arXiv subjects

You-Quan Li

Publications and source records attributed to You-Quan Li.

At least 19 recordsLinked to original sources

arXiv2024

Magnetic-field induced spiral order in the electric polarization

We present a phenomenological model for magnetoelectricity in multiferroic materials. The distinctive feature of the model is a two-component complex order parameter that encodes the electric polarization, along with a direct coupling between the polarization and magnetic field. Our model effectively elucidates that a sufficiently strong magnetic field can destroy electric polarization. Furthermore, the transition field strength diminishes with rising temperature, following a power-law relation with the exponent being precisely worked out. At lower field strength, the electric polarization takes a spiral order in the magnetic field, with the spiral wavelength inversely proportional to the magnetic field strength. We anticipate these predictions can be experimentally tested in future studies on multiferroic materials.

cond-mat.mtrl-sci↗

arXiv2021

Compass-free migratory navigation

How migratory birds can find the right way in navigating over thousand miles is an intriguing question, which much interested researchers in both fields of biology and physics for centuries. There several putative proposals that sound intuitively plausible all remain contested so far because those hypothesis-models of magnetoreceptor to sense geomagnetic field need either extremely high sensitivity or humankind-like intelligence to guide. Here we explore theoretically that the birds can navigate to their destination through an entirely new scenario to sense the geomagnetic field. Our proposal is based on separate peaks of the resonance-fluorescence spectrum of a four-level system derived from the ferric sulfide cluster which exists in a protein complex (Drosophila CG8198) of migratory birds. As the separation of spectral peaks contains information about geomagnetic field at both current location and birthland, the change of such separation cues the bird to choose a right direction to move and double-resonance emerges once arrived the destination. Our theoretical mechanism can explain previous experiments on the disorientation of migratory birds caused by oscillating magnetic field naturally and more precisely. This work provides insight to explain migratory navigation and motivates possible manmade practical devices.

physics.bio-ph↗

arXiv2019

Quantum Severalty: Speed-up and Suppress Effect in Searching Problem

The idea that the search efficiency can be increased with the help of a number of autonomous agents is often relevant in many situations, which is known among biologists and roboticists as a stigmergy. This is due to the fact that, in any probability-based search problem, adding information provides values for conditional prpbabilities. We report new findings of speed-up and suppression effects occuring in the quantum search problem through the study of quantum walk on a graph with floating vertices. This effect is a completely counterintuitive phenomenon in comparison to the classical counterpert, and may faciliate new insight in the future information search mechanisms that were never been perceived in classical picture. In order to understand the first passage probability, we also propose a method via ancillary model to bridge the measurement of the time dependence of the total probability of the complementary part and the first passage probability of the original model. This is expected to provide new ideas for quantum simulation by means of qubit chips.

quant-ph↗

arXiv2019

Electric-field control of collective spin excitations in Neel-type skyrmions

We demonstrate that an electric field could activate the three low-energy eigenmodes of the Neel-type skyrmion lattice via the electrically induced Dzyaloshinskii-Moriya interaction. In particular, we predict that the relative intensity of the clockwise rotation mode against the counter-clockwise rotation mode is significantly enhanced for the electrical activation in comparison with the magnetic activation. We also discover that the electrically and magnetically active modes obey unique selection rules. These findings promise a fresh pathway towards energy-efficient electrical manipulation of skyrmion excitations for future skyrmion-based magnonics.

cond-mat.str-el↗

arXiv2019

The role of coherence on two-particle quantum walks

We investigate the dynamical properties of the two-bosons quantum walk in system with different degrees of coherence, where the effect of the coherence on the two-bosons quantum walk can be naturally introduced. A general analytical expression of the two-bosons correlation function for both pure states and mixed states is given. We propose a possible two-photon quantum-walk scheme with a mixed initial state and find that the two-photon correlation function and the average distance between two photons can be influenced by either the initial photon distribution, or the relative phase, or the degree of coherence. The propagation features of our numerical results can be explained by our analytical two-photon correlation function.

