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The Pauli kinetic energy enhancement factor α = (τ − τW)/τunif is an important density ingredient, used to construct many meta-generalized gradient approximations (meta-GGA) exchange–correlation (XC) energy functionals, including the very successful strongly constrained and appropriately normed (SCAN) semilocal functional. Another meta-GGA functional, known as MGGAC (2019 Phys. Rev. B 100 155140), is also proposed in recent time depending only on the α ingredient and based on the generalization of the Becke–Roussel approach with the cuspless hydrogen exchange hole density. The MGGAC functional is proved to be a very useful and competitive meta-GGA semilocal functional for electronic structure properties of solids and molecules. Based on the successful implication of the ingredient α, which is also useful to construct the one-electron self-interaction free correlation energy functional, here we propose revised correlation energy for MGGAC exchange functional which is more accurate and robust, especially for the high and low-density limits of the uniform density scaling. The present XC functional, named as revised MGGAC (rMGGAC), shows an impressive improvement for the structural and energetic properties of solids compared to its previous version. Moreover, the assessment of the present constructed functional shows to be quite useful in solid-state physics in terms of addressing several current challenging solid-state problems. [ABSTRACT FROM AUTHOR]

Ion beams with energies in the keV regime are widely utilized in solid-state physics, but the ultrafast dynamics triggered by an ion impact onto a solid surface is to date exclusively accessible via simulations based on many untested assumptions and model parameters. A possible experimental access rests on the availability of a laser-synchronized ion source delivering sufficiently short ion pulses for time resolved pump–probe experiments. Here, we demonstrate a new miniaturized ion optical bunching setup for the creation of rare gas ion pulses using strong-field femtosecond laser photoionization. Neutral Ar gas atoms at room temperature are intercepted by a 50 fs, 800 nm laser pulse focused to ∼10 μm spot size. We demonstrate the generation of monoenergetic 2 keV Ar+ ion pulses with 180 ps duration (FWHM) at laser peak intensities around 1014 W cm−2 and of multiply charged Arq+ ions (q = 1–5) at higher laser intensities. The results are in good agreement with detailed ion trajectory simulations, which show that the temporal resolution is essentially limited by the initial (thermal) velocity spread of the generated photo-ions, indicating the possibility to achieve even better time resolution by cooling the gas prior to ionization. [ABSTRACT FROM AUTHOR]

In magnetic semiconductors the optical spectrum and, in particular, the absorption edge representing the band-gap are strongly affected by the onset of the magnetic order. This contribution to the band-gap energy has hitherto been described theoretically in terms of a Heisenberg Hamiltonian, in which a delocalized conduction carrier is coupled to the localized magnetic moments by the exchange interaction. Such models, however, do not take into account the strong correlations displayed in a wide variety of magnetic semiconductors, which are responsible for the formation of the local moments. In particular, the itinerant carrier itself contributes to the spin moment. Here, we overcome this simplification in a combined experimental and theoretical study of the antiferromagnetic semiconductor α -MnTe. First, we present a spectroscopic optical investigation as a function of temperature, from which we extract the magnetic contribution to the blue-shift of the band-gap. Second, we formulate a minimal model based on a Hubbard–Kondo Hamiltonian. In this model, the itinerant charge is one of the electrons forming the localized magnetic moment, which properly captures correlation effects in the material. Our theory reproduces the experimental findings with excellent quantitative agreement, demonstrating that the magnetic contribution to the band-gap energy of α -MnTe is mediated solely by the exchange interaction. These results describe an intrinsic property of the material, independent of the thickness, substrate and capping layer of the specimen. One of the key findings of the model is that the basic effect, namely a blue-shift of the band-gap due to the establishment of the magnetic order, is a general phenomenon in charge-transfer insulators. The identification of the relevant magnetic interaction discloses the possibility to exploit the effect here discussed to induce a novel regime of coherent spin dynamics, in which spin oscillations on a characteristic time-scale of 100 fs are triggered and are intrinsically coupled to charges.

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we—surprisingly—find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials. [ABSTRACT FROM AUTHOR]

Generating, controlling and sensing strong magnetic fields at ever shorter time and length scales is important for both fundamental solid-state physics and technological applications such as magnetic data recording. Here, we propose a scheme for producing strong ultrashort magnetic pulses localized at the nanoscale. We show that a bimetallic nanoring illuminated by femtosecond laser pulses responds with transient thermoelectric currents of picosecond duration, which in turn induce Tesla-scale magnetic fields in the ring cavity. Our method provides a practical way of generating intense nanoscale magnetic fields with great potential for materials characterization, terahertz radiation generation and data storage applications. [ABSTRACT FROM AUTHOR]

How a local perturbation affects a propagating wave traveling in a homogeneous medium is a general physics question widely investigated in condensed materials. Intuitively, one might expect that a perturbation would suppress the transport ability of the medium if it is quasi one dimensional. This is generically true as defects and impurities influence numerous nonexcitable systems such as carbon nanotubes, nanowires and DNA double helixes. However, if the system is excitable, such as a neuron, a defect may generate a highly non-trivial dynamical behavior. In this paper, using the Hodgkin-Huxley model, we explored this diversity generated by locally non-uniform ion channel densities caused by toxins, diseases, environmental disorders or artificial manipulations. These channel density defects could induce several exotic behaviors, in contrast with the normal destructive role of defects in solid-state physics. They may behave as an electric signal generator exhibiting spontaneous or stimulated emissions, as well as trap, reflect, rectify, delay or extinguish propagating signals or be switched to different functions by a signal. Nonlinear analysis and phase diagrams were used to quantify this dynamical complexity. The results may contribute to research on signal manipulation in biotechnology, neuronal diseases and damages, channel distribution-related cell functions and defect dynamics in general excitable mathematical models. [ABSTRACT FROM AUTHOR]

Nitrogen-vacancy centers in diamond have outstanding quantum optical properties that enable applications in information processing and sensing. As with most solid-state systems for quantum photonic applications, the great promise lies in the capability to embed them in an on-chip optical network. Here we present basic integrated devices composed of diamond micro-ring resonators coupled to waveguides that are terminated with grating out-couplers. Strong enhancement is observed for the zero-phonon line of nitrogen-vacancy centers coupled to the ring resonance. The zero-phonon line is efficiently coupled from the ring into the waveguide and then scattered out of plane by the grating out- couplers. [ABSTRACT FROM AUTHOR]

We provide a unified geometrical description for analyzing the stability of holonomic quantum gates in the presence of imprecise driving controls (parametric noise). We consider the situation in which these fluctuations do not affect the adiabatic evolution but can reduce the logical gate performance. Using the intrinsic geometric properties of the holonomic gates, we show under which conditions of the noise's correlation time and strength the fluctuations in the driving field cancel out. In this way, we provide theoretical support for previous numerical simulations. We also briefly comment on the error due to the mismatch between the real and the nominal time of the period of the driving fields and show that it can be reduced by suitably increasing the adiabatic time. [ABSTRACT FROM AUTHOR]

The spontaneous creation of electron-positron pairs out of the vacuum due to a strong electric field is a spectacular manifestation of the relativistic energy-momentum relation for the Dirac fermions. This fundamental prediction of quantum electrodynamics has not yet been confirmed experimentally, as the generation of a sufficiently strong electric field extending over a large enough space-time volume still presents a challenge. Surprisingly, distant areas of physics may help us to circumvent this difficulty. In condensed matter and solid state physics (areas commonly considered as low-energy physics), one usually deals with quasi-particles instead of real electrons and positrons. Since their mass gap can often be freely tuned, it is much easier to create these light quasi-particles by an analogue of the Sauter-Schwinger effect. This motivates our proposal for a quantum simulator in which excitations of ultra-cold atoms moving in a bichromatic optical lattice represent particles and antiparticles (holes) satisfying a discretized version of the Dirac equation together with fermionic anti-commutation relations. Using the language of second quantization, we are able to construct an analogue of the spontaneous pair creation which can be realized in an (almost) table-top experiment. [ABSTRACT FROM AUTHOR]

