Ultracold Fermi Gases : The BEC-BCS Crossover

We give an introduction to the present state of research in the field of ultracold atomic Fermi gases. Of particular interest is the search for (BCS) superfluidity in these systems. The existence of Feshbach resonance have made these systems especially flexible, allowing to control at will the strength and the sign of the effective interaction. In particular it has allowed an experimental realization of the Bose-Einstein Condensation (BEC)-BCS crossover allowing to display on one side a Bose condensate of molecules and on the other side a BCS superfluid produced by Cooper pairs. Then we focus more specifically to the shift of the molecular threshold, produced by Pauli exclusion in these degenerate gas. We discuss also briefly collective modes corresponding to oscillations of the gas in the harmonic trap in which it is confined.      

Shell Effects in the First Sound Velocity of an Ultracold Fermi Gas

We investigate the first sound of a normal dilute and ultracold two-component Fermi gas in a cylindrical trap with harmonic radial confinement. We show that the velocity of the sound that propagates along the axial direction strongly varies in the dimensional crossover of the system. In particular, we predict that the first-sound velocity exhibits shell effects: by increasing the density, that is by inducing the crossover from one to three-dimensions, the first-sound velocity shows jumps in correspondence with the filling of harmonic modes. The experimental achievability of these effects is discussed by considering ⁴⁰K atoms.      

Quantum Degenerate Fermi Gases of Ytterbium Atoms

We performed evaporative cooling for dilute gases of ytterbium (Yb) isotopes in a crossed optical dipole trap and successfully cooled two fermionic and two bosonic species down to quantum degenerate regime, following the previous realization of Bose-Einstein condensation (BEC) in ¹⁷⁴Yb. The elastic collision rate of fermionic ¹⁷³Yb atoms with 6 spin components was found to be large enough to carry out efficient evaporation, which enables us to cool the atoms down to 0.6 T _F , where T _F is the Fermi temperature. In this regime, a plunge of evaporation efficiency was observed as an effect of the Fermi degeneracy. The other fermionic isotope ¹⁷¹Yb was cooled down to the temperature below T _F by sympathetic cooling with bosonic ¹⁷⁴Yb atoms. The sympathetic cooling technique has also been applied to ¹⁷⁴Yb-¹⁷⁶Yb Bose-Bose mixture. We have observed almost pure BEC of ¹⁷⁴Yb and the bimodal distribution of ¹⁷⁶Yb, showing the formation of BEC-BEC mixture. Moreover, we performed evaporative cooling of ¹⁷⁰Yb atoms and realized the BEC.     

Large n-Expansion Limit of the Three-Dimensional Ferromagnetic Quantum Phase Transition

We use the large n-expansion method to study the role of the long-range interaction, topological and dissipation effects for the case of an itinerant quantum ferromagnet in the limit of the Landau–Ginzburg–Wilson theory. In the one-loop approximation, we calculate the explicit form of the electronic self-energy as a result of electron–fluctuation interaction and extract the temperature dependence of the scattering time. The temperature dependence of the relative resistivity shows that both the dissipative and topological terms of the action determine the non-Fermi behavior of the system in the critical region around the quantum phase transition.      

Non-Fermi Behavior of the Itinerant-Electron Ferromagnet near the Quantum Phase Transition Point

We used the Renormalization Group (RG) method in the Hertz–Millis version to study the quantum phase transition of the itinerant-electron ferromagnet. Near the quantum phase transition point the system present a non-Fermi behavior in agreement with the experimental results. The importance of long-range interactions considered by Belitz–Kirkpatrick–Vojta was taken into consideration, showing the importance of the marginal parameters.     

Theory of Interacting Quantum Gases

We present a unified picture of the interaction effects in dilute atomic quantum gases. We consider fermionic as well as bosonic gases and, in particular, discuss for both forms of statistics the fundamental differences between a gas with effectively repulsive and a gas with effectively attractive inter atomic interactions, i.e., between a gas with either a positive or a negative scattering length.     

Kosterlitz-Thouless Transition in Finite-Size BEC Systems

The Kosterlitz-Thouless (KT) transition in two-dimensional BEC systems is calculated taking into account the fact that in experiments these are finite-size systems. The outer boundaries of the condensate effectively act as hard walls, and this has a polarizing effect on the vortex pairs of the KT transition, causing the superfluid fraction to become strongly anisotropic. The decreased pair energy near the walls results in a strongly enhanced vortex density near the boundaries. Since the pair density can now be directly measured, we extend here our previous calculations to include the vortex density as a function of the distance from the boundaries. Possible experiments using sound propagation in the gases are proposed to probe the anisotropic properties of the superfluid density, including an unusual sharp dip in the superfluid density that is predicted to occur down the middle of a long superfluid strip.      

Flow and Critical Velocity of an Imbalanced Fermi Gas through an Optical Potential

Optical lattices offer the possibility to investigate the superfluid properties of both Bose condensates and Fermionic superfluid gases. When a population imbalance is present in a Fermi mixture, this leads to frustration of the pairing, and the superfluid properties will be affected. Here, we investigate how imbalance will influence the flow of a Fermi superfluid through an optical lattice. The flow through the lattice is analysed by taking into account coupling between neighboring layers of the optical lattice up to second order in the interlayer tunneling amplitude for single atoms. We find that the critical velocity of flow through the lattice decreases monotonically to zero as the imbalance is increased to 100%. Closed-form analytical expressions are given for the tunneling contribution to the action and for the critical velocity as a function of the binding energy of pairs in the (quasi) two-dimensional Fermi superfluid and as a function of the imbalance. These results are obtained in the mean-field approximation which is known to provide only a qualitative picture near unitarity. Nevertheless this mean-field result should provide a useful benchmark for theories that take into account fluctuations beyond the saddle-point.      

Thermodynamic Quantities of Imbalanced 2D Fermi Gases Near the Berezinskii-Kosterlitz-Thouless Transition

We investigate the effects of imbalance on the two-dimensional Berezinskii-Kosterlitz-Thouless superfluid transition for a Fermi gas in a parabolic trap. Thermodynamic parameters of the Fermi gas are determined using the functional integral formalism in combination with a hydrodynamic action functional for the phase fluctuations.     

Non-Fermi Behavior Near Ferromagnetic Quantum Phase Transition

We propose a model for the explanation of the non-Fermi behavior near the ferromagnetic quantum phase transition. The scaling equations have been used to calculate the specific heat and we showed that the quantum effects are responsible for the T ln T term from the specific heat. The results are in agreement with recent experimental data obtained in the Ni _x Pd_{1 – x} system. The relation with the other approaches was also discussed.     

Quantifying Quasi‐Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal Dichalcogenides

One of the most fundamental parameters of any photovoltaic material is its quasi‐Fermi level splitting (?µ) under illumination. This quantity represents the maximum open‐circuit voltage (V_oc) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of V_oc that can possibly be achieved from monolayer WS₂, MoS₂, WSe₂, and MoSe₂‐based solar cells, and they are compared with state‐of‐the‐art perovskites. These results show that V_oc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS₂, MoS₂, WSe₂, and MoSe₂ monolayers at 1 Sun illumination, respectively. It is also observed that ?µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ?µ heterogeneity by electrically gating the TMD monolayers in a metal‐oxide‐semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ?µ heterogeneity. The values of ?µ determined from this work reveal the potential of atomically thin TMDs for high‐voltage, ultralight, flexible, and eye‐transparent future solar cells.     

Non-Fermi Behavior Near a Quantum Phase Transition Driven by Disorder

We showed that in a d-wave two-dimensional superconductor the disorder given by non-magnetic impurities at low temperature leads to a non-Fermi behavior for the normal state. The transition is similar to the superconductor–insulator transition in a model with a dissipative term.     

国内外气体市场概况

综述了近两年全球气体市场,着重论述了北美、欧洲、亚洲一些地区的气体市场。     

Photoinduced Phase Transition in Infinite-Layer Nickelates

The quantum phase transition caused by regulating the electronic correlation in strongly correlated quantum materials has been a research hotspot in condensed matter science. Herein, a photon-induced quantum phase transition from the Kondo-Mott insulating state to the low temperature metallic one accompanying with the magnetoresistance changing from negative to positive in the infinite-layer NdNiO₂ films is reported, where the antiferromagnetic coupling among the Ni¹⁺ localized spins and the Kondo effect are effectively suppressed by manipulating the correlation of Ni-3d and Nd-5d electrons under the photoirradiation. Moreover, the critical temperature T_c of the superconducting-like transition exhibits a dome-shaped evolution with the maximum up to ≈42 K, and the electrons dominate the transport process proved by the Hall effect measurements. These findings not only make the photoinduction a promising way to control the quantum phase transition by manipulating the electronic correlation in Mott-like insulators, but also shed some light on the possibility of the superconducting in electron-doped nickelates.     

