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Multi-orbital physics in quasi-two-dimensional electron gases (q2DEGs) triggers intriguing phenomena not observed in bulk materials, such as unconventional superconductivity and magnetism. Here, we investigate the mechanism of orbital selective switching of the spin-polarization in the oxide q2DEG formed at the (001) interface between the LaAlO3, EuTiO3 and SrTiO3 band insulators. By using density functional theory calculations, transport, magnetic and x-ray spectroscopy measurements, we find that the filling of titanium-bands with 3dxz/3dyz orbital character in the EuTiO3 layer and at the interface with SrTiO3 induces an antiferromagnetic to ferromagnetic switching of the exchange interaction between Eu-4f7 magnetic moments. The results explain the observation of the carrier density-dependent ferromagnetic correlations and anomalous Hall effect in this q2DEG, and demonstrate how combined theoretical and experimental approaches can lead to a deeper understanding of emerging electronic phases and serve as a guide for the materials design of advanced electronic applications.

Since the discovery of high-temperature superconductors (HTSs), most efforts of researchers have been focused on the fabrication of superconducting devices capable of immobilizing vortices, hence of operating at enhanced temperatures and magnetic fields. Recent findings that geometric restrictions may induce self-arresting hypervortices recovering the dissipation-free state at high fields and temperatures made superconducting strips a mainstream of superconductivity studies. Here we report on the geometrical melting of the vortex lattice in a wide YBCO submicron bridge preceded by magnetoresistance (MR) oscillations fingerprinting the underlying regular vortex structure. Combined magnetoresistance measurements and numerical simulations unambiguously relate the resistance oscillations to the penetration of vortex rows with intermediate geometrical pinning and uncover the details of geometrical melting. Our findings offer a reliable and reproducible pathway for controlling vortices in geometrically restricted nanodevices and introduce a novel technique of geometrical spectroscopy, inferring detailed information of the structure of the vortex system through a combined use of MR curves and large-scale simulations.

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a ?-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.

Proximity-effect and resistance magneto-fluctuations measurements in submicron Nd1.2Ba1.8Cu3Oz (NBCO) nano-loops are reported to investigate coherent charge transport in the non-superconducting state. We find an unexpected inhibition of cooper pair transport, and a destruction of the induced superconductivity, by lowering the temperature from 6K to 250mK. This effect is accompanied by a significant change in the conductance-voltage characteristics and in the zero bias conductance response to the magnetic field pointing to the activation of a strong pair breaking mechanism at lower temperature. The data are discussed in the framework of mesoscopic effects specific to superconducting nanostructures, proximity effect and high temperature superconductivity., to appear on new journal of Physics

Current trends in data processing have given impetus for an intense search of new concepts of memory devices with emphasis on efficiency, speed, and scalability. A promising new approach to memory storage is based on resistance switching between charge-ordered domain states in the layered dichalcogenide 1T-TaS2. Here we investigate the energy efficiency scaling of such charge configuration memory (CCM) devices as a function of device size and data write time τW as well as other parameters that have bearing on efficient device operation. We find that switching energy efficiency scales approximately linearly with both quantities over multiple decades, departing from linearity only when τW approaches the ∼0.5 ps intrinsic switching limit. Compared to current state of the art memory devices, CCM devices are found to be much faster and significantly more energy efficient, demonstrated here with two-terminal switching using 2.2 fJ, 16 ps electrical pulses.

