Excitation Gap Research Articles

Parallel loop cluster quantum Monte Carlo simulation of quantum magnets based on global union-find graph algorithm

A large-scale parallel loop cluster quantum Monte Carlo simulation is presented. On 24,576 nodes of the K computer, one loop cluster Monte Carlo update of the world-line configuration of the S=1∕2 antiferromagnetic Heisenberg chain with 2.6×106 spins at inverse temperature 3.1×105 is executed in about 8.62 s, in which global union-find cluster identification on a graph of about 1.1 trillion vertices and edges is performed. By combining the nonlocal global updates and the large-scale parallelization, we have virtually achieved about 1013-fold speed-up from the conventional local update Monte Carlo simulation performed on a single core. We have estimated successfully the antiferromagnetic correlation length and the magnitude of the first excitation gap of the S=4 antiferromagnetic Heisenberg chain for the first time as ξ=1.040(7)×104 and Δ=7.99(5)×10−4, respectively.

Nanosecond discharge in atmospheric pressure air with ectonic introduction of iron and copper vapor in plasma and its application in nanotechnology

Introduction: Nanosecond discharges in high-pressure gases between metal electrodes are promising for applications in selective UV lamps. The most investigated working media of such lamps are copper and iron vapors in the air, which are excited by a high-voltage discharge of subnanosecond duration. The use of various metal electrodes in one discharge gap and discharge excitation pulses with an amplitude of 20–40 kV and a duration of 50–100 ns is little studied, which is important for controlling the spectral characteristics of the lamp and producing complex oxide nanostructures based on two different metals. Also important is the use of such discharges for the synthesis of nanostructures of transition metal oxides in assisting the deposition process with hard UV radiation, which allows to improve some of their characteristics. Purpose: The aim of the work is to study the electrical and emission characteristics of a bipolar overstressed nanosecond discharge between stainless steel and copper electrodes in atmospheric pressure and to synthesize and study the optical characteristics of thin films deposited on a glass substrate from the sputtered products of metal electrodes in an air plasma. Methods : We used an optical spectroscopic method for studying the discharge plasma. Results : The results of a study of the electrical characteristics of overstressed nanosecond discharge in air at atmospheric pressure between stainless steel electrodes and copper and stainless electrodes found that it is a source of UV radiation from atoms and iron and copper ions in the spectral range 200-300 nm. The peak value of the electric power of the discharge is ~ 4 MW, and the energy contribution to the plasma per pulse was ~ 0.1 J. The developed point source of UV radiation based on the discharge between the copper and stainless electrode is promising for applications in medicine and sanitation. It has been established that under the assistantance by UV-radiation the developed “point” source in the nanostructures of iron oxides, enlightenment bands appear in the visible region of the spectrum. Conclusion: Experiments revealed the suitability of this discharge for use in a selective UV lamp on iron and copper vapor and deposition of nanostructured films based on iron oxides while assisting the process of deposition by UV radiation.

Single-crystal investigation of the proposed type-II Weyl semimetal CeAlGe

We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in noncentrosymmetric I4$_1$md space group, in line with predictions of non-trivial topology. Magnetization, specific heat and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of $\Delta$ = 9.11 K from heat capacity and hole-like carrier density of 1.44 $\times$ 10$^{20}$ cm$^{-3}$ from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal $\it{ab}$-plane below 7 K. A spin-flop transition to less than 1 $\mu_B$/Ce is observed to occur below 30 kOe at 1.8 K in the $M(H)$ ($\bf{H}\|\bf{a}$) data. Small magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order when applied along the $\it{a}$- and $\it{c}$-axes, respectively, resulting in a complex $\it{T-H}$ phase diagram for $\bf{H}\|\bf{a}$ and a simpler one for $\bf{H}\|\bf{c}$.