Quantum Phase Transition in the Infinite Dimensional Hubbard Model

We investigated the ground state of the infinite dimensional Hubbard model for both attractive and repulsive interactions by applying Gutzwiller type variational wave functions. Our variational wave functions have lower energies than the simple BCS wave function for the attractive case, and lower energies than the Brinkman-Rice state for the repulsive case. We found that the system has several phases depending on the density of electrons and the interaction strength. Investigated phases include antiferromagnetic, Fermi liquid, superconducting, charge density wave, and supersolid phases. The last one is a coexistence phase of superconducting and charge density wave states.     

Quantum Phase Slips of Trapped Superfluid Bose Gases in One Dimension

We discuss transport of trapped one-dimensional superfluids through a single impurity potential, in connection with quantum phase-slip nucleation rate Γ. We specifically consider damping of dipole oscillations induced by sudden displacement of the trapping potential, which has been investigated in previous experiments. Applying the time-evolving block decimation method to the 1D Bose-Hubbard model with an impurity potential in the hardcore limit, we calculate the dynamics of dipole oscillations and extract the damping rate from the oscillations. We show that there is a broad parameter region in which the damping rate G of the oscillation obeys the formula G∝Γ/v∝v ^2K?2 with the Tomonaga-Luttinger parameter K, regardless of whether the impurity potential is repulsive or attractive. We find that in that parameter region the damping rate is almost symmetric with respect to the change of the sign of the impurity strength.     

Discretization of the Fermi surface in thin films

The thin film model in free-electron approximation is considered. A discrete Fermi surface is constructed depending on the film thickness and electron density. The Fermi wavevector, the mean energy per one electron and the density of states at the Fermi surface are calculated for the films with various thicknesses and different crystallographic orientation.     

Universal Properties at the Quantum Superconductor-to-Insulator Transition of Cuprates

We sketch the universal critical properties of a quantum superconductor to insulator transition in two spatial dimensions, using the scaling theory of quantum critical phenomena. Comparison with experimental data reveals that the resulting universal relations among transition temperature, zero temperature penetration depth, and residual resistivity, as well as for the asymptotic linear temperature dependence of the penetration depth, appear to apply in a rather extended doping regime, ranging from the underdoped limit up to the nearly optimum dopant concentration. This behavior uncovers the dominant role of quantum fluctuations in this doping regime.     

Critical Property of the Geometric Phase in the Dicke Model with the Dipole-dipole Interactions

We obtained the ground-state energy level and associated geometric phase in the Dicke model with the dipoledipole interactions analytically by the Holstein-Primakoff transformation and the boson expansion approach in the thermodynamic limit. The nonadiabatic geometric phase induced by the photon field was derived with the time-dependent unitary transformation. It is shown that dipole-dipole interactions have a deep influence on scaled behavior of the geometric phase at the critical point.     

二氧化钛催化剂晶型调控技术的研究进展

综述二氧化钛由锐钛矿向金红石晶型转变调控技术的最新成果,分析温度、氧化物和以及离子掺杂对实现晶型转变的影响规律,重点研究氧化物和离子掺杂对晶型转变的影响。结果显示:复合金属氧化物熔点低于TiO2熔点时,可促进锐钛矿型TiO2向金红石型转变;而金属氧化物熔点高于TiO2熔点时,可阻碍晶型转变;掺杂离子的离子半径、化合价、离子大小对二氧化钛晶型转变及催化性能有明显的影响,当掺杂金属离子半径大于或小于Ti4+半径,使得锐钛矿型TiO2更稳定;当掺杂离子的半径与Ti4+半径相近时,有利于锐钛矿型向金红石型转变,而体积较小的低价阴离子有利于金红石型二氧化钛的的生成,体积较大的高价阴离子则有利于锐钛矿型二氧化钛的的生成;阴阳离子共掺杂可以有效地调控二氧化钛晶型转变,并且能够提高TiO2的光催化活性。探讨二氧化钛多晶之间的协同作用,并基于二氧化钛掺杂改性的计算模拟,指出今后的发展方向。