Pulsed laser deposited superlattices based on perovskite oxides

Recent advances made in the field of pulsed laser deposited superlattices made up of perovskite oxides are reviewed. The superconducting properties and the effect of adjacent layers in ultra-thin YBCO are studied in YBCO/(Pr_xY_1−x)BCO superlattices. We describe the effects of layering in (La_0.7Ba_0.3)MnO₃for the purposes of fundamental studies and possible applications of these newly recognized colossal magnetoresistance materials.     

Reentrant resistance via spin-polarized quasiparticle self injection in CMR/HTSC heterostructures

Epitaxial superlattices of half-metal, colossal magnetoresistive La_2/3Ca_1/3MnO₃ (HM-CMR) and high-T_c superconducting YBa₂Cu₃O_7-δ (HTSC) are grown with thick and thin modulation lengths (Λ) of YBCO/LCMO, with Λ = 280 nm and 12.5 nm; respectively, on SrTiO₃ (0 0 1) single-crystalline substrates by pulsed laser deposition. Transport measurements R(T) show a resistive state below T = 35 K although the superconducting transition temperature is found to be T_c = 60 K and 63 K for both different superlattices, respectively. The onset of the resistive state coincides with a magnetic transition of the samples. This can be explained by a diffusion of spin-polarized quasiparticles into the superconducting film. Which can be considered as evidence for inverse-proximity effects over a wide temperature range in HM-CMR/HTSC heterostructures.     

Microscopic description of superconductor-normal-metal superlattices

We study a model for superconductor-normal-metal superlattices in which adjacent layers are coupled via single-particle hopping. Examples include the high-T _c superconductor Bi₂Sr₂CaCu₂O_8+δ, where the BiO sheets seem to have normal metallic character. Using a BCS treatment, we investigate the influence of the interlayer hopping between the superconducting and the normal-metal slabs on the superconducting density of states, the tunneling characteristics for tunneling into both superconducting and normal-metal slabs as well as the temperature dependence of the London penetration depth.     

Reflectivity oscillations in superconducting superlattices Nb/SiO2

The reflectivity of superconducting superlattices Nb/SiO₂ was experimentally studied at room temperature in a spectral range 450-5000 cm^-1. It was found that the reflectivity as a function of SiO₂ layer thickness oscillates with a period 3.5 Å. This value coincides with the period of oscillations of the superconducting transition temperature found earlier on these superlattices. We suppose that in both cases the oscillations are due to changes of electron state density. So it is possible to considerably influence the superconducting transition temperature of a superconducting film by making sandwich-like structures using thin dielectric layers.     

Suppression of superconductivity in YBCO/LCMO superlattices

The competition of superconductivity and magnetism in superlattices composed of alternating YBa₂Cu₃O_7−d and La_0.67Ca_0.33MnO₃ thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of super lattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symmetry and thus suppress superconductivity.     

π-phase magnetism in ferromagnet-superconductor superlattices

New 0π and ππ Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states with antiferromagnetic orientation of magnetizations in the neighboring layers of a ferromagnetic metal (FM) are predicted for FM/superconductor (FM/S) superlattices. Under certain conditions, the critical temperature T _c of these states is higher than for the known 00 and π0 LOFF states with ferromagnetic ordering of the FM layers. It is shown that the nonmonotonic behavior of T _c in the FM/S superlattices with S-layer thickness d _s less than the threshold value d _s ^π is due to the phase transition cascade 0π-ππ-0π At d _s >d _s ^π , the T _c oscillations are caused by the 00-π0-00 transitions. New logic elements based on the FM/S structures and combining the advantages of the superconducting and magnetic data-record channels in a single sample are proposed.     