We theoretically study the bichromatic driving of a solid-state cavity quantum electrodynamics (QED) system as a means of probing cavity dressed state transitions and observing the coherent interaction between the system and the light field. We show that this method can enable the observation of the higher order cavity dressed states, supersplitting and ac-Stark shift in a solid-state system comprised of a quantum dot (QD) strongly coupled to a photonic crystal cavity for the on- and far off-resonant cases. For the off-resonant case, phonons mediate off-resonant coupling between the QD and the photonic resonator, a phenomenon unique to solid-state cavity QED. [ABSTRACT FROM AUTHOR]

Recently, it was shown that the non-local correlations needed for measurement-based quantum computation (MBQC) can be revealed in the ground state of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model involving nearest-neighbour spin-3/2 interactions on a honeycomb lattice. This state is not singular but resides in the disordered phase of the ground states of a large family of Hamiltonians characterized by short-range-correlated valence bond solid states. By applying local filtering and adaptive single-particle measurements, we show that most states in the disordered phase can be reduced to a graph of correlated qubits that is a scalable resource for MBQC. At the transition between the disordered and Néel ordered phases, we find a transition from universal to non-universal states as witnessed by the scaling of percolation in the reduced graph state. [ABSTRACT FROM AUTHOR]

KLL Auger-energy shifts between free atoms and their solid surfaces were determined from spectra measured simultaneously in identical experimental conditions. Essentially, the shift values obtained for Na, Mg, Al and Si were more accurate than those achieved by combining the results from separate vapour and solid measurements. Using atomic Auger energies and determined shifts, reliable absolute solid state Auger energies with respect to the vacuum level were also obtained. Experimental shift values were also compared with calculations obtained with the excited atom model. 2s and 2p binding energy shifts were estimated from recent high resolution and due to open shell strongly split vapour phase spectra and corresponding published solid state results. Also, the question of the extent to which the 2s and 2p shifts deviate has been discussed here. [ABSTRACT FROM AUTHOR]

Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques—such as angle-resolved photoemission spectroscopy (ARPES)—probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe3, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures. The transient change in the band structure due to the excitation is qualitatively different between the region gapped by the CDW order vector and an ungapped but otherwise equivalent region. Also, we discovered two distinct collective modes at roughly 3.5 and 2.5 THz, the latter of which only occurs in the CDW band near the gapped region, demonstrating the strength of tr-ARPES in discerning the origin of the modes from the way in which they couple to the quasi-particle bands. In addition, a systematic pump fluence dependence in the gapped region documents the crossover from a weakly perturbed to a strongly perturbed regime, which can be related to a crossover from a regime where mainly the amplitude mode gets excited to a regime where the CDW gap closes at least partially. [ABSTRACT FROM AUTHOR]

Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques—such as angle-resolved photoemission spectroscopy (ARPES)—probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe3, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures. The transient change in the band structure due to the excitation is qualitatively different between the region gapped by the CDW order vector and an ungapped but otherwise equivalent region. Also, we discovered two distinct collective modes at roughly 3.5 and 2.5 THz, the latter of which only occurs in the CDW band near the gapped region, demonstrating the strength of tr-ARPES in discerning the origin of the modes from the way in which they couple to the quasi-particle bands. In addition, a systematic pump fluence dependence in the gapped region documents the crossover from a weakly perturbed to a strongly perturbed regime, which can be related to a crossover from a regime where mainly the amplitude mode gets excited to a regime where the CDW gap closes at least partially. [ABSTRACT FROM AUTHOR]

