Quasiparticle Branches Research Articles

Functional renormalisation group approach to the finite-temperature Bose polaron

The functional renormalisation group (FRG) approach is employed to study Bose polarons at finite temperatures in the regime of strong attractive bath-impurity interactions. Both two- and three-dimensional configurations are considered. The appearance of two polaron quasiparticle branches at finite temperatures is revealed, consistent with recent findings by other analytical techniques. Ground-state polaron energies are also reported for selected interactions and temperatures within the gas superfluid phase. The findings of this work present the FRG as a useful tool for studying finite-temperature polarons in quantum gases.

Polariton-polariton interaction induced by the hopping effect between two monolayers

Investigating the quantum many-body interaction in microcavity semiconductor systems is a significant objective in current research. Hence, in this study, Green’s function was employed to investigate the hopping effect between two monolayers denoted as L and R. The interaction between excitons in L monolayer and R monolayer, characterized by spin-like properties, leads to the generation of Josephson currents and emergence of two quasiparticle branches, known as LP+ and LP− polaritons (i.e., two low polariton states). And compared with other papers, such as reference 5, the research shows that presence of hopping interaction and many-body interaction between monolayers causes significant enhancement phenomenon of residue Z, which will further reduce the effective mass. This is more conducive to Bose–Einstein condensates (BEC). Furthermore, we investigated the Parity-Time symmetry (PT symmetry) of the system. The PT broken phase transition point will appear when the condition of PT symmetry is satisfied.

Bose Polaron in a One-Dimensional Lattice with Power-Law Hopping

Polarons, quasiparticles resulting from the interaction between an impurity and the collective excitations of a medium, play a fundamental role in physics, mainly because they represent an essential building block for understanding more complex many-body phenomena. In this manuscript, we study the spectral properties of a single impurity mixed with identical bosons in a one-dimensional lattice with power-law hopping. In particular, based on the so-called T-matrix approximation, we show the existence of well-defined quasiparticle branches for several tunneling ranges and for both repulsive and attractive impurity-boson interactions. Furthermore, we demonstrate the persistence of the attractive polaron branch when the impurity-boson bound state is absorbed into the two-body continuum and that the attractive polaron becomes more robust as the range of the hopping increases. The results discussed here are relevant for the understanding of the equilibrium properties of quantum systems with power-law interactions.

Effect of fermion indistinguishability on optical absorption of doped two-dimensional semiconductors

We study the optical absorption spectrum of a doped two-dimensional semiconductor in the spin-valley polarized limit. In this configuration, the carriers in the Fermi sea are indistinguishable from one of the two carriers forming the exciton. Most notably, this indistinguishability requires the three-body trion state to have p-wave symmetry. To explore the consequences of this, we evaluate the system's optical properties within a polaron description, which can interpolate from the low density limit -- where the relevant excitations are few-body bound states -- to higher density many-body states. In the parameter regime where the trion is bound, we demonstrate that the spectrum is characterized by an attractive quasiparticle branch, a repulsive branch, and a many-body continuum, and we evaluate the doping dependence of the corresponding energies and spectral weights. In particular, at low doping we find that the oscillator strength of the attractive branch scales with the square of the Fermi energy as a result of the trion's p-wave symmetry. Upon increasing density, we find that both the repulsive and attractive branches blueshift, and that the orbital character of the states associated with these branches interchanges. We compare our results with previous investigations of the scenario where the Fermi sea involves carriers distinguishable from those in the exciton, for which the trion ground state is s-wave.

Mobile ion in a Fermi sea

The remarkable single particle control of individual ions combined with the versatility of ultracold atomic gases makes hybrid ion-atom system an exciting new platform for quantum simulation of few- and many-body quantum physics. Here, we study theoretically the properties of a mobile ion immersed in a quantum degenerate gas of fermionic atoms. Using an effective low-energy atom-ion interaction together with a well established approach that includes exactly two-body correlations, we calculate the full spectral response of the ion and demonstrate the existence of several quasiparticle branches, which are charged analogues of the Fermi polaron observed in neutral atomic gases. Due to the long-range nature of the atom-ion interaction, these ionic Fermi polarons have several properties distinct from their neutral counterparts such as the simultaneous presence of several stable states and smooth transitions from repulsive to attractive polarons with increasing interaction strength. Surprisingly, the residue of the ionic polaron is shown to increase with the Fermi density for fixed interaction strength, which is in marked contrast to the neutral polaron. The properties of the ionic polaron approach that of the neutral polaron only in the low density limit where the average interparticle spacing is larger than the characteristic length of the atom-ion interaction. We finally analyse the effects of the Fermi gas on the molecular ions, which are bound atom-dimer states.

Beyond Mean-Field Corrections to the Quasiparticle Spectrum of Superfluid Fermi Gases.