The development of “fault-tolerant” quantum computers, unaffected by noise and decoherence, is one of the fundamental challenges in quantum technology. One of the approaches currently followed is the realization of “topologically protected” qubits which make use of quantum systems characterized by a degenerate ground state of composite particles, known as “non-Abelian anyons”, able to encode and manipulate quantum information in a non-local manner. In this paper, we discuss the potential of quasi-two-dimensional electron gas (q2DEG) at the interface between band insulating oxides, like LaAlO3 and SrTiO3, as an innovative technological platform for the realization of topological quantum systems. Being characterized by a unique combination of unconventional spin-orbit coupling, magnetism, and 2D-superconductivity, these systems naturally possess most of the fundamental characteristics needed for the realization of a topological superconductor. These properties can be widely tuned by electric field effect acting on the orbital splitting and occupation of the non-degenerate 3dxy and 3dxz,yz bands. The topological state in oxide q2DEGs quasi-one-dimensional nanochannels could be therefore suitably controlled, leading to conceptual new methods for the realization of a topological quantum electronics based on the tuning of the orbital degrees of freedom. Copyright © 2021 EPLA

The electric-field effect control of two-dimensional electron gases (2-DEGs) has allowed nanoscale electron quantum transport to be explored in semiconductors. Structures based on transition metal oxides have electronic states that favour the emergence of novel quantum orders that are absent in conventional semiconductors and the 2-DEG formed at a LaAlO3/SrTiO3 interface—a structure in which superconductivity and spin–orbit coupling can coexist—is a promising platform to develop devices for spintronics and topological electronics. However, field-effect control of the properties of this interface at the nanoscale remains challenging. Here we show that a quantum point contact can be formed in a LaAlO3/SrTiO3 interface through electrostatic confinement of the 2-DEG using a split gate. Our device exhibits a quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels. Under a magnetic field, the direct observation of the Zeeman splitting between spin-polarized bands allows the determination of the Lande g-factor, whose value differs strongly from that of the free electrons. Through source–drain voltage measurements, we also performed a spectroscopic investigation of the 3d energy levels inside the quantum point contact. The LaAlO3/SrTiO3 quantum point contact could potentially be used as a spectrometer to probe Majorana states in an oxide 2-DEG. A quantum point contact formed in the two-dimensional electron gas of a LaAlO3/SrTiO3 interface exhibits quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels.

We present a study of the efficiency of LaAlO3/SrTiO3 nanoscale field effect devices realized in the side gate configuration. We show that a change in the resistance of more than four orders of magnitude and a voltage gain of up to 50 can be obtained with the application of a gate voltage smaller than 1V. At dilution temperatures, the nanodevices become superconducting and we demonstrate the possibility to obtain a superconductor to insulator transition by applying only 200 mV. These results are discussed in the view of applications for quantum electronics.

Superconductors are essential in many present and future technologies, from large-scale devices for medical imaging, accelerators, or fusion experiments to ultra-low-power superconducting electronics. However, their potential applicability, and particularly that of high-temperature superconductors (HTS), is severely affected by limited performances at large magnetic fields and high temperatures, where their use is most needed. One of the main reasons for these limitations is the presence of quantized vortices, whose movements result in losses, internal noise, and reduced performances. The conventional strategy to overcome the flow of vortices is to pin them along artificial defects. Here, we theoretically and experimentally demonstrate that critical-current density in high-temperature superconductors can reach unprecedented high values at high fields and temperatures by preventing vortex entry. By tailoring the geometry, that is, reducing the width, W, of nanowire-patterned HTS films, the range of the Meissner state, for which no vortices are present, is extended up to very large applied field values, on the order of ∼1 T. Current densities on the order of the depairing current can be sustained under high fields for a wide range of temperatures. Results may be relevant both for devising new conductors carrying depairing-current values at high temperatures and large magnetic fields and for reducing flux noise in sensors and quantum systems., We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence (SEV-2015-0496), COACHSUPENERGY project (MAT2014-51778-C2-1-R), and SuMaTe (RTI2018-095853-B-C21) cofinanced by the European Regional Development Fund, and project MAT2016-79426-P (Agencia Estatal de Investigacion/Fondo ́ Europeo de Desarrollo Regional). We also thank support from the European Union for NANOCOHYBRI project (Cost Action CA 16218) and from the Catalan Government projects SGR-2017-1519 and 2017-SGR-105. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. V.R. Acknowledges EU H2020 research and innovation programme (Marie Sklodowska-Curie IF grant agreement OXWALD 838693). We acknowledge the use of ICTS-CNM facilities and ICMAB scientific and technical services. We thank Alexey V. Pan for fruitful discussions.