Reversion of thermoremanent magnetization decay is observed in a canonical spin glass; that is, the system is remagnetized despite the absence of a magnetic field. This unexpected phenomenon occurs only when the system is heated or cooled back to the temperature T0 at which it was previously subjected to a magnetic field for a significant length of time. Therefore, this reversion can be interpreted as a restoration of the spin state existing before at T0. This new information will be a key to resolving a long-standing controversy regarding the nature of spin glasses. [ABSTRACT FROM AUTHOR]

The factors that limit both the continuous wave (CW) and the pulsed output power of broad-area laser diodes driven at very high currents are investigated theoretically and experimentally. The decrease in the gain due to self-heating under CW operation and spectral holeburning under pulsed operation, as well as heterobarrier carrier leakage and longitudinal spatial holeburning, are the dominant mechanisms limiting the maximum achievable output power. [ABSTRACT FROM AUTHOR]

An exact algorithm is used to investigate the distributions of the degeneracies of low-energy excited states for the bimodal Ising spin glass on the brickwork lattice. Since the distributions are extreme and do not self-average, we base our conclusions on the most likely values of the degeneracies. Our main result is that the degeneracy of the first excited state per ground state and per spin is finite in the thermodynamic limit. This is very different from the same model on a square lattice where a divergence proportional to the linear lattice size is expected. The energy gap for the brickwork lattice is obviously 2J on finite systems and predicted to be the same in the thermodynamic limit. Our results suggest that a 2J gap is universal for planar bimodal Ising spin glasses. The distribution of the second contribution to the internal energy has a mode close to zero and we predict that the low-temperature specific heat is dominated by the leading term proportional to T -2exp(-2J/kT ). [ABSTRACT FROM AUTHOR]

A fully connected qubit network is considered, where every qubit interacts with every other one. When the interactions between the qubits are homogeneous, the system is a special case of the finite Lipkin-Meshkov-Glick (LMG) model. We propose a natural implementation of this model using superconducting qubits in state-of-the-art circuit QED. The ground state, the low-lying energy spectrum and the dynamical evolution are investigated. We find that, under realistic conditions, highly entangled states of Greenberger-Horne-Zeilinger (GHZ) and W types can be generated. We also comment on the influence of disorder on the system and discuss the possibility of simulating complex quantum systems, such as Sherrington-Kirkpatrick (SK) spin glasses, with superconducting qubit networks. [ABSTRACT FROM AUTHOR]

An exact algorithm is used to compute the degeneracies of the excited states of the bimodal Ising spin glass in two dimensions. It is found that the specific heat at arbitrary low temperature is not a self-averaging quantity and has a distribution that is neither normal nor lognormal. Nevertheless, it is possible to estimate the most likely value and this is found to scale as L3T-2 exp (-4J/kT ), for a L × L lattice. Our analysis also explains, for the first time, why a correlation length ξ ∼ exp (2J/kT ) is consistent with an energy gap of 2J . Our method allows us to obtain results for up to 105 disorder realizations with L ⩽ 96. Distributions of second and third excitations are also shown. [ABSTRACT FROM AUTHOR]

We study the effect of defects in the Mott insulator phase of ultracold atoms in an optical lattice on the dynamics of resonant electronic excitations. These excitations can be described analogous to Frenkel excitons in solids and form polaritons, when coupled to an optical resonator. Defects, which are empty sites in a singly occupied Mott insulator state or singly occupied sites for a filling factor two, change the exciton dynamics. We show that vacancies behave like hard sphere scatterers, while singly occupied sites in a doubly filled region generate either attractive or repulsive interaction potentials. We suggest cavity polaritons as tools which detect such defects and show how the scattering can be controlled by changing the exciton-photon detuning. In the case of aspherical individual lattice sites, we calculate the effective scattering potential as a function of the cavity photon polarization direction which exhibits a crossover from a repulsive into an attractive potential and should give a clearly observable signal. [ABSTRACT FROM AUTHOR]

