Crossover Line Research Articles

Observation of critical scaling in spin glasses below Tc using thermoremanent magnetization

Time-dependent thermoremanent magnetization (TRM) studies have been instrumental in probing energy dynamics within the spin glass phase. In this paper, we review the evolution of the TRM experiment over the last half century and discuss some aspects related to how it has been used in the understanding of spin glasses. We also report on recent experiments using high-resolution DC SQUID magnetometry to probe the TRM at temperatures less than but near to the transition temperature Tc. These experiments have been performed as a function of waiting time, temperature, and five different magnetic fields. We find that as the transition temperature is approached from below, the characteristic time scale of TRM is suppressed up to several orders of magnitude in time. In the highest-temperature region, we find that the waiting time effect subsides, and a waiting time-independent crossover line is reached. We also find that increasing the magnetic field further suppresses the crossover line. Using a first-principles energy argument across the crossover line, we derive an equation that is an excellent fit to the crossover lines for all magnetic fields probed. The data show strong evidence for critical slowing down and an H = 0 Oe phase transition.

Improving the Functional Reliability of an Urban Public Transport Line

In this study we consider correlated and simultaneous interventions regarding: i—the physical infrastructure (by crossover lines between the two tracks of a tram line), ii—the characteristics of the trams (by bi-directional trams), as well as iii—tactical and operative decisions of the line manager. How these interventions are reflected in the functional reliability of the tram line service is demonstrated for both cases of the current operation and for the case of overloads, respectively, for the case of the temporary degradation of circulation caused by random disruptive events. The theoretical analysis, generalizing findings regarding the effectiveness of solutions to improve functional reliability, is supplemented with quantitative evaluations related to certain situations of disruptions. The proposed solutions aim to increase the attractiveness of urban public transport. Even if the evaluations are focused mainly on the interests of the service quality perceived by the user, the beneficial consequences for the line manager (in terms of technical and commercial efficiency) are also addressed.

Quantum scaling for the metal–insulator transition in a two-dimensional electron system

The quantum phase transition observed experimentally in two-dimensional (2D) electron systems has been a subject of theoretical and experimental studies for almost 30 years. We suggest Gaussian approximation to the mean-field theory of the second-order phase transition to explain the experimental data. Our approach explains self-consistently the universal value of the critical exponent 3/2 (found after scaling measured resistivities on both sides of the transition as a function of temperature) as the result of the divergence of the correlation length when the electron density approaches the critical value. We also provide numerical evidence for the stretched exponential temperature dependence of the metallic phase’s resistivities in a wide range of temperatures and show that it leads to correct qualitative results. Finally, we interpret the phase diagram on the density-temperature plane exhibiting the quantum critical point, quantum critical trajectory and two crossover lines. Our research presents a theoretical description of the seminal experimental results.

Chiral condensate and the equation of state at nonzero baryon density from the hadron resonance gas model with a repulsive mean field

We study the QCD equation of state and the chiral condensate using the hadron resonance gas model with repulsive mean-field interactions. We find that the repulsive interactions improve the agreement with the lattice results on the derivatives of the pressure with respect to the baryon chemical potential up to eighth order. From the temperature dependence of the chiral condensate we estimate the crossover temperature as a function of baryon chemical potential, Tpc(μB). We find that the chiral crossover line starts to deviate significantly from the chemical freeze-out line already for μB>400MeV. Furthermore, we find that the chiral pseudocritical line can be parametrized as Tpc(μB)/Tpc(0)=1−κ2[μB/Tpc(0)]2−κ4[μB/Tpc(0)]4 with κ2=0.0150(2) and κ4=3.1(6)×10−5, which are in agreement with lattice QCD results for small values of μB. For the first time we find a tiny but nonzero value of κ4 in our study. Published by the American Physical Society 2024

Thermodynamic crossovers in supercritical fluids

Can liquid-like and gas-like states be distinguished beyond the critical point, where the liquid-gas phase transition no longer exists and conventionally only a single supercritical fluid phase is defined? Recent experiments and simulations report strong evidence of dynamical crossovers above the critical temperature and pressure. Despite using different criteria, many existing theoretical explanations consider a single crossover line separating liquid-like and gas-like states in the supercritical fluid phase. We argue that such a single-line scenario is inconsistent with the supercritical behavior of the Ising model, which has two crossover lines due to its symmetry, violating the universality principle of critical phenomena. To reconcile the inconsistency, we define two thermodynamic crossover lines in supercritical fluids as boundaries of liquid-like, indistinguishable, and gas-like states. Near the critical point, the two crossover lines follow critical scalings with exponents of the Ising universality class, supported by calculations of theoretical models and analyses of experimental data from the standard database. The upper line agrees with crossovers independently estimated from the inelastic X-ray scattering data of supercritical argon, and from the small-angle neutron scattering data of supercritical carbon dioxide. The lower line is verified by the equation of states for the compressibility factor. This work provides a fundamental framework for understanding supercritical physics in general phase transitions.

Broadened quantum critical ground state in a disordered superconducting thin film.

A superconductor-insulator transition (SIT) in two dimensions is a prototypical quantum phase transition (QPT) with a clear quantum critical point (QCP) at zero temperature (T = 0). The SIT is induced by a field B and observed in disordered thin films. In some of weakly disordered or crystalline thin films, however, an anomalous metallic (AM) ground state emerges over a wide B range between the superconducting and insulating phases. It remains a fundamental open question how the QPT picture of the SIT is modified when the AM state appears. Here we present measurements of the Nernst effect N, which has great sensitivity to the fluctuations of the superconducting order parameter. From a thorough contour map of N in the B-T plane, we found a thermal-to-quantum crossover line of the superconducting fluctuations, a so-called ghost-temperature line associated with the QPT, as well as a ghost-field line associated with a thermal transition. The QCP is identified as a T = 0 intercept of the ghost-temperature line inside the AM state, which verifies that the AM state is a broadened critical state of the SIT.

Observation of the possible magnetic correction above the Curie temperature in Cr2Si2Te6 single crystals.

Intrinsic magnetic semiconductors hold great promise in the fields of fundamental magnetization and spintronics. One such semiconductor is Cr2Si2Ti6 (CST), a quasi two-dimensional (2D) magnetic semiconductor with potential applications in future magnetic devices. However, the origin of ferromagnetism in CST remains a mystery. To investigate this, ac/dc susceptibility and electronic spin resonance (ESR) measurements were conducted. Based on ac susceptibility scaling, the critical temperature (TC) for the ferromagnetic (FM) to paramagnetic (PM) phase transition was found to be ∼32.5 K, with a critical exponent of δ = 6.7 from the critical isotherm, β + γ = 1.72 from the temperature dependence of the crossover line, and γ = 1.43 from the temperature dependence of susceptibility along the same line. All critical exponents were found to be consistent with the dc magnetization scaling method. However, above and below TC, the origin of magnetism cannot be explained by a single theory. To explore the origin of abnormal magnetic critical behavior, ESR measurements were performed. Below T* ∼ 130 K, the ESR measurements revealed that the resonance field width (ΔH) tends to increase and decrease for the applied magnetic field H parallel and perpendicular to the c axis, respectively, indicating the onset of magnetic interaction even in the PM state. Meanwhile, the deviation from Curie-Weiss behavior below T* also confirmed the occurrence of magnetic correlation above the TC in CST. These observations suggest that the competition and cooperation among the direct and indirect interactions, the structural distortion and the van der Waals interaction at high temperature should be considered to investigate the origin of anomalous magnetism in CST. The present results provide valuable insights into the nature of ferromagnetism in 2D magnetic semiconductors.

Variational quantum simulation of the quantum critical regime

The quantum critical regime marks a zone in the phase diagram where quantum fluctuation around the critical point plays a significant role at finite temperatures. While it is of great physical interest, simulation of the quantum critical regime can be difficult on a classical computer due to its intrinsic complexity. Herein, we propose a variational approach, which minimizes the variational free energy, to simulate and locate the quantum critical regime on a quantum computer. The variational quantum algorithm adopts an ansatz by performing an unitary operator on a product of a single-qubit mixed state, in which the entropy can be analytically obtained from the initial state, and thus the free energy can be accessed conveniently. With numeral simulation, using the one-dimensional Kitaev model as a demonstration we show that the quantum critical regime can be identified by accurately evaluating the temperature crossover line. Moreover, the dependencies of both the correlation length and the phase coherence time with temperature are evaluated for the thermal states. Our work suggests a practical way as well as a first step for investigating quantum critical systems at finite temperatures on quantum devices with few qubits.

Saturated absorption spectrum of cesium micrometric-thin cell with suppressed crossover spectral lines

Micrometric-thin cells (MCs) with alkali vapor atoms have been valuable for research and applications of hyperfine Zeeman splitting and atomic magnetometers under strong magnetic fields. We theoretically and experimentally study the saturated absorption spectra using a 100-μm cesium MC, where the pump and probe beams are linearly polarized with mutually perpendicular polarizations, and the magnetic field is along the pump beam. Because of the distinctive thin chamber of the MC, crossover spectral lines in saturated absorption spectra are largely suppressed leading to clear splittings of hyperfine Zeeman transitions in experiments, and the effect of spatial magnetic field gradient is expected to be reduced. A calculation method is proposed to achieve good agreements between theoretical calculations and experimental results. This method successfully explains the suppression of crossover lines in MCs, as well as the effects of magnetic field direction, propagation and polarization directions of the pump/probe beam on saturated absorption spectrum. The saturated absorption spectrum with suppressed crossover lines is used for laser frequency stabilization, which may provide the potential value of MCs for high spatial resolution strong-field magnetometry with high sensitivity.

Phase behavior and percolation in an equilibrium system of symmetrically interacting Janus disks on the triangular lattice

A Janus particle has two distinct areas on its surface. Denoting the two areas as P (patch) and N (nonpatch), when two particles come close to each other, the strength of the PP interaction is usually different from that of the NN interaction. Recently the interplay between a rotational-symmetry-breaking continuous phase transition and percolation has been explored for an equilibrium system of asymmetrically interacting (i.e., attractive PP interaction, zero NN and PN interactions) Janus disks on the triangular lattice. By Monte Carlo simulation and finite-size scaling analysis, in this work we study an equilibrium system of symmetrically interacting (i.e., attractive PP and NN interactions with the same strength, zero PN interaction) Janus disks on the same lattice. By definition, the phase diagram in the T–θ plane is symmetric for systems with patch sizes θ below and above 90°. We determine the phase diagram and compare it with that of the asymmetric system. Similar to the latter system, for 60° < θ < 90°, a rotational-symmetry-breaking continuous phase transition and an anisotropic percolation transition are found in the symmetric system, though the transition points in the two systems are quite different. Phase crossover curves are found to be different, e.g., a continuous varying crossover line extends between θ = 0° and 90° for the symmetric model; and in the range 0° < θ ≤ 30°, along the crossover lines of the two models, the trends of 1/T vs. θ are opposite in the two systems. We understand the latter by analytically solving the models with two particles in 0° < θ ≤ 30°. These results are helpful for understanding close-packed systems of Janus disks with more complex interactions.

Shift of the superconducting dome induced by a conduction band of small occupancy: Enhanced d -wave pairing in the overdoped regime

Motivated by the presence of an extremely small electron pocket in the electronic band structure of infinite-layer nickelate superconductors, we systematically explore the superconducting (SC) properties of a two-dimensional Hubbard model influenced by an additional conduction band with small occupancy (dry Fermi sea). Our dynamic cluster quantum Monte Carlo calculations demonstrated the unusual impact of this additional dry metallic layer, which shifts the SC dome of the correlated Hubbard layer towards the overdoped regime, namely the $d$-wave SC can be enhanced (suppressed) in the overdoped (underdoped) regime. Simultaneously, the pseudogap crossover line shifts to the same higher doping regime as well. Our analysis revealed the interplay of the effective pairing interaction, pair-field susceptibility, and antiferromagnetic spin fluctuation in the underlying nonmonotonic physics. We postulate on the physical picture of the SC dome shift in terms of the Kondo-type electron-hole binding in the case of a dry additional conduction band. Our investigation provided valuable insights on tuning the SC properties via external dry bands.

Frustration in a generalized kagomé Ising antiferromagnet: Exact results.

We obtain the exact ground-state phase diagram of a generalized kagomé antiferromagnet with both pair and triplet interactions, J_{2} and J_{3}, respectively, in the presence of a magnetic field h appropriately tuned. We find that when the pair interaction J_{2}<0 dominates, the ground state is geometrically frustrated; on the other hand, the ground state is disordered but not frustrated when the triplet interaction J_{3} dominates, the boundaries between the two cases being at J_{3}=±J_{2}. The exact ground-state crossover lines between the two distinct types of disorder remain identifiable crossover curves at finite temperatures. In the frustrated domain, the ground state of the three-parameter model is identical to the ground state of the prototype one-parameter (J_{2}<0) model of geometrical frustration. Towards further understanding the frustration domain of the three-parameter model, a closed-form approximation (exact at zero temperature) determines solutions on a two-parameter subspace for induced magnetization and parallel magnetic susceptibility at finite fields h and temperatures T, the inverse susceptibility showing a Curie-Weiss behavior. We argue that the existence of an exact T=0 threshold magnetic field, below which the magnetization remains zero, indicates the existence of a gapped spectrum attributable to the presence of the triplet interaction J_{3}.

Gas–liquid crossover in the Lennard-Jones system

It is demonstrated that the crossover between gas- and liquid-like regions on the phase diagram of the Lennard-Jones system occurs at a fixed value of the density divided by its value at the freezing point, ρ/ρfr ≃ 0.35. This definition is consistent with other definitions proposed recently. As a result, a very simple practical expression for the gas-to-liquid crossover line emerges.

A 4 × 4 Beamforming Network for Spatial Diversity and Multiplexing Reconfiguration

A simple 4 × 4 beamforming network with spatial diversity and multiplexing reconfiguration is proposed in this paper. By analyzing and decomposing the simplified transmission matrix of the network, it is found that the 4 × 4 butler matrix can achieve spatial diversity and multiplexing reconfiguration by switching the operating modes of the four-port devices. Therefore, by designing a reconfigurable device with three modes among hybrid coupler, crossover, and transmission line, a simple 4 × 4 reconfigurable beamforming network is achieved. The proposed network can achieve orthogonal spatial diversity responses with equal amplitude and progressive phase distribution responses at the four output ports (State 1) and hybrid spatial diversity and multiplexing responses (State 2) with equal amplitude and progressive phase distribution responses at the two adjacent output ports. For the experimental validation, a particular version of the reconfigurable device and network which work at 4.9 GHz, have been designed, fabricated, and tested. Good results indicate the proposed network can be widely used in wireless communication networks for MIMO applications.

Collinear factorization of diphoton photoproduction at next to leading order

We calculate in the framework of collinear factorization the amplitude for the photoproduction of a near forward large mass diphoton at leading twist and next to leading order (NLO) in $\alpha_s$. We demonstrate the validity of factorization at this order, which was never achieved for such a reaction where the coefficient function describes a $2 \to 3$ hard process. While the Born order amplitude was purely imaginary and only probed the $x=\pm \xi$ cross-over line of generalized parton distributions (GPD) domain, the NLO result contains both a real and an imaginary part and probes the whole domain of definition of quark GPDs. The phenomenology of our results for medium (JLab) and higher energy (EIC) experiments will be developed in a future study.

Reliability Study of Important Cross-Over Transmission Line System Based on Series System

Transmission lines inevitably cross railroads, highways, and other facilities, and in order to ensure the safety and reliability of the cross-crossing section, the influence of various random factors on important cross-crossing transmission lines needs to be fully considered. In this paper, the transmission line crossing section is treated as a tandem system according to the force transmission route, and the reliability calculation method of the cross-crossing transmission line system based on the tandem system is proposed. The reliability calculation method of each component and the reliability calculation method of the tandem system are given. Finally, an example of the reliability calculation of a 220kV cross-crossing transmission line system is given, and the results show that the cross-crossing section has the highest reliability of tower FSJ404, with a reliability index of 8.61, the second highest reliability of insulators, with a reliability index of 7.22, and the lowest reliability of tower FSJ302, with a reliability index of 4.28. The failure probability of the cross-crossing section is 0.000009245, and the reliability index is 4.28.

Corrections to the hadron resonance gas from lattice QCD and their effect on fluctuation-ratios at finite density

The hadron resonance gas (HRG) model is often believed to correctly describe the confined phase of QCD. This assumption is the basis of many phenomenological works on QCD thermodynamics and of the analysis of hadron yields in relativistic heavy ion collisions. We use first-principle lattice simulations to calculate corrections to the ideal HRG. Namely, we determine the sub-leading fugacity expansion coefficients of the grand canonical free energy, receiving contributions from processes like kaon-kaon or baryon-baryon scattering. We achieve this goal by performing a two dimensional scan on the imaginary baryon number chemical potential ($\mu_B$) - strangeness chemical potential ($\mu_S$) plane, where the fugacity expansion coefficients become Fourier coefficients. We carry out a continuum limit estimation of these coefficients by performing lattice simulations with temporal extents of $N_\tau=8,10,12$ using the 4stout-improved staggered action. We then use the truncated fugacity expansion to extrapolate ratios of baryon number and strangeness fluctuations and correlations to finite chemical potentials. Evaluating the fugacity expansion along the crossover line, we reproduce the trend seen in the experimental data on net-proton fluctuations by the STAR collaboration.

Locating the critical endpoint of QCD: Mesonic backcoupling effects

We study the effects of pion and sigma meson backcoupling on the chiral order parameters and the QCD phase diagram and determine their effect on the location of the chiral critical endpoint. To this end, we solve a coupled set of truncated Dyson-Schwinger equations for Landau gauge quark and gluon propagators with ${N}_{\mathrm{f}}=2+1$ dynamical quark flavors and explicitly backcoupled mesons. The corresponding meson bound-state properties and the quark-meson Bethe-Salpeter vertices are obtained from their homogeneous Bethe-Salpeter equation. We find chiral-restoration effects of the pion and/or sigma meson backcoupling and observe a (small) shift of the critical endpoint towards smaller chemical potentials. The curvature of the chiral crossover line decreases. Our results indicate that the location of the critical endpoint in the phase diagram is mainly determined by the microscopic degrees of freedom of QCD (in contrast to its critical properties).

Structural properties of additive binary hard-sphere mixtures. II. Asymptotic behavior and structural crossovers.

The structural properties of additive binary hard-sphere mixtures are addressed as a follow-up of a previous paper [S. Pieprzyk et al., Phys. Rev. E 101, 012117 (2020)]2470-004510.1103/PhysRevE.101.012117. The so-called rational-function approximation method and an approach combining accurate molecular dynamics simulation data, the pole structure representation of the total correlation functions, and the Ornstein-Zernike equation are considered. The density, composition, and size-ratio dependencies of the leading poles of the Fourier transforms of the total correlation functions h_{ij}(r) of such mixtures are presented, those poles accounting for the asymptotic decay of h_{ij}(r) for large r. Structural crossovers, in which the asymptotic wavelength of the oscillations of the total correlation functions changes discontinuously, are investigated. The behavior of the structural crossover lines as the size ratio and densities of the two species are changed is also discussed.

A novel design of compact broadband 4 × 4 Butler matrix-based beamforming antenna array for C-Band applications

In this paper, a compact broadband 4 × 4 Butler matrix (BM) as a feeding network for antenna array is presented and studied. The broadband performance of the BM occurs when it operates in the almost equal difference of phases and coupling throughout the operating band. The different components of the proposed design such as branch-line coupler, crossover, and Schiffman line phase shifter are designed and optimized to be low loss and wideband. The total size of the BM is 37 mm × 47 mm, equal to 0.64λ0 × 0.81λ0 in design frequency at 5.2 GHz. The measurement bandwidth for fabricated structure on the FR4 substrate is 24 % achieved from return loss lower than −10 dB. The beam directions are measured at −15, −43.2, +43, and +15° at 5.2 GHz for each input port 1–4 excitation respectively with the gain value of 10.2 ± 0.2 dB and sidelobe levels of −8.2 ± 1.2 dB. The Final BM exposures phase errors of 3 ± 1.7° with the amplitude imbalance of less than ±1 dB over the operational band.