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A. Schilling

Publications and source records attributed to A. Schilling.

31 records · Page 2Linked to original sources
arXiv2011

Observation of a first-order phase transition deep within the vortex-solid region of YBa2Cu3O7

We have investigated the magnetic phase diagram of a fully oxygenated detwinned YBa2Cu3O7 single crystal by means of magneto-caloric and magnetization measurements, and found thermodynamic evidence for a temperature dependent first-order phase-transition line deep within the vortex-solid region. The associated discontinuities in the entropy are apparently proportional to the magnetic flux density, which may hint at a structural transition of the vortex lattice.

cond-mat.supr-con↗
arXiv2011

Magnetic torque study of the temperature dependent anisotropy parameter in overdoped superconducting single-crystal YBa_2Cu_3O_7

An overdoped YBa2Cu3O7 single crystal was studied by SQUID and torque magnetometry in order to investigate the temperature dependence of the anisotropy parameter close to the transition temperature T_c (0.87 T_c < T < T_c). Angle dependent torque measurements were performed and analyzed with the widely used Kogan model [Phys. Rev. B 38, 7049 (1988)] as well as with an extended model by Hao and Clem [Phys. Rev. Lett. 67, 2371 (1991)], taking into account the influence of the vortex cores on the magnetization. Both approaches yield similar results, with an out-of-plane anisotropy parameter around 6.5 which slightly increases with decreasing temperature, and a temperature independent in-plane anisotropy parameter gamma_ab}= 1.12(5).

cond-mat.supr-con↗
arXiv2010

Symbiotic Optimization of the Nanolithography and RF-Plasma Etching for Fabricating High-Quality Light-Sensitive Superconductors on the 50 nm Scale

We present results of a fabrication-process development for the lithographic pattern transfer into the sub-100nm range by combining electron-beam lithography and reactive dry etching to obtain high quality niobium-based light-sensitive superconducting devices. To achieve this spatial resolution, we systematically investigated the stability of the positive organic etching masks ZEP 520A and PMMA 950k in different properly operated fluoride based plasma discharges. The chemically more robust ZEP 520A was used for defining the nanoscaled superconductors during the dry plasma etching. Our etching recipe is appropriate for a precisely controlled removal of a number of transition metals, their nitrides and a number of lithographic resists. Our process yielded lightsensitive superconducting devices made from NbN with smallest planar lateral dimensions of about 50nm with a critical temperature Tc(0) of about 13K , which is close to the transition temperature of the unstructured thin film. Our ultra-narrow current paths are able to permanently carry bias-currents up to 60% of the theoretical de-pairing current-limit.

cond-mat.mes-hall↗
arXiv2010

Nanoscale Fabrication by Intrinsic Suppression of Proximity-Electron Exposures and General Considerations for Easy & Effective Top-Down Fabrication

We present results of a planar process development based on the combination of electron-beam lithography and dry etching for fabricating high-quality superconducting photosensitive structures in the sub-100nm regime. The devices were fabricated by the application of an intrinsic proximity effect suppression procedure which makes the need for an elaborated correction algorithm redundant for planar design layouts which are orders of magnitude smaller than the backscattering length. In addition, we discuss the necessary considerations for extending the fabrication spatial scale of optical contactlithography with a mercury arc-discharge photon source down to the order of the exposure photon's wavelength ( sub-μm ), thereby minimizing the writing time on the electron-beam lithograph. Finally we developed a unique and novel technique for controlling the undercut during a planar lift-off fabrication procedure without cleaving the wafer.

cond-mat.mes-hall↗
arXiv2009

Current Assisted, Thermally Activated Flux Liberation in Ultrathin Nanopatterned NbN Superconducting Meander Structures

We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature T~0.4Tc(0). The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one. Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal conducting state, which can be registered as a voltage pulse of nanosecond duration. We considered three different models (vortex-antivortex pairs, vortex edge barriers and phase slip centers) to explain the experimental data. Only thermally excited vortices, either via unbinding of vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a satisfactory and consistent description for all measurements.

cond-mat.supr-con↗
arXiv2009

Origin of Ferroelastic Domains in Free-Standing Single Crystal Ferroelectric Films