We investigate the fermionic quasiparticle branch of superfluid Fermi gases in the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover and calculate the quasiparticle lifetime and energy shift due to its coupling with the collective mode. The only close-to-resonance process that low-energy quasiparticles can undergo at zero temperature is the emission of a bosonic excitation from the phononic branch. Close to the minimum of the branch we find that the quasiparticles remain undamped, allowing us to compute corrections to experimentally relevant quantities such as the energy gap, location of the minimum, effective mass, and Landau critical velocity.

Fate of the Bose polaron at finite temperature

We consider an impurity immersed in a Bose-Einstein condensate with tunable boson-impurity interactions. Such a Bose polaron has recently been predicted to exhibit an intriguing energy spectrum at finite temperature, where the ground-state quasiparticle evenly splits into two branches as the temperature is increased from zero [Guenther et al., Phys. Rev. Lett. 120, 050405 (2018)]. To investigate this theoretical prediction, we employ a recently developed variational approach that systematically includes multibody correlations between the impurity and the finite-temperature medium, thus allowing us to go beyond previous finite-temperature methods. Crucially, we find that the number of quasiparticle branches is simply set by the number of hole excitations of the thermal cloud, such that including up to one hole yields one splitting, two holes yields two splittings, and so on. Moreover, this effect is independent of the impurity mass. We thus expect that the exact ground-state quasiparticle will evolve into a single broad peak for temperatures $T>0$, with a broadening that scales as ${T}^{3/4}$ at low temperatures and sufficiently weak boson-boson interactions. In the zero-temperature limit, we show that our calculated ground-state polaron energy is in excellent agreement with recent quantum Monte Carlo results and with experiments.

Phase diagrams and Hofstadter butterflies in the strongly correlated bosonic systems on the lattices with Dirac points

Gauge potentials with different configurations have been recently realized in the optical lattice experiments. It is remarkable that one of the simplest gauge potential can generate particle energy spectrum with the self-similar structure known as a Hofstadter butterfly. We investigate theoretically the impact of strong on-site interaction on such a spectrum in the bosonic Mott insulator within Bose–Hubbard model. In particular, it is shown that the fractal structure is encoded in the quasi-particle and hole bosonic branches for different lattice backgrounds. For example a square lattice and other structures (brick-wall and staggered magnetic flux lattice) which contain Dirac points in energy dispersions are considered. This shows that single-particle physics is still present even in the strong interaction limit for whole Hofstadter spectrum. Additionally we observe, that although in brick-wall and staggered flux lattices the quasi-particle densities of states look qualitatively similar, the corresponding Hofstadter butterfly assumes different forms. In particular, we use a superposition of two different synthetic gauge fields which appears to be a generator of non-trivial phenomena in the optical lattice systems. We also discuss the consequences of these phenomena on the phase diagrams between bosonic Mott insulator and superfluid phase. The analysis is carried out within the strong coupling expansion method on the finite size lattices and also at finite temperatures which are relevant for the currently made experiments.

Experimental Evaluation of Self-Heating Effect in Intrinsic Josephson Junctions Using Pulse Current Measurements

Intrinsic Josephson junctions (IJJs) in layered high-critical-temperature superconductors such as single-crystal Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212), which comprises several atomically stacked Josephson junctions, have been intensively studied, and it is known that the current-voltage (I-V ) characteristics of IJJs are affected by self-heating. Therefore, evaluating the self-heating effect in the IJJs is important. Pulse current measurement techniques are commonly employed to reduce the self-heating effect. However, in mesa-type IJJ samples, self-heating occurs not only owing to the applied voltage in voltage-state IJJs, but also owing to the contact resistance between the electrode and the top of the mesa. It has been found in previous studies, for small samples with good electrode contacts, that an electrode on the top of the mesa can help to cool the mesa. However, the effect of heating owing to the contact resistance has not been well studied in large samples. In this paper, the I-V and self-heating characteristics of mesa-type IJJs were experimentally investigated for devices wherein self-heating was suppressed using pulse current measurements. Furthermore, to evaluate the contact resistance effect on the I-V characteristics, we proposed a new pulse waveform, because when using the conventional pulse measurement, the voltage values of the quasiparticle branch were much larger than those obtained via the dc measurement. We measured the I-V characteristics using this waveform to determine the effect of heat generation that occurs due to the contact resistance between the electrode and the top of the mesa. We found that in this case, the voltage was considerably suppressed.

