A. Walkenhorst

High-superconducting critical current densities in YBa2Cu3O7/PrBa2Cu3O7 superlattices

Well‐ordered [(YBa2Cu3O7)nY/(PrBa2Cu3O7)nPr)]m superlattices with high superconducting critical temperatures were prepared by sequential reactive dc sputtering. The superconducting critical current density at 4.5 K reaches values as high as 1.1×107 A/cm2 for a sample with nY = 2 and nPr = 1 and 3.8×106 A/cm2 for nY = 1 and nPr = 3, indicating that the coupling of the CuO2 layers is not necessary for high‐jc values. The angular dependence jc(φ) is determined by the component of the magnetic field perpendicular to the a‐b planes and shows that these superlattices are two‐dimensional systems.

CeO2: An alternative insulator material for superconducting field effect devices

The properties of high‐Tc superconducting field‐effect devices using CeO2 as an insulating layer have been studied. The dielectric constant and hence the achieved charge transfer for constant gate voltage and equivalent geometry is smaller as compared to the established SrTiO3 dielectric. This is mostly compensated by better insulating properties. The observed field effects for a fixed amount of charge transfer are very similar to those obtained with the SrTiO3 dielectric, indicating that the observed field effects are truly due to changes in the charge carrier density and not due to field‐induced stresses in the multilayer structure.

Growth of high quality YBa2Cu3O7 films on various substrate materials and influence of Zn-doping on superconductivity

Abstract By DC-sputtering from a single stoichiometric target we prepared high quality YBa2Cu3O7 thin films on SrTiO3, LaGaO3, ZrO2, MgO and NdAlCaO4. All films sputtered on these substrates (except NdAlCaO4) showed narrow inductive transitions (ΔTc 86 K), but only on LaGaO3 the transition was as sharp (width dT c dx = −10.5 K (1% Zn) as observed in bulk samples.

ANISOTROPY OF THE DEPINNING FIELD AND THE PINNING FORCE DENSITY OF THIN EPITAXIAL YBA2CU3O7 FILMS

Abstract Measurements of the depinning field and the critical current density on epitaxially grown c -axis oriented YBa 2 Cu 3 O 7 films were performed under variation of the angle ϕ between the field direction and the c -axis of the film with high angular resolution (Δ≈0.04). The critical current densities were measured on a 10 μm wide and 100 μm long strip by applying a 1 μV criterion. The dependence j c (ϕ) obtained for low temperatures can be well explained by a model published by Tachiki and Takahashi. For higher temperatures and higher magnetic fields additional structures appear. The measured angular dependence of the depinning field shows a cusp for ϕ =90°. The complete angular dependence B dp (ϕ) can be explained by simply taking into account the anisotropy of the upper critical field.

Critical current density and upper critical field of YBa2Cu3O7 thin films

Epitaxial YBa2Cu4O7-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO3-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 μm wide strips as function of magnetic field and temperature. The temperature dependences ofjc follow a universal functionjc(B,T)=jc*(T=0,B)·(1−T/Tc(B))α with α=1.5±0.1. ForB=0 andT=77 K we obtainedjc=4·106 A/cm2. The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldBc2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofjc for the magnetic field parallel to the (CuO2)-layers (B⊥c). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-Tc-superconductors the influence of the anisotropy of the upper critical field onjc(B, T, φ) is evident nearTc.

Anisotropy of the pinning force density and the resistive transitions in YBa2Cu3O7/PrBa2Cu3O7 superlattices

Abstract Finely modulated ( YBa 2 Cu 3 O 7 ) nY /( PrBa 2 Cu 3 O 7 ) nPr superlattices (SL) with sharp X-ray satellite lines (up to the fifth order) were prepared by sequential dc-sputtering. The high degree of structural order is also reflected in high superconducting critical temperatures T c and critical current densities j c measured for SL with nY =1 for the first time. The angular dependence of the pinning force density, i.e. j c ab ( B , T , ϕ ), and the resistive transitions ϱ ( B , T , ϕ ), where ϕ is the angle between the external field B and the c -axis, show a strong enhancement of the intrinsic anisotropy due to the superlattice structure. For B parallel to the CuO 2 -layers ϱ ( B , T ) and j c ( B , T ) of SL with nY ≤2 are nearly independent on the magnetic field. Since the superconducting order parameter is zero in the PrBa 2 Cu 3 O 7 layers, we conclude that these SLs act like a stack of ultrathin superconducting film. Consequently, they exhibit a 2-dimensional behavior at low temperatures with j c ( B , ϕ )= j c (Bcosϕ,0°).

