Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpatial BCS-BEC crossover in superconducting p-n junctions
93
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a theory of superconducting p-n junctions. We consider a 2-band model of doped bulk semiconductors with attractive interactions between the charge carriers and derive the superconducting order parameter, the quasiparticle density of states and the chemical potential as a function of semiconductor gap $Delta_0$ and the doping level $varepsilon$. We verify previous results for the quantum phase diagram (QPD) for a system with constant density of states in the conduction and valence band, which show BCS-Superconductor to Bose-Einstein-Condensation (BEC) and BEC to Insulator transitions as function of doping level and band gap. Then, we extend it to a 3D density of states and derive the QPD, finding that a BEC phase can only exist for small band gaps $Delta_0 < Delta_0^*$. For larger band gaps, there is a direct transition from an insulator to a BCS phase. Next, we apply this theory to study the properties of superconducting p-n junctions, deriving the spatial variation of the superconducting order parameter along the p-n junction. We find a spatial crossover between a BCS and BEC condensate, as the density of charge carriers changes across the p-n junction. For the 2D system, we find two regimes, when the bulk is in a BCS phase, a BCS-BEC-BCS junction with a single BEC layer, and a BCS-BEC-I-BEC-BCS junction with two layers of BEC condensates separated by an insulating layer. In 3D there can also be a conventional BCS-I-BCS junction for semiconductors with band gaps exceeding $Delta_0^*$. Thus, there can be BEC layers in the well controlled setting of doped semiconductors, where the doping level can be varied to change the thickness of BEC layers, making Bose Einstein Condensates possibly accessible to experimental transport and optical studies in solid state materials.
rate research
Read More
The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits gives a unified framework of quantum bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature $T_{rm c}=8.5$ K has been found to be deep inside the BCS-BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe below $T^*sim20$ K, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large nonlinear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below $T^*sim20$ K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance (NMR) and transport data give evidence of pseudogap formation at $sim T^*$. The multiband superconductivity along with electron-hole compensation in FeSe may highlight a novel aspect of the BCS-BEC crossover physics.
We present resistivity and thermal-conductivity measurements of superconducting FeSe in intense magnetic fields up to 35 T applied parallel to the $ab$ plane. At low temperatures, the upper critical field $mu_0 H_{c2}^{ab}$ shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at $mu_0 H^{ast}approx 24$ T well below $mu_0 H_{c2}^{ab}$, indicating a first-order phase transition in the superconducting state. This demonstrates the emergence of a distinct field-induced superconducting phase. Moreover, the broad resistive transition at high temperatures abruptly becomes sharp upon entering the high-field phase, indicating a dramatic change of the magnetic-flux properties. We attribute the high-field phase to the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state, where the formation of planar nodes gives rise to a segmentation of the flux-line lattice. We point out that strongly orbital-dependent pairing as well as spin-orbit interactions, the multiband nature, and the extremely small Fermi energy are important for the formation of the FFLO state in FeSe.
The effect of particle-hole fluctuations for the BCS-BEC crossover is investigated by use of functional renormalization. We compute the critical temperature for the whole range in the scattering length $a$. On the BCS side for small negative $a$ we recover the Gorkov approximation, while on the BEC side of small positive $a$ the particle-hole fluctuations play no important role, and we find a system of interacting bosons. In the unitarity limit of infinite scattering length our quantitative estimate yields $T_c/T_F=0.264$. We also investigate the crossover from broad to narrow Feshbach resonances -- for the later we obtain $T_c/T_F=0.204$ for $a^{-1}=0$. A key ingredient for our treatment is the computation of the momentum dependent four-fermion vertex and its bosonization in terms of an effective bound-state exchange.
We theoretically show that a two-band system with very different masses harbors a resonant pair scattering that leads to novel pairing properties, as highlighted by the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Most importantly, the interband pair-exchange coupling induces an effective intraband attraction in each band, enhancing the superfluidity/superconductivity. The effect, a kind of Suhl-Kondo mechanism, is specifically enhanced when the second band has a heavy mass and is incipient (lying close to, but just above, the chemical potential, $mu$), which we call a resonant pair scattering. By elucidating the dependence of the effective interactions and gap functions on $mu$, we can draw an analogy between the resonant pair scattering and the Feshbach resonance.
We study dynamic ground state properties in the crossover from weak (BCS) to strong coupling (BEC) superfluidity. Our approach is based on the attractive Hubbard model which is analyzed by the dynamical mean field theory (DMFT) combined with the numerical renormalization group (NRG). We present an extension of the NRG method for effective impurity models to selfconsistent calculations with superconducting symmetry breaking. In the one particle spectra we show quantitatively how the Bogoliubov quasiparticles at weak coupling become suppressed at intermediate coupling. We also present results for the spin and charge gap. The extension of the NRG method to selfconsistent superconducting solutions opens the possibility to study a range of other important applications.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
