The effects of quantum interferences on the excitation dependence of the intensity of $G$ modes have been investigated on single-walled carbon nanotubes [Duque et al., Phys. Rev. Lett. 108, 117404 (2012)]. In this work, by combining optical absorption spectroscopy and Raman scattering on individual index identified double-walled carbon nanotubes, we examine the experimental excitation dependence of the intensity of longitudinal optical and transverse optical $G$ modes of the constituent inner and outer single-walled carbon nanotubes. The observed striking dependencies are understood in terms of quantum interference effects. Considering such effects, the excitation dependence of the different components of the $G$ modes permits us to unambiguously assign each of them as originating from the longitudinal or transverse $G$ modes of inner and outer tubes.

We have examined the spin ordering of an axial next-nearest-neighbor Ising model in two dimensions (2D) near above the antiphase ($\langle 2\rangle$ phase). We considered an $N_R$-replica system and calculated an overlap function $q_m$ between different replicas, having used a cluster heat bath Monte Carlo method. We determined transition temperature between the $\langle 2\rangle$ phase and a floating incommensurate (IC) phase as $T_{C2}/J=0.89\pm 0.01$ with frustration ratio $\kappa (\equiv -J_2/J_1)=0.6$. We found that the spin state at $T\gtrsim T_{C2}$ may be called a domain state, because the spin structure is characterized by a sequentially arranged four types of domains with different $\langle 2\rangle$ structures. In the domain state, the 2D XY symmetry of the spin correlation in the IC phase weakly breaks, and the diversity of the spin arrangement increases as $T\to T_{C2}$. The Binder ratio $g_L$ exhibits a depression at $T\sim T_{C2}$ and the quasiperiodic spin structure, which is realized in the IC phase, becomes diverse at $T\gtrsim T_{C2}$. We discussed that the domain state is stable against the thermal fluctuation which brings a two-stage development of the spin structure at low temperatures.

We report ${}^{153}\mathrm{Eu}$, ${}^{75}\mathrm{As}$, and ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR) results on ${\mathrm{EuCo}}_2{\mathrm{As}}_2$ single crystal. Observations of ${}^{153}\mathrm{Eu}$ and ${}^{75}\mathrm{As}$ NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\mathrm{N}}=45$ K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in ${\mathrm{EuCo}}_2{\mathrm{As}}_2$. Furthermore, based on ${}^{59}\mathrm{Co}$ NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector $\mathbf{k}=(0,0,0.73\pm 0.07)2\pi /c$, where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.

This work aims to demonstrate and understand the key role of local demagnetizing fields in hybrid structures consisting of a continuous thin film with a stripe modulation on top. To understand the complex spin dynamics of these structures, the magnonic crystal was reconstructed in two different ways—performing micromagnetic simulations based on the structural shape as well as based on the internal demagnetizing field, which both are mapped on the nanoscale using electron holography. The simulations yield the frequency-field dependence as well as the angular dependence revealing the governing role of the internal field landscape around the backward-volume geometry. Simple rules for the propagation vector and the mode localization are formulated in order to explain the calculated mode profiles. Treating internal demagnetizing fields equivalent to anisotropies, the complex angle-dependent spin-wave behavior is described for an in-plane rotation of the external field.

More than a decade after the discovery of the two-dimensional electron system (2DES) at the interface between the band insulators ${\mathrm{LaAlO}}_3$ (LAO) and ${\mathrm{SrTiO}}_3$ (STO) its microscopic origin is still under debate. Several explanations have been proposed, the main contenders being electron doping by oxygen vacancies and electronic reconstruction, i.e., the redistribution of electrons to the interface to minimize the electrostatic energy in the polar LAO film. However, no experiment thus far could provide unambiguous information on the microscopic origin of the interfacial charge carriers. Here we utilize a novel experimental approach combining photoelectron spectroscopy (PES) with highly brilliant synchrotron radiation and apply it to a set of samples with varying key parameters that are thought to be crucial for the emergence of interfacial conductivity. Based on microscopic insight into the electronic structure, we obtain results tipping the scales in favor of polar discontinuity as a generic, robust driving force for the 2DES formation. Likewise, other functionalities such as magnetism or superconductivity might be switched in all-oxide devices by polarity-driven charge transfer.

Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic $3d$ transition metals. We address this issue by investigating the magnetism of a large number of size- and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications.

We study the controlled manipulation of the Jahn-Teller metal state of fulleride compounds using nonequilibrium dynamical mean-field theory. This anomalous metallic state is a spontaneous orbital-selective Mott phase, which is characterized by one metallic and two insulating orbitals. Using protocols based on transiently reduced hopping amplitudes or periodic electric fields, we demonstrate the possibility to switch orbitals between Mott insulating and metallic on a subpicosecond time scale, and to rotate the order parameter between three equivalent states that can be distinguished by their anisotropic conductance. The Jahn-Teller metal phase of alkali-doped fullerides thus provides a platform for ultrafast persistent memory devices.

We present a multipolar model of surface-lattice resonances (SLRs) in two-dimensional arrays of spheres including the electric dipole, magnetic dipole, and electric quadrupole moments of the spheres. We identify SLRs of dipolar and multipolar character, show the importance of nonresonant multipoles in their description, and discuss the sensitivity of SLRs to illumination conditions. We link SLRs to an excitation of modes supported by the array, and we propose a simplified model of the mode dispersion relations that explains the sensitivity of SLRs and the band gap in mode dispersion found at low frequencies. Finally, we discuss the resonant features associated with a direct coupling to a mode which can occur in addition to the diffractive coupling signaled by SLRs.

We study the effects of short-range interactions on a generalized three-dimensional Weyl semimetal, where the band touching points act as the (anti)monopoles of Abelian Berry curvature of strength $n$. We show that any local interaction has a negative scaling dimension $-2/n$. Consequently, all Weyl semimetals are stable against weak short-range interactions. For sufficiently strong interactions, we demonstrate that the Weyl semimetal either undergoes a first-order transition into a band insulator or a continuous transition into a symmetry breaking phase. A translational symmetry breaking axion insulator and a rotational symmetry breaking semimetal are two prominent candidates for the broken symmetry phase. At the one-loop order, the correlation length exponent for continuous transitions is $\nu =n/2$, indicating their non-Gaussian nature for any $n>1$. We also discuss the scaling of the thermodynamic and transport quantities in general Weyl semimetals as well as inside broken symmetry phases.

Electronic structures of ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ alloys $\left(0\le x\le 1\right)$ are theoretically studied by the nonlocal empirical pseudopotential method. For relaxed ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$, a topological semimetal is found for $\mathit{\text{x}}>41\%$ with gapless and band inversion at the $\mathrm{\Gamma }$ point, while there is an indirect-direct band-gap transition at $x=8.5\%$. For strained ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ on a Ge substrate, semimetals with a negative indirect band gap appear for $x>43\%$, and the strained ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ on Ge is always an indirect band-gap semiconductor for $x<43\%$. With appropriate biaxial compressive strains, a topological Dirac semimetal is found with band inversion at $\mathrm{\Gamma }$ and one pair of Dirac cones along the [001] direction.

We investigate the interplay of Coulomb interactions and short-range-correlated disorder in three-dimensional systems where absent disorder the noninteracting band structure hosts a quadratic band crossing. Though the clean Coulomb problem is believed to host a non-Fermi liquid phase, disorder and Coulomb interactions have the same scaling dimension in a renormalization group (RG) sense, and thus should be treated on an equal footing. We therefore implement a controlled $\varepsilon$ expansion and apply it at leading order to derive RG flow equations valid when disorder and interactions are both weak. We find that the non-Fermi liquid fixed point is unstable to disorder, and demonstrate that the problem inevitably flows to strong coupling, outside the regime of applicability of the perturbative RG. An examination of the flow to strong coupling suggests that disorder is asymptotically more important than interactions, so that the low-energy behavior of the system can be described by a noninteracting sigma model in the appropriate symmetry class (which depends on whether exact particle-hole symmetry is imposed on the problem). We close with a discussion of general principles unveiled by our analysis that dictate the interplay of disorder and Coulomb interactions in gapless semiconductors, and of connections to many-body localized systems with long-range interactions.

