previous literature. Thermodynamic data conform to the uniform-spin-chain model with a nearest-neighbor intrachain coupling of about 2300 K, in remarkable agreement with the microscopic magnetic model based on density functional theory band-structure calculations. Using exact diagonalization and the coupled-cluster method we derive a collinear antiferromagnetic order with a strongly reduced ordered moment of about 0.4 ${\mu }_B$, indicating strong quantum fluctuations inherent to this quasi-one-dimensional spin system. We reanalyze the available neutron-scattering data, and conclude that they are not sufficient to resolve or disprove the magnetic order in CuNCN. By contrast, spectroscopic techniques indeed show signatures of long-range magnetic order below 70 K, yet with a rather broad distribution of internal field probed by implanted muons. We contemplate the possible structural origin of this effect and emphasize the strong anisotropic reflection broadening observed with synchrotron powder x-ray diffraction on stoichiometric samples of CuNCN, as well as possible deviations from the ideal CuNCN stoichiometry.

We present the theory of a three-dimensional fractionalized topological insulator in the form of a $U\left(1\right)$ spin liquid with gapped fermionic spinons in the bulk and topologically protected gapless spinon surface states. Starting from a spin-rotation-invariant spin-$1/2$ model on a pyrochlore lattice with frustrated antiferromagnetic and ferromagnetic exchange interactions, we show that decomposition of the latter interactions, within slave-fermion representation of the spins, can naturally give rise to an emergent spin-orbit coupling for the spinons by spontaneously breaking the spin rotation symmetry. The time-reversal symmetry, however, is preserved. This stabilizes a fractionalized topological insulator which also has bulk bond spin-nematic order. We describe the low-energy properties of this state.

We show that orbital currents in a CuO${}_2$ plane, if present, should be described by two independent parity- and time-reversal-odd order parameters, a toroidal dipole (anapole) and a magnetic quadrupole. Based on this, we derive the resonant x-ray diffraction cross section for monoclinic CuO at the antiferromagnetic wave vector and show that the two order parameters can be disentangled. From our analysis, we examine a recent claim of detecting anapoles in CuO.

Although a nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in K${}_x$Fe${}_{2-y}$Se${}_2$, whether its pairing symmetry is $s$ wave or nodeless $d$ wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the $Z$ point in K${}_x$Fe${}_{2-y}$Se${}_2$, which favors the $s$-wave pairing symmetry.

The electrical resistivity ${\rho }_c$ of the underdoped cuprate superconductor YBa${}_2$Cu${}_3$O${}_y$ was measured perpendicular to the CuO${}_2$ planes on ultrahigh quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of ${\rho }_c$ at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBa${}_2$Cu${}_3$O${}_y$ via quantum oscillations, the Hall and Seebeck coefficients, and with the detection of a unidirectional modulation of the charge density as seen by high-field nuclear magnetic resonance measurements. The low coherence temperature is quantitatively consistent with the small hopping integral $t_{\perp }$ inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBa${}_2$Cu${}_3$O${}_y$ at dopings from $p=0.08$ to at least $p=0.15$, attributed to stripe order, produces a metallic state with three-dimensional coherence deep in the underdoped regime.

Using high-resolution resonant inelastic x-ray scattering (RIXS) excited at the Cu $L_3$ edge we have investigated the ligand field ($dd$) and magnetic excitations in (CaCuO${}_2$)${}_n$/(SrTiO${}_3$)${}_n$ superlattices ($n=2,3$) and compared them to those of a 14-nm-thick CaCuO${}_2$ film. The $dd$ excitation spectrum reveals a pyramidal coordination of Cu ions at the CaCuO${}_2$/SrTiO${}_3$ interfaces. In the three samples, spin excitations are in the form of dispersing magnons, with similar spectral intensity but reduced dynamics in SLs with respect to pure CaCuO${}_2$. By fitting the dispersions within linear spin wave theory we have obtained the leading term of the in-plane superexchange parameters: $J=127$ meV, 138 meV, and 157 meV for $n=2$, 3 SLs, and CaCuO${}_2$, respectively. These results demonstrate that the antiferromagnetic order is preserved in these SLs down to very small cuprate layer thickness and despite the chemical and structural alterations at the interface. This finding opens the way to the production of artificial Cu-based high-temperature superconductors where the charge reservoir layer is constituted by the interface itself.

We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking (In,Ga)As quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of $>96\%$, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the $\sim 300$ ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields.

One of the most promising proposals for engineering topological superconductivity and Majorana fermions employs a spin-orbit coupled nanowire subjected to a magnetic field and proximate to an $s$-wave superconductor. When only part of the wire's length contacts to the superconductor, the remaining conducting portion serves as a natural lead that can be used to probe these Majorana modes via tunneling. The enhanced role of interactions in one dimension dictates that this configuration should be viewed as a superconductor-Luttinger liquid junction. We investigate such junctions between both helical and spinful Luttinger liquids, and topological as well as nontopological superconductors. We determine the phase diagram for each case and show that universal low-energy transport in these systems is governed by fixed points describing either perfect normal reflection or perfect Andreev reflection. In addition to capturing (in some instances) the familiar Majorana-mediated “zero-bias anomaly” in a new framework, we show that interactions yield dramatic consequences in certain regimes. Indeed, we establish that strong repulsion removes this conductance anomaly altogether while strong attraction produces dynamically generated effective Majorana modes even in a junction with a trivial superconductor. Interactions further lead to striking signatures in the local density of states and the line shape of the conductance peak at finite voltage, and also are essential for establishing smoking-gun transport signatures of Majorana fermions in spinful Luttinger liquid junctions.

An exact expression is derived for the electron Green function in two-channel Kondo models with one and two impurities, describing the crossover from non-Fermi liquid (NFL) behavior at intermediate temperatures to standard Fermi liquid (FL) physics at low temperatures. Symmetry-breaking perturbations generically present in experiment ensure the standard low-energy FL description, but the full crossover is wholly characteristic of the unstable NFL state. Distinctive conductance lineshapes in quantum dot devices should result. We exploit a connection between this crossover and one occurring in a classical boundary Ising model to calculate real-space electron densities at finite temperature. The single universal finite-temperature Green function is then extracted by inverting the integral transformation relating these Friedel oscillations to the $t$ matrix. Excellent agreement is demonstrated between exact results and full numerical renormalization group calculations.

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3$s$ core level photoemission spectra in the parent and optimally doped CeFeAsO${}_{1-x}$F${}_x$ ($x$ $=$ 0, 0.11) and Sr(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x$ $=$ 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10${}^{-15}$ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3${\mu }_B$ in CeFeAsO and 2.1${\mu }_B$ in SrFe${}_2$As${}_2$. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9${\mu }_B$ in CeFeAsO${}_{0.89}$F${}_{0.11}$ and 1.3${\mu }_B$ in Sr(Fe${}_{0.9}$Co${}_{0.1}$)${}_2$As${}_2$. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.