Contribution of Zn Impurity Atoms to the Anisotropic Pinning Force Density of Thin Epitaxial YBa2(Cu1-xZnx)3O7-δ Films

The in-plane critical current density jcab of thin epitaxial YBa2(Cu1-xZnx)3O7-δ films (x = 0.001 ÷ 0.02) was measured in magnetic fields up to 7 T under variation of the angle between the applied magnetic field and the c-axis of the films (j|| axis of rotation, i.e. j ⊥ B). At high temperatures (i.e. T > 0.5 Tc) the jc() dependences differ significantly from those obtained for undoped films. In order to explain the resulting structures quantitatively, we propose a model based on the additional pinning mechanism arising from pointlike pinning centres due to the Zn dopant atoms.

In situ preparation and transport properties of YBa2Cu3O7 films on sapphire with Zr(Y)O2 buffer layers

YBa2Cu3O7 films on sapphire substrates prepared by laser ablation show c‐axis‐oriented growth and critical temperatures as high as Tc(R=0)=90 K. Due to diffusion processes and the formation of a nonsuperconducting intermediate layer the critical current density Jc is comparatively low. To overcome these difficulties the sapphire substrates were covered by Zr(Y)O2 films acting as diffusion barriers. The preparation of these films was performed by laser ablation using a multitarget system. The structure and the surfaces of the films were characterized by x‐ray diffraction and scanning electron microscopy, respectively. The transport properties of the superconductor films are similar to those on suited substrates. Critical current densities up to Jc (77 K)=5×105 A/cm2 were achieved. In order to gain information about the pinning mechanisms measurements of the dependencies of Jc on the orientation between the crystal c axis and the external magnetic fields were carried out using samples with different buffer ...

Thermally activated flux-flow in epitaxially grown YBa2(Cu1−χZnχ)3O7− thin films

Abstract The influence of Zn-impurity atoms on the superconducting critical temperature T c , the resistive transitions in magnetic fields ϱ ab ( TB , T ) and the critical current density j ab c ( B , T ) of epitaxially grown, c-axis oriented YBa 2 (Cu 1−χ Zn χ ) 3 O 7 films is reported. For very small concentrations χ ≤ 0.005 we observe a narrowing of the resistive transitions which leads to a crossing of the depinning lines and corresponds to an enhancement of the average activation energy 〈 U 〉 by a factor of two. The pinning effectivity of the Zn-dopant atoms can also be deduced from an enhancement of j ab c ( B , T )/ j ab c (0, T ) in large magnetic fields B ≥ 6 T and characteristics features in the angular dependence of j ab c ( B , T , ϕ ) where ϕ is the angle between the external field B and the c - axis.

Preparation and characterization of CeO2‐based superconducting field effect transistors

High‐Tc superconducting field effect transistors using CeO2 as the dielectric layer in combination with ultrathin YBa2Cu3O7−δ layers have been prepared and investigated. The preparation process of these devices is described in detail. CeO2 is very compatible with ultrathin YBa2Cu3O7−δ layers, resulting in Tc values consistent with SrTiO3‐based superconducting field effect transistors. The dielectric constant and the breakdown field of the CeO2 layer are reproducible in all investigated samples, yielding values for er≊20 and Ebd≊2×108 V/m. This results in a charge transfer of ΔN=±2 μC/cm2. In the charge carrier enhancement mode this value is close to SrTiO3‐based superconducting field effect transistors. So far, the maximum achieved modulation of the charge carrier density by the use of CeO2 as dielectric is 1.5%.