Parity-mixing superconducting phase in the Rashba-Hubbard model and its topological properties from dynamical mean-field theory

We investigate parity-mixing superconductivity in the two-dimensional Hubbard model with Rashba spin-orbit coupling, using Cellular Dynamical Mean-Field Theory (CDMFT). A superconducting state with mixed singlet $d$-wave and triplet $p$-wave character is found in a wide range of doping. The singlet component decreases with the amplitude of the Rashba spin-orbit coupling, whereas the triplet component increases, but both disappear at about 20\% doping. The effect of a Zeeman field is also investigated; it tends to suppress both types of superconductivity, but induces nontrivial topological properties: the computed bulk Chern number is nonzero in the mixed superconductivity phase, at least in the underdoped region. A strong suppression of the excitation gap occurs slightly after optimal doping; this might be the sign of a topological transition within the superconducting dome.

Quantum anomalous Hall phase stabilized via realistic interactions on a kagome lattice

We study the quantum phases of spinless fermion at one-third filling on a Kagome lattice featuring a quadratic band touching Fermi point. In the presence of weak first and second nearest-neighbor repulsive interactions ($V_1$ and $V_2$), we demonstrate an interaction driven quantum anomalous Hall effect by employing exact diagonalization and density-matrix renormalization group methods. The time-reversal symmetry is broken spontaneously by forming loop currents that exhibit long-range correlation. Quantized Hall conductance corresponding to Chern number of $\pm1$ is obtained by measuring the pumped charge through inserting flux in a cylinder geometry. We find that the energy gap, which topologically protects the emerging ground states, can be enhanced remarkably by a moderate $V_2 < V_1$ via calculating the spectrum and charge excitation gaps, which highlights the experimentally feasible scheme of realizing interaction driven topological phase by spatially decaying interactions on topologically trivial lattice models.

Characterization of Majorana-Ising phase transition in a helical liquid system

Abstract We map an interacting helical liquid system, coupled to an external magnetic field and s-wave superconductor, to an XYZ spin system, and it undergoes Majorana-Ising transition by tuning of parameters. In the Majorana state lowest excitation gap decays exponentially with system size, and the system has degenerate ground state in the thermodynamic limit. On the contrary, the gap opens in the Ising phase even in the thermodynamic limit. We study various criteria to characterize the transition, such as edge spin correlation with its neighbor C ( r = 1 ) , local susceptibility χ i , superconducting order parameter of edge spin P ( r = 1 ) , and longitudinal structure factor S ( k ) . All these criteria lead to the same critical value of parameters for Majorana-Ising phase transition in the thermodynamic limit. We study the entanglement spectrum of the reduced density matrix of the helical liquid system. The system shows finite Schmidt gap and non-degeneracy of the entanglement spectrum in the Ising limit. The Schmidt gap closes in the Majorana state, and all the eigenvalues are either doubly or multiply degenerate.

Gapless magnetic excitation in a heavily electron-doped antiferromagnetic phase of LaFeAsO0.5D0.5

Magnetic excitations in a heavily electron-doped antiferromagnet, ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$, have been investigated using powder inelastic neutron scattering. Unlike other parent compounds of the iron-based superconductors, the magnetic excitation gap in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ was not detected down to the lowest measured temperature of 4 K. This result can be understood as a result of quasi-isotropy within the $ab$ plane, which is consistent with the band calculation result that the ${d}_{xy}$ orbital plays the dominant role in the magnetism of ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{H}}_{0.5}$. In addition, the intensities of the magnetic excitations in this phase are much stronger than those in nondoped LaFeAsO. Even in the paramagnetic phase, the magnetic excitation in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ persists. These results corroborate recent studies showing that the electron doping enhances the localized nature in this system.