Photon-activated electron hopping in a single-electron trap enhanced by Josephson radiation

Using a Josephson junction interferometer (DC SQUID) as a microwave source for irradiating a single-electron trap, both devices fabricated on the same chip, we study the process of photon-assisted tunneling as an effective mechanism of single photon detection. High sensitivity down to a very small oscillation amplitude vJ∼10 nV≪Eact≲hfJ and down to low photon absorption rates Γph ∼ (1–50) Hz, as well as a clear threshold type of operation with an activation energy Eact ∼ 400 μeV, is demonstrated for the trap with respect to the microwave photons of frequency fJ ∼ (100–200) GHz. Tunable generation is demonstrated with respect to the power and frequency of the microwave signal produced by the SQUID source biased within the subgap voltage range. A much weaker effect is observed at the higher junction voltages along the quasiparticle branch of the I–V curve; this response mostly appears due to the recombination phonons.

Fine structure of spectra in the antiferromagnetic phase of the Kondo lattice model

We study the antiferromagnetic phase of the Kondo lattice model on bipartite lattices at half-filling using the dynamical mean-field theory with numerical renormalization group as the impurity solver, focusing on the detailed structure of the spectral function, self-energy, and optical conductivity. We discuss the deviations from the simple hybridization picture, which adequately describes the overall band structure of the system (four quasiparticle branches in the reduced Brillouin zone), but neglects all effects of the inelastic-scattering processes. These lead to additional structure inside the bands, in particular asymmetric resonances or dips that become more pronounced in the strong-coupling regime close to the antiferromagnet-paramagnetic Kondo insulator quantum phase transition. These features, which we name "spin resonances", appear generically in all models where the $f$-orbital electrons are itinerant (large Fermi surface) and there is N\'eel antiferromagnetic order (staggered magnetization), such as periodic Anderson model and Kondo lattice model with antiferromagneitc Kondo coupling, but are absent in antiferromagnetic phases with localized $f$-orbital electrons (small Fermi surface), such as the Kondo lattice model with ferromagnetic Kondo coupling. We show that with increasing temperature and external magnetic-field the spin resonances become suppressed at the same time as the staggered magnetization is reduced. Optical conductivity $\sigma(\Omega)$ has a threshold associated with the indirect gap, followed by a plateau of low conductivity and the main peak associated with the direct gap, while the spin resonances are reflected as a secondary peak or a hump close to the main optical peak. This work demonstrates the utility of high-spectral-resolution impurity solvers to study the dynamical properties of strongly correlated fermion systems.

Size Dependence of Terahertz Electromagnetic Wave Radiation from Intrinsic Josephson Junctions

We report the experimental results of in situ detection of terahertz radiation from large stacks with a few hundreds of intrinsic Josephson junctions (IJJs) using a high sensitivity detector composed of a small IJJ mesa. The dimensions of the fabricated IJJ oscillators are 290 μm in length, 50, 70, and 90 μm in width and between 255 and 600 nm in height. The current-voltage characteristics of the oscillators are strongly influenced by self-heating and show negative resistance. We have successfully observed terahertz radiation from stacks with 170-400 IJJs. We find that the strong emission occurs when the oscillator is biased in the negative resistance region on the outermost quasi-particle branch. The characteristic velocity estimated from the emission voltage is consistent with that of the in-phase mode. This implies that the observed emission is related to the in-phase cavity resonance. Furthermore, we find that for a smaller number of junctions the emission corresponds to higher harmonics of the cavity resonance.

Phononic self-energy effects and superconductivity in CaC6

We study the graphite intercalated compound CaC${}_{6}$ by means of Eliashberg theory. We perform an analysis of the electron-phonon coupling and define a minimal six-band anisotropic structure, which leads to a Fermi-surface dependence of the superconducting gap. A comparison of the superconducting gap structure obtained using the Eliashberg and the superconducting density functional theory is performed. We further report the anisotropic properties of the electronic spectral function, the polaronic quasiparticle branches, and their interplay with Bogoljubov excitations.

Intrinsic Josephson Junctions in Bi-2212 Thin Films Fabricated by Metal-Organic Decomposition

We fabricated a mesa <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10\ \mu{\rm m}\times 10\ \mu{\rm m}$</tex></formula> in area and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 150 nm in height on a single grain in a 240-nm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{8+\delta}/{\rm MgO}$</tex></formula> thin-film grown by metal-organic decomposition (MOD) with face-to-face annealing. A multi-branch structure peculiar to intrinsic Josephson junctions (IJJs) was observed in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I-V$</tex> </formula> characteristics of the fabricated mesa, and this structure was characterized by approximately 100 quasiparticle branches with critical current densities of 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm kA/cm}^{2}$</tex> </formula> and voltage-jump intervals of 20 mV. Moreover, the temperature dependence of the critical current was in good agreement with a theoretical result based on <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$c$</tex> </formula> -axis Josephson tunneling in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$d$</tex></formula> -wave superconductors. These results imply that the thin films grown by MOD have sufficient quality to fabricate IJJs for practical terahertz applications.