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

The Kohn-Luttinger effect and anomalous pairing in new superconducting systems and graphene

We present a review of theoretical investigations into the Kohn-Luttinger nonphonon superconductivity mechanism in various 3D and 2D repulsive electron systems described by the Fermi-gas, Hubbard, and Shubin-Vonsovsky models. Phase diagrams of the superconducting state are considered, including regions of anomalous s-, p-, and d-wave pairing. The possibility of a strong increase in the superconducting transition temperature T _c even for a low electron density is demonstrated by analyzing the spin-polarized case or the two-band situation. The Kohn-Luttinger theory explains or predicts superconductivity in various materials such as heterostructures and semimetals, superlattices and dichalcogenides, high-T _c superconductors and heavy-fermion systems, layered organic superconductors, and ultracold Fermi gases in magnetic traps. This theory also describes the anomalous electron transport and peculiar polaron effects in the normal state of these systems. The theory can be useful for explaining the origin of superconductivity and orbital currents (chiral anomaly) in systems with the Dirac spectrum of electrons, including superfluid ³He-A, doped graphene, and topological superconductors.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Superconducting probe of electronic correlations and exchange field based on the proximity effect in F/S nanostructures

The proximity effect and competition between the BCS and LOFF states are studied in the Cooper limit for thin F/S and F/S/F nanostructures, where F is a ferromagnet and S is a superconductor. The dependences of the critical temperature on the exchange field I, electron correlations λ _f, and the thickness d _f of the F layer are derived for F/S bilayers and F/S/F trilayers. In addition, two new π-phase superconducting states with electron-electron repulsion in the F layers of F/S/F trilayers are predicted. A two-dimensional LOFF state in F/S/F trilayers is possible only in the presence of a weak magnetic field and the appropriate parameters of the F and S layers. The absence of the suppression of three-dimensional superconductivity in short-period Gd/La superlattices is explained and the electron-electron coupling constant in gadolinium is predicted. A method of superconducting sounding spectroscopy based on the proximity effect is proposed for determining the symmetry of the order parameter, the magnitude and sign of electron correlations, and the exchange field in various nanomagnets F.     

Current-carrying states in superconductor/insulator and superconductor/semiconductor superlattices in the mesoscopic regime

We discuss some of the basic theoretical aspects of current-carrying states in superconducting superlattices with tunnel barriers in the mesoscopic regime, when p₀ ^− 1 ⪡ a ⪡ ξ₀(a is the superconducting layer thickness, p₀is the Fermi momentum, ξ₀is the BCS coherence length and ℏ =  1). We establish the necessary conditions for the observation of the classical Josephson effect (with sinusoidal current–phase dependence) and derive self-consistent analytical expressions for the critical Josephson current. These expressions are proportional to the small factor a / ξ₀and have unusual temperature dependence as compared with the single-junction case. For certain parameter values, the superconducting gap exhibits an exponential decrease due to pair-breaking effect of the supercurrent. The supercurrent can completely destroy the superconductivity of the system above a certain characteristic temperature T ^* . In this paper, we also study the effect of intrabarrier exchange interactions. We show that this effect is strongly enhanced compared with the single-junction case and can manifest itself in an exponential decrease of the critical temperature.     

Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

We consider in detail Raman scattering by vibration of the apical oxygen ions in the RBa₂Cu₃O₇ superconducting cuprates. The scattering intensity is very sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our results show a large quantitative difference between the results of frozen-phonon and perturbational approach to the Raman process. The discrepancy becomes especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of phonon-induced ion charge fluctuations shows an analogous discrepancy. The reason for these effects is the possibility of carrier redistribution between different parts of the Fermi surface arising in the frozen-phonon approximation. Our results show that Raman scattering in superconducting superlattices is very sensitive to the properties of the states near the Fermi level. For this reason experiments performed on the superlattices can help to resolve the discrepancy. Received 8 December 1999     

Critical thickness and stress relaxation in YBaCuO (123) strained epitaxial layers and YBaCuO based strained superlattices

An energy model has been used to calculate the critical thickness h _c of YBaCuO thin films and YBaCuO based superlattices within an isotropic or anisotropic approximation. The critical thickness of single layers calculated from the anisotropic model (16 nm) is in good agreement with the previously published experimental values which are spread out from 4 to 20 nm. In the case of superlattices, relaxation appears to be governed by the critical thickness of the elementary sub-layers and is then better evaluated through the calculation performed for YBaCuO single layers. XRD measurements on YBa₂Cu₃O₇/PrBa₂Cu_3?xGa_xO₇ superlattices grown on {100{ SrTiO₃ have evidenced a tetragonal stress in the YBaCuO ab plane which remains expanded when the YBaCuO elementary layer thickness is lower than 4.8 nm (4 YBaCuO cells). However the critical temperature of the shortest period superlattices is only slightly affected by this expanded stress in contrast to the effect of an elastic stress externally applied along the ab plane of YBaCuO thin films.     

