Superlattice Of Quantum Stripes Research Articles

Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes

The doping dependent isotope effect on the critical temperature (Tc) is calculated for multi-band multi-condensate superconductivity near a 2.5 Lifshitz transition. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for the appearance of a new sub-band making a circular electron-like Fermi surface pocket. We describe a particular type of BEC (Bose–Einstein Condensate) to BCS (Bardeen–Cooper–Schrieffer condensate) crossover in multi-band/multi-condensate superconductivity at a metal-to-metal transition that is quite different from the standard BEC–BCS crossover at an insulator-to-metal transition. The results show that the isotope coefficient strongly deviates from the standard BCS value 0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for the metal-to-metal transition. The critical temperature Tc shows a minimum due to the Fano antiresonance in the superconducting gaps and the isotope coefficient diverges at the point where a BEC coexists with a BCS condensate. In contrast Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the newly appearing sub-band.

Evidence for a striped phase of high T c superconductor at the Fermi surface

A complete Fermi surface in the normal state of superconducting Bi2Sr2CaCu2O8+δ at the optimum doping (T c = 91K) has been measured by angle scanning photoemission spectroscopy using synchrotron radiation. The Fermi surface reveals broken segments and hot spots of high photointensity along lines of k y = n:0:23A−1 in the (π; π) direction. The pseudogap observed at (π,0) in underdoped materials is one of this missing segments. The results indicate the presence of characteristic subbands of a superlattice of quantum stripes. This is formed by an ordered array of ∼ 14A large stripes.

The isotope effect in a superlattice of quantum stripes

We have calculated the isotope effect coefficient and the critical temperature in a particular superconducting heterostructure formed by a bidimensional (2D) superlattice of metallic quantum stripes. We model the superlattice with a 2D free electron gas confined in a ID periodic potential barrier. Within the BCS theory we calculate the shape resonance for the critical temperature and the isotope coefficient as a function of the chemical potential. The results provide a good description of the anomalous behavior of the isotope coefficient in La{2-x}SrxCuO4 as a function of doping with a sharp peak α ~0.8 in the underdoped regime and a rapid drop to α ~0.2 at 0.125 holes per copper site.

T c amplication and pseudogap at a shape resonance in a superlattice of quantum stripes

The shape resonance of the superconducting gap for a 2D electron gas in a superlattice of quantum stripes with a finite 1 D periodic potential barrier is studied when the Fermi level is tuned near the bottom of the third superlattice subband. The maximumTc. appears at a critical charge densityTc. Forp > pc (in the underdoped regime) the chemical potential is tuned in a pseudogap region characterized by a decrease of the density of states and an opening of the partial gap at the Fermi surface. The pseudogap is closed forp< pc (in the overdoped regime).

High Tc superconductivity in a superlattice of quantum stripes

The superconducting gap for 2D electron gas in a superlattice of quantum stripes with a finite 1D periodic potential barrier is calculated. The resonant enhancement of the superconducting gap is found when the Fermi level is tuned near the bottom of the nth superlattice subband with n ≥ 2. The critical charge density ϱ c for maximum T c at the n = 2 resonance is mainly determined by the stripe width L and vice versa, while it is nearly independent on the amplitude of the potential barrier. The width of the resonance is determined by the interaction cutoff h ̵ ω 0 and its shape by the ratio ΔE n⊥ h ̵ ω 0 , where ΔE n⊥ is the transversal energy dispersion of the n subband due to the transverse hopping between stripes.

The gap amplification at a shape resonance in a superlattice of quantum stripes: A mechanism for high Tc

The amplification of the superconducting critical temperature T c from the low temperature range in homogeneous 2D planes ( T c < 23 K) to the high temperature range (23K < T c < 150K) in an artificial heterostructure of quantum stripes is calculated. The high T c is obtained by tuning the chemical potential μ near the bottom of the nth subband ( E n ), at a “shape resonance”, in a range μ- E n ∼ ω D , where ω D is the energy cutoff for the pairing interaction. The resonance for the gap Δ n at the nth “shape resonance” is studied for a free electron gas in the BCS approximation imation as a function of the stripe width L, and of the number of electrons ϱ per unit surface. An amplification factor 650 > Δ 3 Δ ∞ > 6 for coupling 0.1 < λ < 0.3 is obtained at the third shape resonance raising the critical temperature in the high T c range.

The instability close to the 2D generalized wigner polaron crystal density: A possible pairing mechanism indicated by a key experiment

Here we show that the metallic phase of Bi 2Sr 2CaCu 2O y is given by a phase separation. It appears at the phase transition from a Fermi-gas to a Wigner-solid by decreasing the density of a 2D electron gas in the presence of a polaronic electron-lattice interaction. Two phases are in equilibrium: an electron gas confined in a superlattice of quantum stripes and a one-dimensional (1D) incommensurate generalized Wigner charge density wave (CDW) of condensed polarons at T<T ∗∼120K. The 1D confinement of the electron gas gives the non Fermi liquid behavior of the normal state as in 1D conductors. The superconducting phase competes with the phase separation below T c. The pairing mechanism is given by the coupling of polaron pairs with electron pairs. The superconducting energy gap is of the order of the short range polaron-polaron attraction E b for condensed polarons. The anisotropic gap arises because the electrons in the n=2 subband of the stripe ( k Fy =2 π/ L, where L is the stripe width) with Fermi wave vector k F in the ΓM direction, k Fx = k Fy , i.e, at 45° degree with the stripe direction, condense at T c. The resonant confinement of the electron gas in the stripes increases T c. The superlattice with barrier width W∼ξ 0 quenches the fluctuations in D<3.

On the possibility of new high T c superconductors by producing metal heterostructures as in the cuprate perovskites

Here we propose a possible way to produce superconductors with high critical temperature T c by confinement of a Fermi liquid in a superlattice of quantum stripes (wells, wires or dots). The enhancement of T c is obtained by tuning the size L of the stripes (wells, wires or dots), and the wavelength of electrons at the Fermi level λ F at the resonance condition L∼λ F. The 3D superconducting phase is stabilized by the condition that the separation W between the stripes (wells, wires or dots) should be of the order of the coherence length ξ 0. This quantum state is realized in Bi 2Sr 2Ca 2Cu 2O 8+y superconductor with T c =84 K, where the resonance condition k Fy∼π/ L for the component of the Fermi wavevector normal to the stripe direction holds. This work gives the parameters for the design of new superconducting materials with higher T c made of metal heterostructures approaching the atomic limit.