Relevant crystal field interaction in the magnetically ordered state

Abstract Experimental data are discussed showing that in contrast to the paramagnetic phase, in the magnetically ordered state the action of the crystal electric field on the spin dynamics is quantized. In the Curie-Weiss regime of the paramagnetic susceptibility the spin dynamics is determined by local exchange interactions between individual pairs of spins and by single particle anisotropies (crystal field interaction). As we know from Renormalization Group (RG) theory, these local interactions are of no importance on the spin dynamics in the long range ordered state. On the other hand, a sufficiently strong crystal electric field is known to decrease the saturation magnetic moment for T → 0. In the critical paramagnetic range and for all lower temperatures the spin dynamics is controlled by a field of delocalized bosons instead by exchange interactions between spins. As we could show, the bosons are essentially magnetic dipole radiation emitted by the precessing spins. It is observed that the spontaneous generation of magnetic dipole radiation involves all N = 2S + 1 spin states, and is different in magnets with an integer and a half-integer spin. The dynamics of the boson field therefore is quantized and can be characterized by a limited number of universality classes. The effect of a relevant crystal field interaction is to reduce the number of thermodynamically relevant spin states per magnetic atom by ΔN = 1 or multiples thereof. This happens as discrete crossover events and reduces the saturation magnetic moment for T → 0 in discrete steps. The dynamics remains quantized. Each reduction by ΔN = 1 changes the universality class. Since a crossover is a threshold induced event, we have to distinguish between a relevant and a non-relevant crystal field interaction. Only a sufficiently strong crystal field interaction can become relevant. The crossover from S to Seff = S − 1/2 can occur in the critical paramagnetic range, and manifests as a functional change in the temperature dependence of either the longitudinal or the transverse susceptibility. A very particular observation is that in the insulating magnets a relevant crystal field interaction lets the magnetic heat capacity collapse to its absolute minimum. The magnetic entropy saturates at the lowest possible value of R·ln(2), irrespective of the value of Seff (R = gas constant). This does not mean that a crossover to atomistic Ising behavior has occurred. For the metallic magnets the action of a relevant crystal electric field is also to reduce the number of relevant spin states for T → 0 but the magnetic entropy saturates only gradually below the expected value of R·ln(2S + 1). Indications are discussed that each reduction of the spin by ΔS = 1/2 generates an additional energy band in the magnon excitation spectrum. In the metals the gap energy of the lowest magnon band is lower than in the insulators. Due to the lower excitation gap, spin dynamics and magnetic heat capacity are less suppressed in the metals compared to the insulators.

Insensitivity of bulk properties to the twisted boundary condition

The symmetry and the locality are the two major sources of various nontrivial statements in quantum many-body systems. We demonstrate that, in gapped phases of a U(1) symmetric Hamiltonian with finite-range interactions, the bulk properties such as the expectation value of local operators, the ground state energy and the excitation gap, and the static and low-frequency dynamical responses in general, do not depend on the U(1) phase of the twisted boundary condition in the limit of the large system size. Specifically, their dependence on the twisted angle is exponentially suppressed with the linear dimension of the system. The argument is based on the exponential decay of various types of equal-time correlation functions.

Topological Gap States of Rectangular Armchair Ribbon.

We consider a rectangular graphene armchair ribbon with an excitation gap. The boundary of this system consists of two short zigzag edges and two long armchair edges. Within such a ribbon, topological gap states exist that are localized along the zigzag edges. The end charge of these states is an integer, which can be related to the Zak phase of the periodic armchair ribbon constructed from the rectangular armchair ribbon by connecting its zigzag edges together. In this paper, we provide an explicit analytical computation of the Zak phase of a periodic armchair ribbon, and show that its value is consistent with the integer end charges that are computed numerically. In the presence of a staggered potential, non-integer end charges are possible. We discuss its relation to the Zak phase.

Accurate Spin-State Energetics for Aryl Carbenes.

A test set of 12 aryl carbenes (AC12) is compiled with the purpose of establishing their adiabatic singlet-triplet energy splittings using correlated wave function based methods. The set covers both singlet and triplet ground state aryl carbenes, as well as a range of magnitudes for the ground state to excited state gap. The performance of coupled cluster methods is examined with respect to the reference wave function, the basis set, and a number of additional methodological parameters that enter the calculation. Inclusion of perturbative triples and basis set extrapolation with a combination of triple and quadruple-ζ basis sets are both required to ensure high accuracy. When canonical coupled cluster calculations become too expensive, the domain-based local pair natural orbital approach DLPNO-CCSD(T) can be used as a reliable method for larger systems, as it achieves a mean absolute error of only 0.2 kcal/mol for the singlet-triplet gaps in the present test set. Other first-principles wave function methods and selected density functional methods are also evaluated. Second-order Møller-Plesset perturbation theory approaches are only applicable in conjunction with orbital optimization (OO-MP2). Among the representative density functional methods tested, only double hybrid functionals perform sufficiently accurately to be considered useful for systems with small singlet-triplet gaps. On the basis of the reference coupled cluster results, projected gas-phase free energies are reported for all aryl carbenes. Finally, the treatment of singlet-triplet gaps in solution is discussed in terms of both implicit and explicit solvation.