We investigate the effect of spin-orbit coupling on the in-gap bound states localized at magnetic impurities in multiband superconductors with unconventional (sign-changed) and conventional (sign-unchanged) $s$-wave pairing symmetry, which may be relevant to iron-based superconductors. Without spin-orbit coupling, for spin-singlet superconductors it is known that such bound states cross zero energy at a critical value of the impurity scattering strength and acquire a finite spin polarization. Moreover, the degenerate, spin-polarized, zero-energy bound states are unstable to applied Zeeman fields as well as a deviation of the impurity scattering strength away from criticality. Using a $T$-matrix formalism as well as analytical arguments, we show that, in the presence of spin-orbit coupling, the zero-energy bound states localized at magnetic impurities in unconventional, sign-changed, $s$-wave superconductors acquire surprising robustness to applied Zeeman fields and variation in the impurity scattering strength, an effect which is absent in the conventional, sign-unchanged, $s$-wave superconductors. Given that the iron-based multiband superconductors may possess a substantial spin-orbit coupling as seen in recent experiments, our results may provide one possible explanation to the recent observation of surprisingly robust zero bias scanning tunneling microscope peaks localized at magnetic impurities in iron-based superconductors provided the order parameter symmetry is sign changing $s_{+-}$-wave.

Transition-metal dichalcogenides are unique semiconductors because of their exclusive coupling between the spin and the valley degrees of freedom. The spin flip simultaneously requires a large amount of the crystal momentum variation; hence most of the carrier scattering is expected to be the spin-conserving intravalley scattering. Analysis of the quantum interference effects on the magnetoconductivity in ${\mathrm{WSe}}_2,{\mathrm{MoSe}}_2$, and ${\mathrm{MoS}}_2$ reveals that the spin-relaxation time is orders of magnitude longer than the carrier momentum scattering time, indicating that the valley-spin coupling robustly protects the spin polarization from carrier scatterings. In addition, the electron-spin-relaxation time of ${\mathrm{MoSe}}_2$ is found to be anomalously short compared to other members, which is likely the origin of the ultrafast valley scattering of excitons in ${\mathrm{MoSe}}_2$.

We propose a continuum model to predict long-wavelength vibrational modes of empty and liquid-filled tubules that are very hard to reproduce using the conventional force-constant matrix approach based on atomistic ab initio calculation. We derive simple quantitative expressions for long-wavelength longitudinal and torsional acoustic modes, flexural acoustic modes, as well as the radial breathing mode of empty or liquid-filled tubular structures that are based on continuum elasticity theory expressions for a thin elastic plate. We furthermore show that longitudinal and flexural acoustic modes of tubules are well described by those of an elastic beam resembling a nanowire. Our numerical results for biological microtubules and carbon nanotubes agree with available experimental data.

Recent pump-probe experiments demonstrate the possibility that Dirac materials may be driven into transient excited states describable by two chemical potentials, one for the electrons and one for the holes. Given the Dirac nature of the spectrum, such an inverted population allows the optical tunability of the density of states of the electrons and holes, effectively offering control of the strength of the Coulomb interaction. Here we discuss the feasibility of realizing transient excitonic instabilities in optically pumped Dirac materials. We demonstrate, theoretically, the reduction of the critical coupling leading to the formation of a transient condensate of electron-hole pairs and identify signatures of this state. Furthermore, we provide guidelines for experiments by both identifying the regimes in which such exotic many-body states are more likely to be observed and estimating the magnitude of the excitonic gap for a few important examples of existing Dirac materials. We find a set of material parameters for which our theory predicts large gaps and high critical temperatures and which could be realized in future Dirac materials. We also comment on transient excitonic instabilities in three-dimensional Dirac and Weyl semimetals. This study provides an example of a transient collective instability in driven Dirac materials.

