We report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photoinduced ultrafast excitations. Following transient optical excitations the effective lattice temperature of the film rises close to the Néel temperature and the charge-density wave (CDW) amplitude is reduced but does not appear to ever be fully destroyed. The persistence of the CDW diffraction signal demonstrates that the CDW, if destroyed by the laser pulse, must be reestablished within the 100-ps time resolution of the synchrotron x-ray pulses. Furthermore, at all times after photoexcitation, the CDW retains its low-temperature periodicity, rather than regenerating with its high-temperature period shortly after photoexcitation. The long-term evolution shows that the CDW reverts to its ground state on a time scale of $370\pm 40\mathrm{ps}$. We attribute the apparent persistence of the CDW to the long-lived periodic lattice displacement in chromium. This study highlights the fundamental role of the lattice distortion and its impact on the recondensation dynamics of the charge ordered state in strongly correlated materials.

Graphene bilayer systems are known to exhibit a band gap when the layer symmetry is broken by applying a perpendicular electric field. The resulting band structure resembles that of a conventional semiconductor with a parabolic dispersion. Here, we introduce a bilayer graphene heterostructure, where single-layer graphene is placed on top of another layer of graphene with a regular lattice of antidots. We dub this class of graphene systems GOAL: graphene on graphene antidot lattice. By varying the structure geometry, band-structure engineering can be performed to obtain linearly dispersing bands (with a high concomitant mobility), which nevertheless can be made gapped with a perpendicular field. We analyze the electronic structure and transport properties of various types of GOALs, and draw general conclusions about their properties to aid their design in experiments.

We investigate the nonlinear scattering theory for quantum systems with strong Seebeck and Peltier effects, and consider their use as heat engines and refrigerators with finite power outputs. This paper gives detailed derivations of the results summarized in a previous paper [R. S. Whitney, Phys. Rev. Lett. 112, 130601 (2014)]. It shows how to use the scattering theory to find (i) the quantum thermoelectric with maximum possible power output, and (ii) the quantum thermoelectric with maximum efficiency at given power output. The latter corresponds to a minimal entropy production at that power output. These quantities are of quantum origin since they depend on system size over electronic wavelength, and so have no analog in classical thermodynamics. The maximal efficiency coincides with Carnot efficiency at zero power output, but decreases with increasing power output. This gives a fundamental lower bound on entropy production, which means that reversibility (in the thermodynamic sense) is impossible for finite power output. The suppression of efficiency by (nonlinear) phonon and photon effects is addressed in detail; when these effects are strong, maximum efficiency coincides with maximum power. Finally, we show in particular limits (typically without magnetic fields) that relaxation within the quantum system does not allow the system to exceed the bounds derived for relaxation-free systems, however, a general proof of this remains elusive.

Recent experiments on the role of electron-electron interactions in fractal Dirac systems have revealed a host of interesting effects, in particular, the unique nature of the magnetic field dependence of butterfly gaps in graphene. The novel gap structure recently observed in the integer quantum Hall effect is quite intriguing [G. L. Yu et al., Nat. Phys. 10, 525 (2014)], where one observes a suppression of the ferromagnetic state at one value of the commensurable flux but a reentrant ferromagnetic state at another. In our present work we introduce the magnetic translation symmetry in the integer quantum Hall effect regime and consider the interplay between the electron-electron interaction and the periodic potential. In this approach, we explain the underlying physical processes that can lead to such a unique behavior of the butterfly gaps as observed in that system where we invoke the spin-flip transitions in the ground state.

We present evidence that subsurface carbon nanoparticles in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ can be manipulated with nanometer precision using a scanning tunneling microscope. High-resolution images indicate that most of the carbon particles remain subsurface after transport observable as a local increase in height as the particle pushes up on the surface. Tunneling spectra in the vicinity of these protrusions exhibit semiconducting characteristics with a band gap of approximately 1.8 eV, indicating that the incorporation of carbon locally alters the electronic properties near the surface.

Concealing objects by making them invisible to an external electromagnetic probe is coined by the term “cloaking.” Cloaking devices, having numerous potential applications, are still facing challenges in realization, especially in the visible spectral range. In particular, inherent losses and extreme parameters of metamaterials required for the cloak implementation are the limiting factors. Here, we numerically demonstrate nearly perfect suppression of scattering from arbitrary-shaped objects in spatially dispersive metamaterial acting as an alignment-free concealing cover. We consider a realization of a metamaterial as a metal-dielectric multilayer and demonstrate suppression of scattering from an arbitrary object in forward and backward directions with perfectly preserved wave fronts and less than 10% absolute intensity change, despite spatial dispersion effects present in the composite metamaterial. Beyond the usual scattering suppression applications, the proposed configuration may be used for a simple realization of scattering-free detectors and sensors.

Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependence and excitation power dependence of emission intensity. We have obtained the values of exciton diffusion length and absorption cross section for different chiralities, and diameter-dependent photoluminescence quantum yields have been observed. The simulations have also revealed the nature of a one-dimensional coalescence process, and an analytical expression for the power dependence of emission intensity is given.

${\mathrm{Th}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ and ${\mathrm{U}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ crystallize with primitive cubic ${\mathrm{Tm}}_3{\mathrm{Co}}_4{\mathrm{Ge}}_{13}$ and noncentrosymmetric rhombohedral ${\text{HT-Y}}_3{\mathrm{Pt}}_4{\mathrm{Ge}}_{13}$ type of structures, respectively, which are derivatives of the cubic ${\mathrm{Yb}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ prototype. Measurements of magnetic susceptibility, electrical resistivity, specific heat, thermopower, and thermal conductivity reveal that ${\mathrm{Th}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ is a diamagnetic bad metal and undergoes a first-order phase transition at ≈200 K. Charge-density wave (CDW) and structural phase-transition scenarios for ${\mathrm{Th}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ are discussed. ${\mathrm{U}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ is Curie paramagnetic $({\mu }_{\mathrm{eff}}=4.05$ ${\mu }_{\mathrm{B}})$ and orders ferromagnetically at $T_C=15$ K. For ${\mathrm{U}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ the oxidation state of U is investigated by x-ray absorption spectroscopy. The U $5f$ electrons in this compound are predominantly of itinerant nature.

EuPtP undergoes two valence transitions and has two kinds of valence states of Eu ions at low temperatures. In the charge-ordered state, this compound shows an antiferromagnetic order ascribed to magnetic divalent Eu ions. We investigated the antiferromagnetically ordered state of EuPtP by nuclear magnetic resonance (NMR) measurement and nuclear quadrupole resonance (NQR) measurement in a zero external magnetic field. The observed ${}^{153}\mathrm{Eu}$ NMR signals of a magnetic divalent state and ${}^{151,153}\mathrm{Eu}$ NQR signals of a nonmagnetic trivalent state clearly demonstrate that the spins order in the hexagonal basal plane and the internal magnetic field is not canceled out, even at the ${\mathrm{Eu}}^{3+}$ layers which are in the middle of magnetic ${\mathrm{Eu}}^{2+}$ layers. In addition, the observation of ${}^{31}\mathrm{P}$ and ${}^{195}\mathrm{Pt}$ NMR spectra allowed us to discuss a possible magnetic structure. We also evaluated the nuclear quadrupole frequencies for both ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ion states.

We report the results on the structure, transport, and magnetic properties of a layered oxysulfide CaOFeS with a stacked triangle lattice of Fe ions. The susceptibility data show a broad maximum near 120 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. Features associated with long-range antiferromagnetic (AFM) phase transition are seen below 40 K. Meanwhile, a very small heat-capacity anomaly is detected around 35 K, and most of the measured magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 125, 40, and 6 K. The structure was refined based on space group $P{6}_{3}mc$ with $a=3.75998\left(4\right)$ and $c=11.38351\left(16\right)\AA$ at ambient temperature. Low-temperature diffraction reveals 2D magnetic correlations between Fe moments without showing significant structural distortion. Warren peak shape analysis of the neutron-diffraction data at $2\theta$ near 18° is employed to characterize the correlation length in the 2D magnetic state with lowering temperature. The geometrically frustrated compound is found to gradually condense into a partial long-range ordered state with AFM coupled Fe layers between 40.6 and 26 K. The resulting partially ordered magnetic structure is a $G$-type Ising AFM with a propagation vector of $\mathbf{k}=(1/2,1/2,0)$ and an ordered magnetic moment of $2.59\left(3\right){\mu }_{\mathrm{B}}/\mathrm{Fe}$ along $c$ at 6 K.

We evaluate Rényi entropy flows from generic quantum heat engines (QHE) to a weakly coupled probe environment kept in thermal equilibrium. We show that the flows are determined not only by heat flow but also by a quantum coherent flow that can be separately measured in experiment apart from the heat flow measurement. The same pertains to Shannon entropy flow. This appeals for a revision of the concept of entropy flows in quantum nonequlibrium thermodynamics.

Spin-wave spectra of a double magnetic layer are calculated theoretically in the macroscopic limit. Magnetic dynamics is described in terms of the Landau-Lifshitz-Gilbert equation, and both static (of the Ruderman-Kittel-Kasuya-Yosida type) and dynamic (via spin pumping) interlayer couplings are taken into account. The influence of spin pumping and spin transfer torque on the spin-wave spectra (frequency and damping factor) has been studied for both parallel and antiparallel magnetic configurations. The spin-wave spectrum in the parallel magnetic state is reciprocal, while in the antiparallel configuration it is nonreciprocal. In both cases, a substantial reduction of the spin-wave lifetimes due to spin pumping to the nonmagnetic metallic layers has been found. In the parallel configuration, this reduction appears mainly for optical modes, while in the antiparallel configuration, it is remarkable for all modes. In turn, the spin torque due to spin current flowing from a metallic layer, created for instance by the spin Hall effect, gives rise to significant changes in the damping factors as well, but these modifications depend on the sign of spin current. For one spin current orientation, the spin-wave damping becomes reduced and may disappear for some modes at a specific threshold value of the spin current, indicating magnetic instability in the system due to spin transfer torque. For the opposite spin current, the damping is enhanced, which indicates stabilization of the corresponding magnetic state.

Using a combination of muon-spin relaxation $\left(\mu \mathrm{SR}\right)$, inelastic neutron scattering (INS), and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure ${\left({\mathrm{CaFe}}_{1-x}{\mathrm{Pt}}_x\mathrm{As}\right)}_{10}{\mathrm{Pt}}_3{\mathrm{As}}_8$ ($T_{\text{c}}=13$ K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the $\mu \mathrm{SR}$ relaxation-rate measurements indicates the presence of two superconducting gaps, ${\Delta }_{\text{1}}\gg {\Delta }_{\text{2}}$. According to our INS measurements, commensurate spin fluctuations are centered at the $(\pi ,0)$ wave vector, like in most other iron arsenides. Their intensity remains unchanged across $T_{\text{c}}$, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around $\hslash {\omega }_0=7$ meV at the same wave vector, which persists above $T_{\text{c}}$ and is characterized by the ratio $\hslash {\omega }_0/k_{\text{B}}{T}_{\text{c}}\approx 6.2$, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides $(\sim 4.3)$. The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around $T^{*}=45$ K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, $1/T_{1}T$, observed by NMR immediately below $T^{*}$ without any notable subsequent anomaly at $T_{\mathrm{c}}$, indicates that $T^{*}$ could mark the onset of a pseudogap in ${\left({\mathrm{CaFe}}_{1-x}{\mathrm{Pt}}_x\mathrm{As}\right)}_{10}{\mathrm{Pt}}_3{\mathrm{As}}_8$, which is likely associated with the emergence of preformed Cooper pairs.

The gap equation for Dirac quasiparticles in monolayer graphene in constant magnetic and pseudomagnetic fields, where the latter is due to strain, is studied in a low-energy effective model with contact interactions. By analyzing solutions of the gap equation, the phase diagram of the system in the plane of pseudomagnetic and parallel magnetic fields is obtained in the approximation of the lowest Landau level. The three quantum Hall states, ferromagnetic, antiferromagnetic, and canted antiferromagnetic, are realized in different regions of the phase diagram. It is found that the structure of the phase diagram is sensitive to signs and values of certain four-fermion interaction couplings which break the approximate spin-valley $SU\left(4\right)$ symmetry of the model.

In this work, we numerically study critical phases in translation-invariant ${\mathbb{Z}}_N$ parafermion chains with both nearest- and next-nearest-neighbor hopping terms. The model can be mapped to a ${\mathbb{Z}}_N$ spin model with nearest-neighbor couplings via a generalized Jordan-Wigner transformation and translational invariance ensures that the spin model is always self-dual. We first study the low-energy spectrum of chains with only nearest-neighbor coupling, which are mapped onto standard self-dual ${\mathbb{Z}}_N$ clock models. For $3\le N\le 6$, we match the numerical results to the known conformal field theory(CFT) identification. We then analyze in detail the phase diagram of a $N=3$ chain with both nearest and next-nearest-neighbor hopping and six critical phases with central charges being $4/5$, 1, or 2 are found. We find continuous phase transitions between $c=1$ and 2 phases, while the phase transition between $c=4/5$ and 1 is conjectured to be of Kosterlitz-Thouless type.

The presence of a ${\mathrm{SiO}}_2$ layer on Si nanowires (SiNWs) has been found through molecular dynamics simulation to reduce their thermal conductivity $\left(\kappa \right)$, with $\kappa$ approaching the amorphous limit of Si as the oxide layer thickness is increased. Through analysis of the phonon energy dispersion and vibrational density of states (VDOS) spectrum, this decrease in $\kappa$ was attributed to dispersionless vibrational states that appear in the low energy range below 4 THz as a result of the lattice vibration of Si atoms near the ${\mathrm{SiO}}_2/\mathrm{Si}$ interface. The ${\mathrm{SiO}}_2$ layer also induced a low-frequency tail in the VDOS spectrum, the length of which was more closely correlated to the reduction in $\kappa$ than the frequency-integrated value of the VDOS spectrum. These findings provide a more refined explanation for the decrease in $\kappa$ than has been previously observed, and contribute to providing a greater understanding of the anomalistic vibration near the interface that is critical to determining the heat conductivity in nanoscale materials.

Probabilities deduced from quantum information studies are usually based on averaging many identical experiments separated by an initialization step. Such initialization steps become experimentally more challenging to implement as the complexity of quantum circuits increases. To better understand the consequences of imperfect initialization on the deduced probabilities, we study the effect of not initializing the system between measurements. For this we utilize Landau-Zener-Stückelberg oscillations in a double quantum dot circuit. Experimental results are successfully compared to theoretical simulations.

A double peak structure in the magnetophonon resonance (MPR) spectrum of uniaxial strained graphene, under crossed electric and magnetic fields, is predicted. We focus on the $\Gamma$ point optical phonon modes coupled to the inter-Landau level transitions $0\leftrightarrows \pm 1$ where MPR is expected to be more pronounced at high magnetic field. We derive the frequency shifts and the broadenings of the longitudinal and transverse optical phonon modes taking into account the effect of the strain modified electronic spectrum on the electron-phonon coupling. We show that the MPR line for a given phonon mode acquires a double peak structure originating from the twofold valley degeneracy lifting. The latter is due to the different Landau level spacings in the two Dirac valleys resulting from the simultaneous action of the inplane electric field and the strain-induced Dirac cone tilt. We discuss the role of some key parameters such as disorder, strain, doping, and electric field amplitude on the emergence of the double peak structure.