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"Scheer, E."
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The public work of Christmas : difference & belonging in multicultural societies
\"Christmas is not a holiday just for Christians anymore, if it ever was. Embedded in calendars around the world and long a lucrative merchandising opportunity, Christmas enters multicultural, multi-religious public spaces, provoking both festivity and controversy, hospitality and hostility. The Public Work of Christmas takes a comparative historical and ethnographic perspective on the politics of Christmas in multicultural contexts ranging from a Jewish museum in Berlin to a shopping boulevard in Singapore. A seasonal celebration that is at once inclusive and assimilatory, Christmas offers a clarifying lens for considering the historical and ongoing intersections of multiculturalism, Christianity, and the nationalizing and racializing of religion. The essays gathered here examine how cathedrals, banquets, and carols serve as infrastructures of memory that hold up Christmas as a civic, yet unavoidably Christian holiday. At the same time, the authors show how the public work of Christmas depends on cultural forms that mark, mask, and resist the ongoing power of Christianity in the lives of Christians and non-Christians alike. Legislated into paid holidays and commodified into marketplaces, Christmas has arguably become more cultural than religious, making ever wider both its audience and those who do the work to make it happen every year. The Public Work of Christmas articulates a fresh reading of Christmas--as fantasy, ethos, consumable product, site of memory, and terrain for the revival of exclusionary visions of nation and whiteness--at a time of renewed attention to the fragility of belonging in diverse societies.\"-- Provided by publisher.
Book
Creation of equal-spin triplet superconductivity at the Al/EuS interface
In conventional superconductors, electrons of opposite spins are bound into Cooper pairs. However, when the superconductor is in contact with a non-uniformly ordered ferromagnet, an exotic type of superconductivity can appear at the interface, with electrons bound into three possible spin-triplet states. Triplet pairs with equal spin play a vital role in low-dissipation spintronics. Despite the observation of supercurrents through ferromagnets, spectroscopic evidence for the existence of equal-spin triplet pairs is still missing. Here we show a theoretical model that reveals a characteristic gap structure in the quasiparticle density of states which provides a unique signature for the presence of equal-spin triplet pairs. By scanning tunnelling spectroscopy we measure the local density of states to reveal the spin configuration of triplet pairs. We demonstrate that the Al/EuS interface causes strong and tunable spin-mixing by virtue of its spin-dependent transmission.
Spectroscopic evidence of equal-spin triplet Cooper pairs is still missing so far. Here, Diesch et al. propose a unique signature for the presence of equal-spin triplet pairs and experimentally reveal the spin configuration of triplet pairs at the Al/EuS interface.
Journal Article
Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces
The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.
In the proximity of noncollinear magnetization, the Cooper-paired electrons of a superconductor may exist in a spin-triplet state. Here, the authors use scanning tunnel methods to directly observe this effect in Niobium as an adjacent film of Holmium is driven between helical and ferromagnetic order.
Journal Article
A current-driven single-atom memory
The possibility of fabricating electronic devices with functional building blocks of atomic size is a major driving force of nanotechnology
1
. The key elements in electronic circuits are switches, usually realized by transistors, which can be configured to perform memory operations. Electronic switches have been miniaturized all the way down to the atomic scale
2
,
3
,
4
,
5
,
6
,
7
,
8
,
9
. However, at such scales, three-terminal devices are technically challenging to implement. Here we show that a metallic atomic-scale contact can be operated as a reliable and fatigue-resistant two-terminal switch. We apply a careful electromigration protocol to toggle the conductance of an aluminium atomic contact between two well-defined values in the range of a few conductance quanta. Using the nonlinearities of the current–voltage characteristics caused by superconductivity
10
in combination with molecular dynamics and quantum transport calculations, we provide evidence that the switching process is caused by the reversible rearrangement of single atoms. Owing to its hysteretic behaviour with two distinct states, this two-terminal switch can be used as a non-volatile information storage element.
Electromigration is used to rearrange single atoms in an atomic-sized metal contact and to switch its conductance between two well-defined values, enabling memory device functionality.
Journal Article
Unveiling unconventional magnetism at the surface of Sr2RuO4
Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr
2
RuO
4
, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr
2
RuO
4
is still ongoing, a deeper understanding of the Sr
2
RuO
4
normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr
2
RuO
4
in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr
2
RuO
4
surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry.