${\mathrm{K}}_2{\mathrm{CuF}}_4$ is magnetically described as a spin-$\frac{1}{2}$, quasi-two-dimensional (2D), square-lattice XXZ ferromagnet with weak easy-plane anisotropy. The magnetic ordering for an applied magnetic field $H$ parallel to the $c$ axis is equivalent to the Bose-Einstein condensation (BEC) of lattice bosons, as discussed by Matsubara and Matsuda [T. Matsubara and H. Matsuda, Prog. Theor. Phys. 16, 569 (1956)]. Magnetization and specific-heat measurements were performed to obtain the temperature versus magnetic field phase diagram for $H\parallel c$. The phase boundary between polarized and ordered phases was found to be expressed by the power law $H_{\mathrm{c}}\left(T\right)-H_{\mathrm{c}}\left(0\right)\propto T^{\varphi }$ with exponent $\varphi \approx 1.0$ in a wide temperature range, in agreement with the theory of quasi-2D BEC.

We investigate the onset of superconductivity in a magnetic field for a clean two-dimensional multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Due to the small number of carriers in this band, the quasiclassical Werthamer-Helfand approximation breaks down and Landau quantization has to be taken into account. We found that the transition temperature $T_{C2}\left(H\right)$ has giant oscillations and is resonantly enhanced at the magnetic fields corresponding to the matching of the chemical potential with the Landau levels in the shallow band. This enhancement is especially pronounced for the lowest Landau level. As a consequence, the reentrant superconducting regions in the temperature-field phase diagram emerge at low temperatures near the magnetic fields at which the shallow-band Landau levels cross the chemical potential. The specific behavior depends on the relative strength of the intraband and interband pairing interactions and the reentrance is most pronounced in the purely interband coupling scenario. The reentrant behavior is suppressed by the Zeeman spin splitting in the shallow band; the separated regions disappear already for very small spin-splitting factors. On the other hand, the reentrance is restored in the resonance cases when the spin-splitting energy exactly matches the separation between the Landau levels. The predicted behavior may be realized in the gate-tuned FeSe monolayer.

The symmetry of single-molecule magnets dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry $\left[\mathrm{M}{\mathrm{n}}_{12}{\mathrm{O}}_{12}{\left({\mathrm{O}}_2\mathrm{CM}\mathrm{e}\right)}_{16}{\left(\mathrm{M}\mathrm{e}\mathrm{OH}\right)}_4\right]·\mathrm{M}\mathrm{e}\mathrm{OH}$ single-molecule magnet as a function of uniaxial pressure applied either parallel or perpendicular to the sample's “easy” magnetization axis. At temperatures between 1.8 and 3.3 K, magnetic hysteresis loops exhibit the characteristic steplike features that signal the occurrence of QTM. After applying uniaxial pressure to the sample in situ, both the magnitude and field position of the QTM steps changed. The step magnitudes were observed to grow as a function of pressure in both arrangements of pressure, while pressure applied along (perpendicular to) the sample's easy axis caused the resonant-tunneling fields to increase (decrease). These observations were compared with simulations in which the system's Hamiltonian parameters were changed. From these comparisons, we determined that parallel pressure induces changes to the second-order axial anisotropy parameter as well as either the fourth-order axial or fourth-order transverse parameter, or to both. In addition, we find that pressure applied perpendicular to the easy axis induces a rhombic anisotropy $E\approx D/2000\mathrm{per}\mathrm{kbar}$ that can be understood as deriving from a symmetry-breaking distortion of the molecule.

In the present work, we report the growth of fully untwinned high-quality $a$-axis-oriented ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ films on (100) ${\mathrm{SrLaGaO}}_4$ substrates by using ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ as a buffer layer. We also fabricated nanowires at different angles $\gamma$ with respect to the [0,1,0] direction of the substrate and studied the in-plane anisotropy of the critical current density, which we explained by considering the anisotropy in the coherence length $\xi$ and London penetration depth ${\lambda }_L$. Finally, half-integer Shapiro-like steps measured in slightly underdoped $c$-axis oriented ($\gamma ={90}^{\circ }$) nanowires point towards a different transport regime, which could shed light on intriguing issues of high-critical-temperature superconductors.