Strongly coupled bacteriochlorin dyad studied using phase-modulated fluorescence-detected two-dimensional electronic spectroscopy.

Fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) projects the third-order non-linear polarization in a system as an excited electronic state population which is incoherently detected as fluorescence. Multiple variants of F-2DES have been developed. Here, we report phase-modulated F-2DES measurements on a strongly coupled symmetric bacteriochlorin dyad, a relevant 'toy' model for photosynthetic energy and charge transfer. Coherence map analysis shows that the strongest frequency observed in the dyad is well-separated from the excited state electronic energy gap, and is consistent with a vibrational frequency readily observed in bacteriochlorin monomers. Kinetic rate maps show a picosecond relaxation timescale between the excited states of the dyad. To our knowledge this is the first demonstration of coherence and kinetic analysis using the phase-modulation approach to F-2DES.

Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap.

Defect States Control Effective Band Gap and Photochemistry of Graphene Quantum Dots.

Graphene quantum dots (GQDs) have emerged as a new group of quantum-confined semiconductors in recent years, with possible applications as light absorbers, luminescent labels, electrocatalysts, and photoelectrodes for photoelectrochemical water splitting. However, their semiconductor characteristics, such as the effective band gap, majority carrier type, and photochemistry, are obscured by defects in this material. Herein, we use surface photovoltage spectroscopy (SPS) in combination with photoelectrochemical measurements to determine the parameters that are essential to the use of GQDs as next-generation semiconductor devices and photocatalysts. Our results show that ordered GQDs (1-6 nm) behave as p-type semiconductors, based on the positive photovoltage in the SPS measurements on Al, Au, and fluorine-doped tin oxide substrates, and generate mobile charge carriers under excitation of defect states at 1.80 eV and under band gap excitation at 2.62 eV. Chemical reduction with hydrazine removes some defects and increases the effective band gap to 2.92 eV. SPS measurements in the presence of sacrificial electron donor and acceptors show that photochemical charge carriers can be extracted and promote redox reactions. A reduced GQDs photocathode supports an unprecedented photocurrent of 50 μA cm-2 using K3Fe(CN)6 as sacrificial electron acceptor. Additionally, while pristine GQDs do not photoreduce protons under visible light, hydrazine-treated GQDs generate H2 from aqueous methanol under visible and UV light (0.04% quantum efficiency at 375 nm) without added co-catalysts. These findings are relevant to the use of GQDs in photochemical and photovoltaic energy-conversion systems.

Experimental investigation of the magnetorheological polishing process with roller

PurposeThis paper aims to develop a new magnetorheological polishing (MRP) device with roller and investigate the polishing mechanism of MRP fluids using this new device.Design/methodology/approachThe principle of MRP process with roller is discussed, and then the structure of the polishing device is designed in detail. The polishing experiments of K9 glass are carried out using MRP device with roller.FindingsA series of tests are performed to evaluate the effect of the excitation gaps, working gap and polishing time on the polishing characteristics, and the optimized polishing parameters can be obtained. The surface roughness Ra of the sample is reduced from 359 to 38 nm under optimized polishing parameters.Originality/valueMRP method with roller is proposed in this paper, and the MRP device with roller can reduce the surface roughness Ra significantly. After being polished, the circular ring-shaped polishing belt disappears on the workpiece surface, which means it has great potential for polishing.