Strontium Ruthenate, Sr
2
RuO
4
, displays a remarkable number of intriguing physical phenomena, from superconductivity, to strain-induced ferromagnetism. Here, using low-energy muon spectroscopy, Fittipaldi et al. demonstrate the existence of unconventional magnetism at the surface of Sr
2
RuO
4
in its normal state and without any applied strain.
Journal Article
Calorimetry of a phase slip in a Josephson junction
Josephson junctions are a central element in superconducting quantum technology; in these devices, irreversibility arises from abrupt slips of the quantum phase difference across the junction. This phase slip is often visualized as the tunnelling of a flux quantum in the transverse direction to the superconducting weak link, which produces dissipation. Here we detect the instantaneous heat release caused by a phase slip in a Josephson junction, signalled by an abrupt increase in the local electronic temperature in the weak link and subsequent relaxation back to equilibrium. Beyond the advance in experimental quantum thermodynamics of observing heat in an elementary quantum process, our approach could allow experimentally investigating the ubiquity of dissipation in quantum devices, particularly in superconducting quantum sensors and qubits.Superconducting currents around a loop containing a weak link can be quantized and only change during discrete events called phase slips. Now, the heat generated by a single phase slip and the subsequent relaxation have been experimentally observed.
Journal Article
Critically Charged Superfluid 4He Surface in Inhomogeneous Electric Fields
We have studied the spatial distribution of charges trapped at the surface of superfluid helium in the inhomogeneous electric field of a metallic tip close to the liquid surface. The electrostatic pressure of the charges generates a deformation of the liquid surface, leading to a “hillock” (called “Taylor cone”) or “dimple”, depending on whether the tip is placed above or below the surface. We use finite element simulations for calculating the surface profile and the corresponding charge density in the vicinity of the tip. Typical electric fields E are in the range of a few kV/cm, the maximum equilibrium surface deformations have a height on the order of (but somewhat smaller than) the capillary length of liquid
4
He (0.5 mm), and the maximum number density of elementary charges in a hillock or dimple, limited by an electrohydrodynamic instability, is some 10
13
m
−2
. These results can be used to determine the charge density at a liquid helium surface from the measured surface profile. They also imply that
inhomogeneous
electric fields at a bulk helium surface do not allow one to increase the electron density substantially beyond the limit for a homogeneous field, and are therefore not feasible for reaching a density regime where surface state electrons are expected to show deviations from the classical behavior. Some alternative solutions are discussed.
Journal Article
Stability of Surface State Electrons on Helium Films
Electrons on helium substrates form a model Coulomb system in which the transition from classical electron liquid to Wigner crystal is readily observed. However, attempts to increase the electron density in order to observe the ‘quantum melting’ of the system to a Fermi degenerate gas are hindered by an instability of the helium surface. Here we describe experimental efforts to reach the degenerate regime on thin helium films and microstructured substrates, for which the surface instability is suppressed. We demonstrate that, although the electron densities obtained exceed those for bulk helium substrates, observation of quantum melting remains challenging. We discuss possible solutions to the technical challenges involved.
Journal Article
The Helium Field Effect Transistor (II): Gated Transport of Surface-State Electrons Through Micro-constrictions
We present transport measurements of surface-state electrons on liquid helium films in confined geometry. The measurements are taken using split-gate devices similar to a field effect transistor. The number of electrons passing between the source and drain areas of the device can be precisely controlled by changing the length of the voltage pulse applied to the gate electrode. We find evidence that the effective driving potential depends on electron–electron interactions, as well as the electric field applied to the substrate. Our measurements indicate that the mobility of electrons on helium films can be high and that microfabricated transistor devices allow electron manipulation on length scales close to the interelectron separation. Our experiment is an important step toward investigations of surface-state electron properties at much higher densities, for which the quantum melting of the system to a degenerate Fermi gas should be observed.
Journal Article
Flux-periodicity crossover from h/2e to h/e in aluminium nano-loops
We study the magnetoresistance of aluminium 'double-networks' formed by connecting the vertexes of nano-loops with relatively long wires, creating two interlaced subnetworks of small and large loops (SL and LL, respectively). Far below the critical temperature, Aharonov-Bohm like quantum interference effects are observed for both the LL and the SL subnetworks. When approaching Tc, both exhibit the usual Little-Parks oscillations, with periodicity of the superconducting flux quantum Φ0=h/2e. For one sample, with a relatively large coherence length, ξ, at temperatures very close to Tc, the Φ0 periodicity of the SL disappears, and the waveform of the first period is consistent with that predicted recently for loops with a size a < ξ, indicating a crossover to 2Φ0 periodicity.
Journal Article