quant-ph↗

arXiv2019

The switching effect of the side chain on quantum walks on triple graphs

We consider a continuous-time quantum walk on a triple graph and investigate the influence of the side chain on the propagation in the main chain. Calculating the interchange of the probabilities between the two parts of the main chain, we find that a switching effect appears if there are odd number of points on the side chain when concrete conditions between the length of the main chain and the position of the side chain are satisfied. Whereas, such an effect does not occur if there are even number of points on the side chain. We also suggest two proposals for experiment to observe such an effect, which may be employed to design new type of switching device.

quant-ph↗

arXiv2019

Quantum Intelligence on Protein Folding Pathways

We study the protein folding problem on the base of the quantum approach we proposed recently by considering the model of protein chain with nine amino-acid residues. We introduced the concept of distance space and its projections on a $XY$-plane, and two characteristic quantities, one is called compactness of protein structure and another is called probability ratio involving shortest path. Our results not only confirmed the fast quantum folding time but also unveiled the existence of quantum intelligence hidden behind in choosing protein folding pathways.

physics.bio-ph↗

arXiv2019

Quantum Approach to Fast Protein-Folding Time

In the traditional random-conformational-search model, various hypotheses with a series of meta-stable intermediate states were often proposed to resolve the Levinthal paradox. Here we introduce a quantum strategy to formulate protein folding as a quantum walk on a definite graph, which provides us a general framework without making hypotheses. Evaluating it by the mean of first passage time, we find that the folding time via our quantum approach is much shorter than the one obtained via classical random walks. This idea is expected to evoke more insights for future studies.

physics.bio-ph↗

arXiv2014

Spin-orbit coupling and electronic charge effects in Mott insulators

We derive the effective charge- and current-density operators for the strong-coupling limit of a single-band Mott insulator in the presence of spin-orbit coupling and show that the spin-orbit contribution to the effective charge density leads to novel mechanisms for multiferroic behavior. In some sense, these mechanisms are the electronic counterpart of the ionic-based mechanisms, which have been proposed for explaining the electric polarization induced by spiral spin orderings. The new electronic mechanisms are illustrated by considering cycloidal and proper screw magnetic orderings on sawtooth and kagome lattices. As for the isotropic case, geometric frustration is crucial for achieving this purely electronic coupling between spin and charge degrees of freedom.

cond-mat.str-el↗

arXiv2014

Electric field control of multiferroic domain wall motion

The dynamics of a multiferroic domain wall in which an electric field can couple to the magnetization via inhomogeneous magnetoelectric interaction is investigated by the collective-coordinate framework. We show how the electric field is capable of delaying the onset of the Walker breakdown of the domain wall motion, leading to a significant enhancement of the maximum wall velocity. Moreover, we show that in the stationary regime the chirality of the domain wall can be efficiently reversed when the electric field is applied along the direction of the magnetic field. These characteristics suggest that the multiferroic domain wall may provide a new prospective means to design faster and low-power-consumption domain wall devices.

cond-mat.mtrl-sci↗

arXiv2013

Theory of magneto-electric susceptibility in multiferroic chiral magnets

We present a theoretical examination of the magneto-electric response in the recently discovered multiferroic insulator Cu$_2$OSeO$_3$. Combining Monte Carlo simulation and Ginzburg-Landau analysis we predict the response in each of the magnetic phases, including helical, conical, ferromagnetic and Skyrmion crystal phases, both for thin film and bulk systems. A common feature for all non-collinear phases is that the magneto-electric susceptibility increases linearly with the applied magnetic field. Being both calculable and measurable, the magneto-electric susceptibility can serve as a new powerful probe to detect and investigate magnetic phases and phase transitions in multiferroic chiral magnets in general.

cond-mat.str-el↗

arXiv2013

Spin-Orbital Driven Ferroelectricity

Proposing a general formulism in terms of local coordinates representing the tilting of the ligands' octahedra, we evaluate the electric polarization in a chain of transition metal ions with unpolar octahedron rotation. We find the orbital ordering produced by the ligands's rotation and the spin order, together, determine the polarization features, manifesting that nonvannishing polarization appears in collinear spin order and the direction of polarization is no more restricted in the plane of spin rotation in cycloidal ordering.

cond-mat.str-el↗

arXiv2012

Quantum phase transition in an atom-molecule conversion system with atomic hopping