X-ray diffraction investigation of BaTiO3/(Ba,Sr)TiO3 superlattices

Samples of BaTiO₃/(Ba_{1 ? x} Sr _x )TiO₃ (BT/BST-x) superlattices have been studied using X-ray diffraction analysis. A complete parallel orientation of all the studied films and the substrate has been revealed, and the modulation periods ?? of the superlattices, the parameters of the unit cell averaged over the ?? period, and the parameters of individual components of the superlattices have been determined.     

Thermodynamic instability of Ag/Au and Cu/Pd metal superlattices

We show how the formation energies of A_pB_q superlattices with arbitrary periods p and q and layer orientation Ĝ can be predicted via a 'cluster expansion' technique, given the formation energies of short period structures from first-principles calculations. We predict both bulk and epitaxial energies as well as the energies of the fully intermixed (alloyed) superlattices. Applications to Ag/Au and Cu/Pd superlattices illustrate our method, as well as a global classification scheme for superlattice stability.     

Comparison of the calculated electronic structures of [111] and [001] GaAs/AℓxGa1-xAs semiconductor superlattices

A theoretical comparison of the electronic structures of long-period (about 300 Å) N_GaAs×N_AℓGaAs GaAs/Aℓ_0.3Ga_0.7As superlattices grown along the [111] direction and superlattices grown along the [001] direction is presented. Almost all qualitative features of the theoretical results are in good agreement with experiments by Hayakawa et al. The observed optical transition enhancement in the [111]-oriented quantum-well structures is caused by the topological difference in the superlattices, and is only partly due to the fact that the heavy holes in the [111] superlattices have larger transverse effective masses, and therefore have larger two-dimensional valence band densities of states.     

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)_n/(BAs)_n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.     

Intrinsic tunneling spectroscopy for Bi2−xPbxSr2CaCu2O8+δ of nm-thickness mesa structure

We have investigated intrinsic tunneling spectroscopy (ITS) with short-pulse bias to mesa structures consisting of several layers of intrinsic Josephson junction superlattices of Bi_1.8Pb_0.2Sr₂CaCu₂O_8+δ(PbBi2212). Through ITS, the superconducting gap 2Δ = 75 meV (at 10 K) is obtained for a PbBi2212 crystal. The large 2Δ value corresponds to the underdoped property of Pb-free Bi2212, which is consistent with the ab-plane transport measurement results performed simultaneously. The normal tunneling resistance R_N derived from the high bias region of the I–V characteristics is significantly small in comparison with underdoped Bi2212. Moreover, J_c of PbBi2212 is less deviated from the Ambegaokar–Baratoff value than the case of underdoped Bi2212. It is interpreted that the Pb substitution makes the tunnel barrier lower, resulting in a reduced anisotropy and a high J_c even with a lower doping.     

The 2s exciton in intermediate dimensionality structures

We report the observation of the 1s and 2s ground state excitons in small-offset superlattices with alternating layers of non-magnetic (ZnSe) and diluted magnetic (Zn_xMn_1 − xSe) semiconductors. Due to the large Zeeman splitting of the electron states in ZnMnSe, valence and conduction band offsets in these structures can be tuned by external magnetic field over the scale of tens of meV. Two series of superlattices were investigated, with Mn concentrations close to 10 and 15 atomic percent, respectively. Each series consisted of five superlattices with different layer thicknesses. The 2s exciton of the ground state is observed in all samples, and exhibits a diamagnetic shift that is consistent with earlier studies. The ability to determine the energy separation between the 1s and 2s exciton states allows us to analyze our results in the framework of thefractional dimension analysis , and to extract the values of exciton binding energies for the superlattices from the analysis.     

Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices

YBa₂Cu₃O₇/PrBa₂Cu₃O₇ (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U₀ was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U₀ is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U₀ with the current I in the specimen: U₀ exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep – elastic (collective) creep induced by the transport current.