The origin of the unusual 90^o ferroelectric / ferroelastic domains, consistently observed in recent studies on meso and nanoscale free-standing single crystals of BaTiO3 [Schilling et al., Physical Review B, 74, 024115 (2006); Schilling et al., Nano Letters, 7, 3787 (2007)], has been considered. A model has been developed which postulates that the domains form as a response to elastic stress induced by a surface layer which does not undergo the paraelectric-ferroelectric, cubic-tetragonal phase transition. This model was found to accurately account for the changes in domain periodicity as a function of size that had been observed experimentally. The physical origin of the surface layer might readily be associated with patterning damage, seen in experiment; however, when all evidence of physical damage is removed from the BaTiO3 surfaces by thermal annealing, the domain configuration remains practically unchanged. This suggests a more intrinsic origin, such as the increased importance of surface tension at small dimensions. The effect of surface tension is also shown to be proportional to the difference in hardness between the surface and the interior of the ferroelectric. The present model for surface tension induced twinning should also be relevant for finely grained or core-shell structured ceramics.

cond-mat.mtrl-sci↗
arXiv2008

LaBaNiO4: A Fermi glass

Polycrystalline samples of LaSr1-xBaxNiO4 show a crossover from a state with metallic transport properties for x = 0 to an insulating state as x -> 1. The end member LaBaNiO4 with a nominal nickel Ni 3d7 configuration might therefore be regarded as a candidate for an antiferromagnetic insulator. However, we do not observe any magnetic ordering in LaBaNiO4 down to 1.5 K, and despite its insulating transport properties several other physical properties of LaBaNiO4 resemble those of metallic LaSrNiO4. Based on an analysis of electrical and thermal-conductivity data as well as magnetic-susceptibility and low-temperature specific-heat measurements we suggest that LaBaNiO4 is a Fermi glass with a finite electron density of states at the Fermi level but these states are localized.

cond-mat.str-el↗
arXiv2007

Low-temperature differential-thermal analysis to measure variations in entropy

We describe how we can precisely measure variations in the entropy S of small solid samples below room temperature, as a function the temperature T or the external magnetic field H, respectively. A simple differential-thermal analysis (DTA) technique allows, in principle, for the measurement of variations in S without any instrumental broadening of the data. The method is particularly well suited for the detection of sharp phase transitions in samples of milligram size.

physics.ins-det↗
arXiv2007

Domains in Three-dimensional Ferroelectric Nanostructures: Theory and Experiment

Ferroelectric random access memory cells (FeRAMs) have reached 450 x 400 nm production (0.18 micron^2) at Samsung with lead zirconate-titanate (PZT), 0.13 micron^2 at Matsushita with strontium bismuth tantalate (SBT), and comparable sizes at Fujitsu with BiFeO3. However, in order to increase storage density, the industry roadmap requires by 2010 that such planar devices be replaced with three-dimensional structures. Unfortunately, little is known yet about even such basic questions as the domain scaling of 3-d nanodevices, as opposed to 2-d thin films. Here we report the experimental measurement of nano-domains in ferroelectric nanocolumns, together with a theory of domain size in 3-d structures which explains the observations.

cond-mat.mtrl-sci↗
arXiv2006

Universality of the Scaling Law for Ferroic Domains

We show how the periodicity of 180^{o} domains as a function of crystal thickness scales with the thickness of the domain walls both for ferroelectric and for ferromagnetic materials. We derive an analytical expression for the universal scaling factor and use this to calculate domain wall thickness and gradient coefficients (exchange constants) in some ferroic materials. We then use these to discuss some of the wider implications for the physics of ferroelectric nano-devices and periodically poled photonic crystals.

cond-mat.mtrl-sci↗
arXiv1999

A generalization of the q-Saalschutz sum and the Burge transform

A generalization of the q-(Pfaff)-Saalschutz summation formula is proved. This implies a generalization of the Burge transform, resulting in an additional dimension of the ``Burge tree''. Limiting cases of our summation formula imply the (higher-level) Bailey lemma, provide a new decomposition of the q-multinomial coefficients, and can be used to prove the Lepowsky and Primc formula for the A_1^{(1)} string functions.

math.QA↗
arXiv1997

Bailey flows and Bose-Fermi identities for the conformal coset models $(A^{(1)}_1)_N\times (A^{(1)}_1)_{N'}/(A^{(1)}_1)_{N+N'}$

We use the recently established higher-level Bailey lemma and Bose-Fermi polynomial identities for the minimal models $M(p,p')$ to demonstrate the existence of a Bailey flow from $M(p,p')$ to the coset models $(A^{(1)}_1)_N\times (A^{(1)}_1)_{N'}/(A^{(1)}_1)_{N+N'}$ where $N$ is a positive integer and $N'$ is fractional, and to obtain Bose-Fermi identities for these models. The fermionic side of these identities is expressed in terms of the fractional-level Cartan matrix introduced in the study of $M(p,p')$. Relations between Bailey and renormalization group flow are discussed.

hep-th↗