The quantum phase transition in an atom-molecule conversion system with atomic hopping between different hyperfine states is studied. In mean field approximation, we give the phase diagram whose phase boundary only depends on the atomic hopping strength and the atom-molecule energy detuning but not on the atomic interaction. Such a phase boundary is further confirmed by the fidelity of the ground state and the energy gap between the first-excited state and the ground one. In comparison to mean field approximation, we also study the quantum phase transition in full quantum method, where the phase boundary can be affected by the particle number of the system. Whereas, with the help of finite-size scaling behaviors of energy gap, fidelity susceptibility and the first-order derivative of entanglement entropy, we show that one can obtain the same phase boundary by the MFA and full quantum methods in the limit of $N\rightarrow \infty$. Additionally, our results show that the quantum phase transition can happens at the critical value of the atomic hopping strength even if the atom-molecule energy detuning is fixed on a certain value, which provides one a new way to control the quantum phase transition.

cond-mat.quant-gas↗

arXiv2012

Manipulation of atom-to-molecule conversion in a magnetic lattice

The atom-to-molecule conversion by the technique of optical Feshbach resonance in a magnetic lattice is studied in the mean-field approximation. For the case of shallow lattice, we give the dependence of the atom-to-molecule conversion efficiency on the tunnelling strength and the atomic interaction by taking a double-well as an example. We find that one can obtain a high atom-to-molecule conversion by tuning the tunnelling and interaction strengths of the system. For the case of deep lattice, we show that the existence of lattice can improve the atom-to-molecule conversion for certain initial states.

physics.atom-ph↗

arXiv2012

Skyrmion Dynamics in Multiferroic Insulator

Recent discovery of Skyrmion crystal phase in insulating multiferroic compound Cu$_2$OSeO$_3$ calls for new ways and ideas to manipulate the Skyrmions in the absence of spin transfer torque from the conduction electrons. It is shown here that the position-dependent electric field, pointed along the direction of the average induced dipole moment of the Skyrmion, can induce the Hall motion of Skyrmion with its velocity orthogonal to the field gradient. Finite Gilbert damping produces longitudinal motion. We find a rich variety of resonance modes excited by a.c. electric field.

cond-mat.str-el↗

arXiv2012

A mechanism to pin skyrmions in chiral magnets

We propose a mechanism to pin skyrmions in chiral magnets by introducing local maximum of magnetic exchange strength, which can be realized in chiral magnetic thin films by engineering the local density of itinerate electrons. Thus we find a way to artificially control the position of a single skyrmion in chiral magnetic thin films. The stationary properties and the dynamical pinning and depinning processes of an isolated skyrmion around a pinning center are studied. We do a series of simulations to show that the critical current to depin a skyrmion has linearly dependence on the pinning strength. We also estimate the critical current to have order of magnitude 10^{7}\sim10^{8}A/m^{2} .

cond-mat.str-el↗

arXiv2012

Bond distortion effects and electric orders in spiral multiferroic magnets

We study in this paper bond distortion effect on electric polarization in spiral multiferroic magnets based on cluster and chain models. The bond distortion break inversion symmetry and modify the $d$-$p$ hybridization. Consequently, it will affect electric polarization which can be divided into spin-current part and lattice-mediated part. The spin-current polarization can be written in terms of $\vec{e}_{i,j}\times(\vec{e}_{i}\times\vec{e}_{j}) $ and the lattice-mediated polarization exists only when the M-O-M bond is distorted. The electric polarization for three-atom M-O-M and four-atom M-O$_{2}$-M clusters is calculated. We also study possible electric ordering in three kinds of chains made of different clusters. We apply our theory to multiferroics cuprates and find that the results are in agreement with experimental observations.

cond-mat.str-el↗

arXiv2011

The role of inter-well tunneling strength on coherence dynamics of two-species Bose-Einstein condensates

Coherence dynamics of two-species Bose-Einstein condensates in double wells is investigated in mean field approximation. We show that the system can exhibit decoherence phenomena even without the condensate-environment coupling and the variation tendency of the degree of coherence depends on not only the parameters of the system but also the initial states. We also investigate the time evolution of the degree of coherence for a Rosen-Zener form of tunneling strength, and propose a method to get a condensate system with certain degree of coherence through a time-dependent tunneling strength.

cond-mat.quant-gas↗