Magnetization-dependent critical current of intrinsic Josephson junctions in Co/Au/Bi2Sr2CaCu2Oy mesa structures

We have investigated the critical current Ic of intrinsic Josephson junctions (IJJs) in Co/Au/Bi2Sr2CaCu2Oy mesa structures as a function of magnetic field. The current-voltage characteristics of the mesas before a magnetization of Co layer showed the multiple quasiparticle branches with hysteresis similar to that of conventional IJJs. On the other hand, the magnetic field dependence of Ic clearly showed hysteretic behavior different from that of conventional IJJs. It is found that such dependence corresponds to the magnetization curve of the Co layer and the Ic has a maximum at the coercive field of the Co layer. The observed modulation of Ic of the Co/Au/Bi2Sr2CaCu2Oy mesa is attributed to the injection of the spin-polarized current.

The Behavior of Subgap Structures of Intrinsic Josephson Junctions in $({\rm Bi},{\rm Pb})_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{\rm y}$Under Magnetic Field and Microwave Irradiation

We have extensively studied the behavior of pronounced subgap structures on the quasiparticle branches of the current-voltage (I-V) characteristics of intrinsic Josephson junctions (IJJs) in (Bi1-xPbx)2Sr2CaCu2Oy (x=0.15, 0.2) single crystals for their applications in THz range. Six subgap structures are successfully observed between 6.3 and 26.2 mV on the first quasiparticle branch at 4.2 K. The intensities of two typical subgap structures at 6.3 and 8.2 mV are found to be approximately proportional to Ic 2 (critica current)2 over a wide range of temperature between 4.2 and ~ 80 K. They are interpreted with regard to the interference of Raman-active optical phonons with ac Josephson currents in IJJs. Under magnetic field up to 2.5 T they have been affected somewhat in the respect to return current, though the vortex flow also has been induced in IJJs. By microwave application of 2-100 GHz they have been strongly affected so that their peak intensities decrease and then each peak splits into two, with increasing microwave power.

RF-Current Effects on Intrinsic Josephson Junctions

We have fabricated intrinsic Josephson junction arrays using Tl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> thin films. The junctions have dimensions of order 1 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and therefore high slope resistance in the range 1 kOmega to 10 kOmega per junction on the quasiparticle branches. We have irradiated the junctions with microwaves at a frequency of 6.37 GHz. We observe both microwave-induced vortex-flow branches and strong suppression of the switching current but only when no junctions are in the voltage state. We attribute this observation to the shunting of the high-resistance junctions by the environmental impedance, and discuss the significance of this result for the application of intrinsic junctions as THz sources and qubits.

Investigation of I– V characteristics of Bi-2212 intrinsic Josephson junctions for THz-wave generation

Voltage jumps have been observed in the quasiparticle branch I– V characteristics of Bi 2Sr 2CaCu 2O 8+ x intrinsic Josephson junctions fabricated as 60 μm × 300 μm × 0.5 μm mesas. The Josephson frequency at the highest jump voltage was estimated to be 0.65 THz, close to the fundamental cavity resonance frequency of the transverse Josephson plasma mode. Numerical simulations using coupled sine-Gordon equations demonstrated voltage jumps similar to the above experimental results, which appeared in the I– V characteristics, and the electric field distribution at the highest jump voltage exhibited resonance in the junctions. Moreover, the maximum radiation power, which was obtained in the vicinity of the highest voltage, was proportional to the square of the number of junctions. This can be explained by reduction of the impedance mismatch between the junctions and free space with increasing the number of junctions.

Microwave Response of Bi2Sr2CaCu2O8+δSurface Intrinsic Josephson Junctions with Bending Pancake Vortices

We study the microwave response of surface intrinsic Josephsonjunctions on Bi2Sr2CaCu2O8+δ, in which bendingpancake vortex lines are introduced in a controllable way. It isfound that the bending vortices can greatly influence theresponse. In some cases, typical Shapiro steps that lie far abovethe quasiparticle branch are observed, with the step intervalsatisfying the Josephson relation and their amplitude versus thesquare root of microwave power following the Bessel functionbehaviour. In the other cases, current steps that lie on thequasiparticle branch are observed, but only one or two stepsappear at the same time under the variation of the microwavepower.

Effect of magnetic field on quasiparticle branches of intrinsic Josephson junctions with ferromagnetic layer

The interlayer tunneling spectroscopy has been performed on micron-sized mesa arrays of HgBr 2 intercalated superconducting Bi2212 single crystals. A ferromagnetic multilayer (Au/Co/Au) is deposited on top of the mesas. The spin-polarized current is driven along the c-axis of the mesas through a ferromagnetic Co layer and the hysteretic quasiparticle branches are observed at 4.2 K. Magnetic field evolution of hysteretic quasiparticle branches is obtained to examine the effect of injected spin-polarized current on intrinsic Josephson junction characteristics. It is observed that there is a gradual distribution in quasiparticle branches with the application of magnetic field and increasing field reduces the switching current progressively.