Who would have imagined a supercurrent flowing between two phase coherent electrodes at 100 K? Josephson junctions (JJs) based on high temperature superconductors (HTS) can actually sustain large supercurrents at such unprecedented high temperatures. They are as well the first example of a wide class of devices based on exotic materials. In this chapter, we examine the properties of different types of HTS-based junctions and discuss notions on fabrication which is useful to classify junction properties.

We report on the magnetotransport properties of a spin-polarized two-dimensional electron system (2DES) formed in LaAlO3(LAO)/EuTiO3/SrTiO3 (STO) heterostructures. We show that, at low temperature, the 2DES magnetoconductance exhibits weak antilocalization corrections related to Rashba spin-orbit scattering, in analogy with the LAO/STO 2DES. However, the characteristic spin-orbit scattering field decreases substantially for carrier density higher than 1.9×1013cm-2. We attribute this behavior to the masking effect of ferromagnetism, which sets in at the same carrier density and at a temperature below 10 K. Our work shows that, while weak antilocalization corrections to the magnetoconductance are strongly reduced by the emergence of ferromagnetism, they persist in a large part of the phase diagram of a spin-polarized oxide 2DES.

High critical temperature superconductors (HTS) based devices can have impact in the study of the phase dynamics of Josephson junctions (JJs) thanks to the wide range of junction parameters they offer and to their unconventional properties. Measurements of current-voltage characteristics and of switching current distributions constitute a direct way to classify different regimes of the phase dynamics and of the transport, also in nontrivial case of the moderately damped regime (MDR). MDR is going to be more and more common in JJs with advances in nanopatterning superconductors and synthesizing novel hybrid systems. Distinctive signatures of macroscopic quantum tunneling and of thermal activation in presence of different tunable levels of dissipation have been detected in YBCO grain boundary JJs. Experimental data are supported by Monte Carlo simulations of the phase dynamics, in a wide range of temperatures and dissipation levels. This allows us to quantify dissipation in the MDR and partially reconstruct a phase diagram as guideline for a wide range of moderately damped systems. (C) 2014 Elsevier B.V. All rights reserved.

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides(1,2). Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (T-FM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.

In this paper, an experimental investigation of the main characteristics of a niobium nano Superconducting QUantum Interference Device (nanoSQUID) as a function of the temperature (9-0.3 K) is presented. The nanosensor consists of a niobium superconducting loop (0.4 × 1.0μm2) interrupted by two sandwich nanojunctions (Nb/Al-AlOx/Nb) having an area of about (300 × 300) nm2. These nanodevices have been fabricated by means of a focused ion beam sculpting method, which is used as a lithographic technique to define the various elements of the SQUID. We have performed measurements of current-voltage, critical current-magnetic flux characteristics, and switching current distributions from the zero voltage state for different temperatures. The high critical current modulation depths and the low intrinsic dissipation exhibited by these devices ensure a suitable sensitivity for nanoscale applications in the whole temperature range investigated.

We describe the main ideas and some experimental outcomes of two experiments, both aimed to study quantum tunneling effects in high-T-C structures. In the first experiment, macroscopic quantum tunneling is demonstrated in grain boundary YBaCuO Josephson junctions revealing dissipation levels lower than expected and opening novel perspectives for quantum circuitry. In the second, the low temperature dissipation is dominated by quantum tunneling of individual Pearl vortices in ultra-thin CaBaCuO systems characterized by extremely long Pearl lengths. (c) 2006 Elsevier B.V. All rights reserved.

We have developed a fabrication process that allows us to realize pure YBCO nanowires displaying robust superconductivity at widths w as low as 160 nm. We can modify the process in order to maintain a Au protective layer. This allows us to scale our nanowires even further to widths as low as 50 nm. We have studied how the presence of vortices and the occurrence of phase slips affect the transport properties of nanowires in the width range xi < w < lambda, being xi the coherence length and lambda the magnetic penetration depth. Magnetoresistance curves present features which are related to the effect of screening currents. Vortex entry barrier is found to scale with the width. Our findings confirm that for widths xi < w < lambda nanowires are better protected against phase slips and vortex flow. (C) 2014 Elsevier B.V. All rights reserved.

The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation which characterize a large majority of low critical current Josephson devices and are therefore an avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation.

Dissipation encodes the interaction of a quantum system with the environment and regulates the activation regimes of a Brownian particle. We have engineered grain boundary biepitaxial YBaCuO junctions to drive a direct transition from a quantum activated running state to a phase diffusion regime. The crossover to the quantum regime is tuned by the magnetic field and dissipation is described by a fully consistent set of junction parameters. To unravel phase dynamics in moderately damped systems is of general interest for advances in the comprehension of retrapping phenomena and in view of quantum hybrid technology.

YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA/cm2 at T = 4.2 K and of 10 MA/cm2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices., 8 pages, 3 figures. Accepted for publication in Superconductor Science and Technology

We report on the occurrence of multiple hopping and retrapping of a Brownian particle in a tilted washboard potential. The escape dynamic has been studied experimentally by measuring the switching current distributions as a function of temperature in a moderately damped NbN/MgO/NbN Josephson junction. At low temperatures the second moment of the distribution increases in agreement with calculations based on Kramers thermal activation regime. After a turn-over temperature T*, the shape of the distributions starts changing and width decreases with temperature. We analyze the data through fit of the switching probability and Monte Carlo simulations and we find a good agreement with a model based on a multiple retrapping process.

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d = 1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm(2) have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624471]

We have extended the off-axis biepitaxial technique to produce YBCO grain boundary junctions on low loss substrates. Excellent transport properties have been reproducibly found, with remarkable values of the quality factor I(c)R(n) (with I(c) the critical current and R(n) the normal state resistance) above 10 mV, far higher than the values commonly reported in the literature for high temperature superconductor (HTS) based Josephson junctions. The outcomes are consistent with a picture of a more uniform grain boundary region along the current path. This work supports a possible implementation of grain boundary junctions for various applications including terahertz sensors and HTS quantum circuits in the presence of microwaves.

Self-assembled YBaCuO diffusive grain boundary submicron Josephson junctions offer a realization of a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Their persistence at large voltages is evidence of the long lifetime of the antinodal (high energy) quasiparticles.

We report on super conducting behavior of Sr(3)Ru(2)O(7) macrodomain taken out from Sr(2)RuO(4)-Sr(3)Ru(2)O(7) eutectic crystals. Transport measurements performed down to 300 mK provide evidence of a supercurrent owing through the whole sample. Structural and compositional analyses are used to estimate the percentage of possible Sr(2)RuO(4) inclusions. On the base of a model that describes the system as a proximity network made of superconducting Sr(2)RuO(4) grains dispersed in the normal Sr(3)Ru(2)O(7) matrix we determine the conditions for having a percolating superflow through the system.

We report on measurements of the switching current probability both in the thermal and quantum regime on moderately damped NbN / MgO / NbN Josephson junctions with very low critical current density (’ 2 A / cm 2 ). We observed three distinct regimes: below 90 mK the width of the switching distributions is constant within the experimental error, while from 90 mK to 1.6 K the width increases with temperature, this change of behavior signaling the standard transition from macroscopic quantum tunneling (MQT) to thermal activation (TA) regime respectively. Above 1.6 K we find effects of anti-correlation between the bath temperature and thermal fluctuations since the width of the histograms starts to collapse, and indications of a transition to a third regime called Phase Diffusion (PD) are evident, such as the change of the switching distributions symmetry. Experimental data have been analyzed through numerical fitting of the switching probability and Monte Carlo simulations, and we found a good agreement with theoretical expectations based on multiple re-trapping processes, which strongly depend on the junction damping. The parameters which completely characterize the dynamics of such moderately damped Josephson junctions have been also extracted.