Geometrically induced broadening for phonon blocking at low temperatures

Because of the presence of robust massless modes with no excitation gap, the heat current carried by phonons tends to survive at low temperatures even when scattering mechanisms are incorporated into the system. This becomes a fundamental obstacle to thermoelectric applications at low temperatures. In this study, we investigate the effect of energy broadening on phonon transport in mesoscopic systems coupled to leads or probes in various geometries using a nonequilibrium Green's function formalism. An analytic theory derived from a minimal model consisting of a harmonic chain shows that geometrically induced broadening sizably suppresses low-temperature phonon transport. It is also demonstrated from numerical calculations that this scheme for phonon blocking is viable for realistic systems in higher dimensions.

Reverse Saturable Absorption Induced by Phonon-Assisted Anti-Stokes Processes.

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (Elaser ) is higher than the band gap (EB ). Here, it is shown that indirect gap semiconductor WSe2 exhibits high RSA on exposure to a femtosecond laser under Elaser > EB near band gap excitation, which is attributed to the longitudinal optical phonon-assisted anti-Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm-2 ) lower than those reported for other optical-limiting materials currently for femtosecond laser at 800 nm is observed.

Accurate Treatment of Charge-Transfer Excitations and Thermally Activated Delayed Fluorescence Using the Particle-Particle Random Phase Approximation.

Thermally activated delayed florescence (TADF) is a mechanism that increases the electroluminescence efficiency in organic light-emitting diodes by harnessing both singlet and triplet excitons. TADF is facilitated by a small energy difference between the first singlet (S1) and triplet (T1) excited states (Δ E(ST)), which is minimized by spatial separation of the donor and acceptor moieties. The resultant charge-transfer (CT) excited states are difficult to model using time-dependent density functional theory (TDDFT) because of the delocalization error present in standard density functional approximations to the exchange-correlation energy. In this work we explore the application of the particle-particle random phase approximation (pp-RPA) for the determination of both S1 and T1 excitation energies. We demonstrate that the accuracy of the pp-RPA is functional dependent and that, when combined with the hybrid functional B3LYP, the pp-RPA computed Δ E(ST) have a mean absolute deviation (MAD) of 0.12 eV for the set of examined molecules. A key advantage of the pp-RPA approach is that the S1 and T1 states are characterized as CT states for all of experimentally reported TADF molecules examined here, which allows for an estimate of the singlet-triplet CT excited state energy gap (Δ E(ST) = 1CT - 3CT). For experimentally known TADF molecules with a small (<0.2 eV) Δ E(ST) in this data set, a high accuracy is demonstrated for the prediction of both the S1 (MAD = 0.18 eV) and T1 (MAD = 0.20 eV) excitation energies as well as Δ E(ST) (MAD = 0.05 eV). This result is attributed to the consideration of correct antisymmetry in the particle-particle interaction leading to the use of full exchange kernel in addition to the Coulomb contribution, as well as a consistent treatment of both singlet and triplet excited states. The computational efficiency of this approach is similar to that of TDDFT, and the cost can be reduced significantly by using the active-space approach.

Microstrip Center-Fed Bi-plane Printed Dipole Antenna Design and the Excitation Gap Contribution for Impedance Control

Source-antenna transition modeling differential feed of microstrip center-feed bi-plane printed dipole antenna is presented. Design considerations are focused on the feed mode of the studied antenna at its center by the means of ideal plane parallel-plate microstrip transmission line that supports a quasi-TEM mode of propagation. Numerical experiments of the excitation gap effects on the antenna peak resonant resistance were carried out. The calculations of the antenna input impedance using voltage and current wave formalism and the radiated field pattern using retarded potential approach for a given current distribution are performed. It is concluded that bi-plane printed dipole antenna exhibits excellent directional performances such as good selectivity, HPBW of 60° and large band-width of 22%.

Erratum to: Non-hydrodynamic collective modes in liquid metals and alloys

A misprint has been found in paper [1] in expression for the propagation gap for transverse collective excitations, equation (24). The power index “2” at the shear viscosity was missing. The correct expression reads: