Effects Of Quantum Fluctuations Research Articles

Emergent Potts order in the kagome J1−J3 Heisenberg model

Motivated by the physical properties of Vesignieite BaCu$_3$V$_2$O$_8$(OH)$_2$, we study the $J_1-J_3$ Heisenberg model on the kagom\'e lattice, that is proposed to describe this compound for $J_1<0$ and $J_3\gg|J_1|$. The nature of the classical ground state and the possible phase transitions are investigated through analytical calculations and parallel tempering Monte Carlo simulations. For $J_1<0$ and $J_3>\frac{1+\sqrt{5}}4|J_1|$, the ground states are not all related by an Hamiltonian symmetry. Order appears at low temperature via the order by disorder mechanism, favoring colinear configurations and leading to an emergent $q=4$ Potts parameter. This gives rise to a finite temperature phase transition. Effect of quantum fluctuations are studied through linear spin wave approximation and high temperature expansions of the $S=1/2$ model. For $J_3$ between $\frac14|J_1|$ and $\frac{1+\sqrt{5}}4|J_1|$, the ground state goes through a succession of semi-spiral states, possibly giving rise to multiple phase transitions at low temperatures.

Rotor Jackiw-Rebbi Model: A Cold-Atom Approach to Chiral Symmetry Restoration and Charge Confinement

Understanding the nature of confinement, as well as its relation with the spontaneous breaking of chiral symmetry, remains one of the long-standing questions in high-energy physics. The difficulty of this task stems from the limitations of current analytical and numerical techniques to address non-perturbative phenomena in non-Abelian gauge theories. In this work, we show how similar phenomena emerge in simpler models, and how these can be further investigated using state-of-the-art cold-atom quantum simulators. More specifically, we introduce the rotor Jackiw-Rebbi model, a (1+1)-dimensional quantum field theory where interactions between Dirac fermions are mediated by quantum rotors. Starting from a mixture of ultracold atoms in an optical lattice, we show how this quantum field theory emerges in the long-wavelength limit. For a wide and experimentally-relevant parameter regime, the Dirac fermions acquire a dynamical mass via the spontaneous breakdown of chiral symmetry. Moreover, we study the effect of both quantum and thermal fluctuations, which lead to the phenomenon of chiral symmetry restoration. Finally, we uncover a confinement-deconfinement quantum phase transition, where meson-like fermions fractionalise into quark-like quasi-particles bound to topological solitons of the rotor field. The proliferation of these solitons at finite chemical potentials again serves to restore the chiral symmetry, yielding a clear analogy with the quark-gluon plasma in quantum chromodynamics, where this symmetry coexists with the deconfined fractional charges. Our results show how the interplay between these phenomena could be analyse in realistic atomic experiments.

Thermodynamics of dilute Bose gases: Beyond mean-field theory for binary mixtures of Bose-Einstein condensates

We study the thermodynamic properties of binary Bose mixtures, by developing a beyond mean-field Popov theory which properly includes the effects of quantum and thermal fluctuations in both the density and spin channels. Results for key thermodynamic quantities, such as the isothermal compressibility and the magnetic susceptibility, are derived from a perturbative calculation of the grand-canonical potential. We find that thermal fluctuations can play a crucial role on the miscibility condition of a binary mixture, favoring phase separation at finite temperature even if the mixture is soluble at zero temperature, as already anticipated in a previous work [Ota et al., Phys. Rev. Lett. 123, 075301 (2019)]. We further investigate the miscibility condition for binary mixtures in the presence of asymmetry in the intra-species interactions, as well as in the masses of the two components. Furthermore, we discuss the superfluid behavior of the mixture and the temperature dependence of the Andreev-Bashkin effect.

Strong-coupling effects of pairing fluctuations, and Anderson-Bogoliubov mode in neutron S01 superfluids in neutron stars

We investigate effects of thermal and quantum fluctuations of the superfluid order parameter in $^1S_{0}$ superfluids in neutron stars. We construct a separable potential to reproduce the $^1S_{0}$ phase shift reconstructed by using the partial wave analysis from nucleon scattering data. We include superfluid fluctuations within a strong-coupling approximation developed by Nozi\`eres and Schmitt-Rink and determine self-consistently the superfluid order parameter as well as the chemical potential. We show that the quantum depletion, which gives a fraction of noncondensed neutrons at zero temperature due to quantum pairing fluctuations, plays an important role not only near the critical temperature from superfluid states to normal states but also at zero temperature. We derive the dispersion relation of Anderson-Bogoliubov modes associated with phase fluctuations, and show also that there is a nonzero fraction of noncondensed components in the neutron number as a result of the strong-coupling effect. Our results indicate that superfluid fluctuations are important for thermodynamic properties in neutron stars.

Hierarchy of energy scales and field-tunable order by disorder in dipolar-octupolar pyrochlores

Dipolar-octupolar pyrochlore magnets in a strong external magnet field applied in the [110] direction are known to form a `chain' state, with subextensive degeneracy. Magnetic moments are correlated along one-dimensional chains carrying effective Ising degrees of freedom which are noninteracting on the mean-field level. Here, we investigate this phenomenon in detail, including the effects of quantum fluctuations. We identify two distinct types of chain phases, both featuring distinct subextensive, classical ground state degeneracy. Focussing on one of the two kinds, we discuss lifting of the classical degeneracy by quantum fluctuations. We map out the ground-state phase diagram as a function of the exchange couplings, using linear spin wave theory and real-space perturbation theory. We find a hierarchy of energy scales in the ground state selection, with the effective dimensionality of the system varying in an intricate way as the hierarchy is descended. We derive an effective two-dimensional anisotropic triangular lattice Ising model with only three free parameters which accounts for the observed behavior. Connecting our results to experiment, they are consistent with the observation of a disordered chain state in Nd$_2$Zr$_2$O$_7$. We also show that the presence of two distinct types of chain phases has consequences for the field-induced breakdown of the apparent $U(1)$ octupolar quantum liquid phase recently observed in Ce$_2$Sn$_2$O$_7$.

Quantum corrections to the accretion onto a Schwarzschild black hole in the background of quintessence

It is well known that quantum effects may lead to removal of the intrinsic singularity point of back holes. Also, the quintessence scalar field is a candidate model for describing late-time acceleration expansion. Accordingly, Kazakov and Solodukhin considered the existence of back-reaction of the spacetime due to the quantum fluctuations of the background metric to deform a Schwarzschild black hole, which led to a change of the intrinsic singularity of the black hole to a 2-sphere with a radius of the order of the Planck length. Also, Kiselev rewrote the Schwarzschild metric by taking into account the quintessence field in the background. In this study, we consider the quantum-corrected Schwarzschild black hole inspired by Kazakov–Solodukhin’s work, and the Schwarzschild black hole surrounded by quintessence deduced by Kiselev to study the mutual effects of quantum fluctuations and quintessence on the accretion onto the black hole. Consequently, the radial component of the 4-velocity and the proper energy density of the accreting fluid have a finite value on the surface of its central 2-sphere due to the presence of quantum corrections. Also, by comparing the accretion parameters in different kinds of black holes, we infer that the presence of a point-like electric charge in the spacetime is somewhat similar to some quantum fluctuations in the background metric.

Ultracold Bose mixtures with spin-dependent fermion-mediated interactions

We develop a functional integral formulation for binary Bose-Einstein condensates coupled to polarized fermions. We find that spin-dependent fermion-mediated interactions have dramatic effects on the properties of the binary condensates. The quasiparticle spectrum features two branches. The upper branch, which is of density nature, gets modified by the induced interactions, while the lower branch, which is of spin nature, is left intact. The ground-state phase diagram consists of stable region of miscible phases and unstable region toward phase separation. In the stable region, it is further classified by the damping of excitations of the upper branch. We show that it is possible to find region of well-defined, long-lived quasiparticle excitations by tuning relevant parameters, such as boson-fermion mass ratio, boson-fermion number density ratio, and interspecies interactions between bosons as well. We explore the effects of quantum fluctuation due to the effective potential on the binary condensates. It turns out that both the density structure factor and spin density structure factor fulfill the Feynman relation, except that the latter is immune to the fermion-mediated interactions.

Upper critical field and superconductor-metal transition in ultrathin niobium films

Recent studies suggest that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Here we investigate the nature of SC state in ultrathin Nb films, of thickness d ranging from 1.2 to 20 nm, which undergo a transition from amorphous to polycrystalline structure at the thickness d simeq 3.3 nm. We show that the properties of SC state are very different in polycrystalline and amorphous films. The upper critical field (H_{c2}) is orbitally limited in the first case, and paramagnetically limited in the latter. The magnetic field induced superconductor-metal transition is observed, with the critical field approximately constant or decreasing as a power-law with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity exists above H_{c2}, particularly pronounced in amorphous films, signaling the presence of fluctuating Cooper pairs. These observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximity-coupled SC islands. An usual evolution of SC correlations on cooling is observed in amorphous films, likely related to the effect of quantum fluctuations on the proximity-induced phase coherence.

Emergent PT symmetry in a double-quantum-dot circuit QED setup

Open classical and quantum systems with effective parity-time ($\mathcal{PT}$) symmetry, over the past five years, have shown tremendous promise for advances in lasers, sensing, and non-reciprocal devices. And yet, how such effective $\mathcal{PT}$-symmetric non-Hermitian models emerge out of Hermitian quantum mechanics is not well understood. Here, starting from a fully Hermitian microscopic Hamiltonian description, we show that a non-Hermitian Hamiltonian emerges naturally in a double-quantum-dot-circuit-QED (DQD-circuit QED) set-up, which can be controllably tuned to the $\mathcal{PT}$-symmetric point. This effective Hamiltonian governs the dynamics of two coupled circuit-QED cavities with a voltage-biased DQD in one of them. Our analysis also reveals the effect of quantum fluctuations on the $\mathcal{PT}$ symmetric system. The $\mathcal{PT}$-transition is, then, observed both in the dynamics of cavity observables as well as via an input-output experiment. As a simple application of the $\mathcal{PT}$-transition in this set-up, we show that loss-induced enhancement of amplification and lasing can be observed in the coupled cavities. By comparing our results with two conventional local Lindblad equations, we demonstrate the utility and limitations of the latter. Our results pave the way for an on-chip realization of a potentially scalable non-Hermitian system with a gain medium in quantum regime, as well as its potential applications for quantum technology.

Barrow black hole corrected-entropy model and Tsallis nonextensivity

The quantum scenario concerning Hawking radiation, gives us a precious clue that a black hole has its temperature directly connected to its area gravity and that its entropy is proportional to the horizon area. These results have shown that there exist a deep association between thermodynamics and gravity. The recently introduced Barrow formulation of back holes entropy, influenced by the spacetime geometry, shows the quantum fluctuations effects through Barrow exponent, Δ, where Δ=0 represents the usual spacetime and its maximum value, Δ=1, characterizes a fractal spacetime. The quantum fluctuations are responsible for such fractality. Loop quantum gravity approach provided the logarithmic corrections to the entropy. This correction arises from quantum and thermal equilibrium fluctuations. In this paper we have analyzed the nonextensive thermodynamical effects of the quantum fluctuations upon the geometry of a Barrow black hole. We discussed the Tsallis' formulation of this logarithmically corrected Barrow entropy to construct the equipartition law. Besides, we obtained a master equation that provides the equipartition law for any value of the Tsallis q-parameter and we analyzed several different scenarios. After that, the heat capacity were calculated and the thermal stability analysis was carried out as a function of the main parameters, namely, one of the so-called pre-factors, q and Δ.

Avoided quasiparticle decay and enhanced excitation continuum in the spin- 12 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9

We explore the magnetic excitations of the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal high-resolution inelastic neutron scattering. Sharp magnons with no decay are observed throughout reciprocal space, with a strongly renormalized dispersion and multiple soft modes compared to linear spin wave theory. We propose an empirical parametrization that can quantitatively capture the complete dispersions in the three-dimensional Brillouin zone and explicitly show that the dispersion renormalizations have the direct consequence that one to two magnon decays are avoided throughout reciprocal space, whereas such decays would be allowed for the unrenormalized dispersions. At higher energies, we observe a very strong continuum of excitations with highly-structured intensity modulations extending up at least 4x the maximum one-magnon energy. The one-magnon intensities decrease much faster upon increasing energy than predicted by linear spin wave theory and the higher-energy continuum contains much more intensity than can be accounted for by a two-magnon cross-section, suggesting a significant transfer of spectral weight from the high-energy magnons into the higher-energy continuum states. We attribute the strong dispersion renormalizations and substantial transfer of spectral weight to continuum states to the effect of quantum fluctuations and interactions beyond the spin wave approximation, and make connections to theoretical approaches that might capture such effects. Finally, through measurements in a strong applied magnetic field, we find evidence for magnetic domains with opposite senses for the spin rotation in the 120 degree ordered ground state, as expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense of spin rotation is selected via spontaneous symmetry breaking.

Variational theory for the ground state and collective excitations of an elongated dipolar condensate

We develop a variational theory for a dipolar condensate in an elongated (cigar shaped) confinement potential. Our formulation provides an effective one-dimensional extended meanfield theory for the ground state and its collective excitations. We apply our theory to investigate the properties of rotons in the system comparing the variational treatment to a full numerical solution. We consider the effect of quantum fluctuations on the scattering length at which the roton excitation softens to zero energy.

Bosonic impurity in a one-dimensional quantum droplet in the Bose–Bose mixture

We study an impurity immersed in the mixture of Bose ultracold gases in the regime where a quantum droplet (QD) exists. The quasi-one-dimensional geometry is considered. We find an effective attractive potential, which acts by the QD onto the impurity. The bound states of the impurity in this potential are investigated. These impurity-bound states can provide potential probes for the presence of quantum fluctuation effects on the droplet properties. In the case of strong impurity–Bose–Einstein condensate coupling we study the properties of the nonlinear local modes on the impurity induced by the quantum fluctuations.

First-order corrected thermodynamic potentials characterizing BTZ black hole in massive gravity

In this paper, we explore the effect of quantum fluctuations on the thermodynamic potentials, characterizing the (2[Formula: see text]+[Formula: see text]1)-dimensional AdS black hole with the negative cosmological constant in massive gravity. The (2[Formula: see text]+[Formula: see text]1)-dimensional black hole solution was discovered by three pioneer scientists, Banados, Teitelboim, and Zanelli in 1992 and hence is named as BTZ black hole.[Formula: see text] First, we present the brief idea of the BTZ black hole solution in massive gravity and then estimate the leading order corrections to thermodynamic potentials characterizing BTZ black hole in massive gravity. The qualitative analysis is done by plotting the corrected thermodynamic potentials against the event horizon radius for distinct values of the correction parameter. The correction parameter [Formula: see text] measures the strength of quantum fluctuations.

Tricritical Physics in Two-Dimensional p-Wave Superfluids.

We study effects of quantum fluctuations on two-dimensional p+ip superfluids near resonance. In the standard paradigm, phase transitions between superfluids and zero density vacuum are continuous. When strong quantum fluctuations near resonance are present, the line of continuous phase transitions terminates at two tricritical points near resonance, between which the transitions are expected to be first-order ones. The size of the window where first-order phase transitions occur is shown to be substantial when the coupling is strong. Near first-order transitions, superfluids self-contract due to phase separations between superfluids and vacuum.

Low-Lying Collective Modes of a One-Dimensional Bose Gas with Quantum Fluctuation Effect

We theoretically investigate the collective excitations of a one-dimensional Bose–Einstein condensate with both interparticle interaction and quantum fluctuation effect in an anharmonic trap. We derive the coupled equations of motions for the center-of-mass coordinate of the condensate and its width by using variational approaches. We find a reentrant behavior of the breathing mode and demonstrate that the anharmonic distortion may give rise to the collapse and revival of the collective excitations.

Quantum fluctuation of entanglement for accelerated two-level detectors**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402600), the National Natural Science Foundation of China (Grant Nos. 11690023, 11373014, and 11633001), the Beijing Talents Fund of Organization Department of Beijing Municipal Committee of the CPC, and the Strategic Priority Research Program of the Chinese Academy of

It is well known that the quantum fluctuation of entanglement (QFE) between Unruh–De Witt detector (modeled by a two-level atom) is always investigated in a relativistic setting. However, both of the Unruh radiation and quantum fluctuation effects play an important role in precise measurements of quantum entanglement. In this paper, we have quantitatively analyzed how the relativistic motion affects the QFE for two entangled Unruh–De Witt detectors, one of which is accelerated and interacting with the neighbor external scalar field. Our results show that the QFE, which initially increases by the Unruh thermal noise, will suddenly decay when the acceleration reaches to a considerably large value. Therefore, the relativistic effect will lead to non-negligible QFE effect. We also find that the initial QFE (without acceleration effect) reaches its minimum value at the maximally entangled state and the separable state. More importantly, our analysis demonstrates that although the QFE has a huge decay when the acceleration is greater than ∼0.96, the ratio of ΔE/C is still very large, due to the simultaneous decay of concurrence to a very low value. Finally, enlightened by the well-known equivalence principle, we discuss the possibility of applying the above findings to the dynamics of QFE under the influence of gravitation field.

Quantum fluctuation and interference effect in a single atom–cavity QED system driven by a broadband squeezed vacuum

Squeezed vacuum, as a nonclassical field, has many interesting properties and results in many potential applications for quantum measurement and information processing. Here, we investigate a single atom–cavity quantum electrodynamics (QED) system driven by a broadband squeezed vacuum. In the presence of the atom, we show that both the mean photon number and the quantum fluctuations of photons in the cavity undergo a significant depletion due to the additional transition pathways generated by the atom–cavity interaction. By measuring these features, one can detect the existence of atoms in the cavity. We also show that two-photon excitation can be significantly suppressed by the quantum destructive interference when the squeezing parameter is very small. These results presented here are helpful in understanding the quantum nature of the broadband squeezed vacuum.

Large excitonic effect on van der Waals interaction between two-dimensional semiconductors.

An exceptionally large excitonic effect on the van der Waals (vdW) interaction between two-dimensional semiconductors is unraveled using the Lifshitz theory in conjunction with the ab initio GW plus Bethe-Salpeter equation formalism. Upon consideration of the electron-hole interaction, the vdW energy between two atomistic layers separated by 10 000 angstroms can be larger by a ratio of ∼30%, which is an order of magnitude greater than that seen for semi-infinite silicon surfaces. The large influence of the short-range electron-hole interaction on the long-range effect of quantum fluctuations is rooted in the ultra-thin nature of two-dimensional semiconductors which results in not only large exciton binding energy but also amplified roles of low-frequency dielectric responses.

Weak deflection angle of extended uncertainty principle black holes * * Supported by Comisión Nacional de Ciencias y Tecnología of Chile through FONDECYT (3170035) (A. Ö.)

We discuss the effects of quantum fluctuations spewed by a black hole on its deflection angle. The Gauss-Bonnet theorem (GBT) is exploited with quantum corrections through the extended uncertainty principle (EUP), and the corresponding deflection angle is obtained. Moreover, we have attempted to broaden the scope of our work by subsuming the effects of plasma medium on the deflection angle. To demonstrate the degree of difference, the acquired results are compared with the prevailing findings.