This editorial provides a short overview of the focus issue on Bose condensation phenomena in atomic and solid state physics sponsored by the POLATOM ESF Research Networking Programme. [ABSTRACT FROM AUTHOR]

It remains challenging to preserve entanglement between distant solid-state qubits with high-fidelity, such as nitrogen vacancy centers (NVCs). We propose a Floquet engineering strategy to protect the maximal entanglement between two weakly interacting NVCs separated in long spatial distance by locally applying periodic strong driving on the NVCs. It is found that entanglement of the Floquet states of the NVCs resonantly reaches its maximum during the whole driving period at certain values of the driving parameters. Our analysis reveals that it is the occurrence of the avoided level crossing in the Floquet quasienergy spectrum which results in such entanglement resonance. The dissipation effect on the generated entanglement has also been analyzed. Our results may be of both theoretical and experimental interests in exploring the long-lasting entanglement between weakly interacting spins to long distances in realistic environments. [ABSTRACT FROM AUTHOR]

Control and utilization of coherent states of microwave photons is a ubiquitous requirement for the present and near-future implementations of solid-state quantum computers. The rate at which the photon state responds to external driving is limited by the relaxation rate of the storage resonator, which poses a trade-off between fast control and long storage time. Here, we present a control scheme that is designed to drive an unknown photon state to a desired coherent state much faster than the resonator decay rate. Our method utilizes a tunable environment which acts on an ancillary qubit coupled to the resonator. By periodically resetting the qubit and tuning it into resonance with the resonator, possible photon loss and dephasing of the resonator mode are corrected without measurements or active feedback. In general, our method is suitable for accelerating the control of coherent states in high-fidelity resonators. [ABSTRACT FROM AUTHOR]

We report on a source of heralded narrowband ( MHz) single photons compatible with solid-state spin-wave quantum memories based on praseodymium doped crystals. Widely non-degenerate narrow-band photon pairs are generated using cavity enhanced down conversion. One photon from the pair is at telecom wavelengths and serves as heralding signal, while the heralded single photon is at 606 nm, resonant with an optical transition of Pr3+:Y2SiO5. The source offers a heralding efficiency of 28% and a generation rate exceeding 2000 pairs mW−1 in a single-mode. The single photon nature of the heralded field is confirmed by a direct antibunching measurement, with a measured antibunching parameter down to 0.010(4). Moreover, we investigate in detail photon cross- and autocorrelation functions proving non-classical correlations between the two photons. The results presented in this paper offer prospects for the demonstration of single photon spin-wave storage in an on-demand solid state quantum memory, heralded by a telecom photon. [ABSTRACT FROM AUTHOR]

While it is often thought that the geometric phase is less sensitive to fluctuations in the control fields, a very general feature of adiabatic Hamiltonians is the unavoidable dynamic phase that accompanies the geometric phase. The effect of control field noise during adiabatic geometric quantum gate operations has not been probed experimentally, especially in the canonical spin qubit system that is of interest for quantum information. We present measurement of the Berry phase and carry out adiabatic geometric phase gate in a single solid-state spin qubit associated with the nitrogen-vacancy center in diamond. We manipulate the spin qubit geometrically by careful application of microwave radiation that creates an effective rotating magnetic field, and observe the resulting Berry phase signal via spin echo interferometry. Our results show that control field noise at frequencies higher than the spin echo clock frequency causes decay of the quantum phase, and degrades the fidelity of the geometric phase gate to the classical threshold after a few (∼10) operations. This occurs inspite of the geometric nature of the state preparation, due to unavoidable dynamic contributions. We have carried out systematic analysis and numerical simulations to study the effects of the control field noise and imperfect driving waveforms on the quantum phase gate. [ABSTRACT FROM AUTHOR]

The long term scaling prospects for solid-state quantum computing architectures relies heavily on the ability to simply and reliably measure and control the coherent electron interaction strength, known as the tunnel coupling, tc. Here, we describe a method to extract the tc between two quantum dots (QDs) utilising their different tunnel rates to a reservoir. We demonstrate the technique on a few donor triple QD tunnel coupled to a nearby single-electron transistor (SET) in silicon. The device was patterned using scanning tunneling microscopy-hydrogen lithography allowing for a direct measurement of the tunnel coupling for a given inter-dot distance. We extract and between each of the nearest-neighbour QDs which are separated by 14.5 nm and 14.0 nm, respectively. The technique allows for an accurate measurement of tc for nanoscale devices even when it is smaller than the electron temperature and is an ideal characterisation tool for multi-dot systems with a charge sensor. [ABSTRACT FROM AUTHOR]

The Bose–Hubbard model provides an excellent platform for exploring exotic quantum coherence. Interaction blockade is an important fundamental phenomenon in the two-site Bose–Hubbard system (BHS), which gives a full quantum description for the atomic Bose–Josephson junction. Using the analogy between the two-site BHS and the quadrupolar nuclear magnetic resonance (NMR) crystal, we experimentally simulate a two-site Bose–Hubbard system in a NMR quantum simulator composed of the quadrupolar spin-3/2 sodium nuclei of a NaNO3 single crystal, and observe the interesting phenomenon of interaction blockade via adiabatic dynamics control. To our best knowledge, this is the first experimental implementation of the quantum simulation of the interaction blockade using quadrupolar nuclear system. Our work exhibits important applications of quadrupolar NMR in the quantum information science, i.e. a spin-3/2 system can be used as a full 2-qubit su(4) system, if the quadrupole moment is not fully averaged out by fast tumbling in the liquid phase. [ABSTRACT FROM AUTHOR]

We introduce the works collected in the focus issue on Cavity and circuit quantum quantum electrodynamics in solids. [ABSTRACT FROM AUTHOR]

The effect of a transparent conductive oxide (TCO) buffer layer on the insulator matrix and on the resistive switching process in the metal/TiO2/TCO/metal assembly was studied depending on the material of the TCO (ITO-(In2O3)0.9(SnO2)0.1 or SnO2 or ZnO). For the first time electro-physical studies and near edge x-ray absorption fine structure (NEXAFS) studies were carried out jointly and at the same point of the sample, providing direct experimental evidence that the switching process strongly influences the lowest unoccupied bands and the local atomic structure of the TiO2 layers. It was established that a TCO layer in a metal/TiO2/TCO/metal assembly is an additional source of oxygen vacancies for the TiO2 film. The RL (RH) states are achieved presumably with the formation (rupture) of the electrically conductive path of oxygen vacancies. Inserting an Al2O3 thin layer between the TiO2 and TCO layers to some extent restricts the processes of migration of the oxygen ions and vacancies, and does not allow the anti-clockwise bipolar resistive switching in a Au/TiO2/Al2O3/ITO/Au assembly. The greatest value of the ratio RH/RL is observed for the assembly with a SnO2 buffer layer that will provide the maximum set of intermediate states (recording analog data) and increase the density of information recording in this case. [ABSTRACT FROM AUTHOR]

We report an interference experiment of spontaneous emission of light from two distant solid-state ensembles of atoms that are coherently excited by a short laser pulse. The ensembles are erbium ions doped into two LiNbO3 crystals with channel waveguides, which are placed in the two arms of a Mach-Zehnder interferometer. The light that is spontaneously emitted after the excitation pulse shows first-order interference. By a strong collective enhancement of the emission, the atoms behave as ideal two-level quantum systems and no which-path information is left in the atomic ensembles after emission of a photon. This results in a high fringe visibility of 95%, which implies that the observed spontaneous emission is highly coherent. [ABSTRACT FROM AUTHOR]