Phase-Tunable Temperature Amplifier

Coherent caloritronics, the thermal counterpart of coherent electronics, has drawn growing attention since the discovery of heat interference in 2012. Thermal interferometers, diodes, transistors and nano-valves have been theoretically proposed and experimentally demonstrated by exploiting the quantum phase difference between two superconductors coupled through a Josephson junction. So far, the quantum-phase modulator has been realized in the form of a superconducting quantum interference device (SQUID) or a superconducting quantum interference proximity transistor (SQUIPT). Thence, an external magnetic field is necessary in order to manipulate the heat transport. Here, we theoretically propose the first on-chip fully thermal caloritronic device: the phase-tunable temperature amplifier. Taking advantage of a recent thermoelectric effect discovered in spin-split superconductors coupled to a spin-polarized system, by a temperature gradient we generate the magnetic flux controlling the transport through a temperature biased SQUIPT. By employing commonly used materials and a geometry compatible with state-of-the-art nano-fabrication techniques, we simulate the behavior of the temperature amplifier and define a number of figures of merit in full analogy with voltage amplifiers. Notably, our architecture ensures infinite input thermal impedance, maximum gain of about 11 and efficiency reaching the 95%. This device concept could represent a breakthrough in coherent caloritronic devices, and paves the way for applications in radiation sensing, thermal logics and quantum information.Comment: 7 pages, 3 figure

Phase-Tunable Thermal Logic: Computation with Heat

Boolean algebra, the branch of mathematics where variables can assume only true or false value, is the theoretical basis of classical computation. The analogy between Boolean operations and electronic switching circuits, highlighted by Shannon in 1938, paved the way to modern computation based on electronic devices. The grow of computational power of such devices, after an exciting exponential -Moore trend, is nowadays blocked by heat dissipation due to computational tasks, very demanding after the chips miniaturization. Heat is often a detrimental form of energy which increases the systems entropy decreasing the efficiency of logic operations. Here, we propose a physical system able to perform thermal logic operations by reversing the old heat-disorder epitome into a novel heat-order paradigm. We lay the foundations of heat computation by encoding logic state variables in temperature and introducing the thermal counterparts of electronic logic gates. Exploiting quantum effects in thermally biased Josephson junctions (JJs), we propound a possible realization of a functionally complete dissipationless logic. Our architecture ensures high operation stability and robustness with switching frequencies reaching the GHz

Impact of classical forces and decoherence in multi-terminal Aharonov-Bohm networks

Multi-terminal Aharonov-Bohm (AB) rings are ideal building blocks for quantum networks (QNs) thanks to their ability to map input states into controlled coherent superpositions of output states. We report on experiments performed on three-terminal GaAs/Al_(x)Ga_(1-x)As AB devices and compare our results with a scattering-matrix model including Lorentz forces and decoherence. Our devices were studied as a function of external magnetic field (B) and gate voltage at temperatures down to 350 mK. The total output current from two terminals while applying a small bias to the third lead was found to be symmetric with respect to B with AB oscillations showing abrupt phase jumps between 0 and pi at different values of gate voltage and at low magnetic fields, reminiscent of the phase-rigidity constraint due to Onsager-Casimir relations. Individual outputs show quasi-linear dependence of the oscillation phase on the external electric field. We emphasize that a simple scattering-matrix approach can not model the observed behavior and propose an improved description that can fully describe the observed phenomena. Furthermore, we shall show that our model can be successfully exploited to determine the range of experimental parameters that guarantee a minimum oscillation visibility, given the geometry and coherence length of a QN.Comment: 7 pages, 8 figure

High operating temperature in V-based superconducting quantum interference proximity transistors

Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52$\ \textrm{mV}/\Phi_0$) and flux-to-current (above 12$\ \textrm{nA}/\Phi_0$) transfer functions, with the best estimated flux sensitivity $\sim$2.6$\ \mu\Phi_0/\sqrt{\textrm{Hz}}$ reached under fixed voltage bias, where $\Phi_0$ is the flux quantum. The interferometers operate up to $T_\textrm{bath}\simeq$ 2 $\textrm{K}$, with an improvement of 70$\%$ of the maximal operating temperature with respect to early SQUIPTs design. The main features of the V-based SQUIPT are described within a simplified theoretical model. Our results open the way to the realization of SQUIPTs that take advantage of the use of higher-gap superconductors for ultra-sensitive nanoscale applications that operate at temperatures well above 1 K.Comment: Published version with Supplementary Informatio

Majorana bound states in hybrid 2D Josephson junctions with ferromagnetic insulators

We consider a Josephson junction consisting of superconductor/ferromagnetic insulator (S/FI) bilayers as electrodes which proximizes a nearby 2D electron gas. By starting from a generic Josephson hybrid planar setup we present an exhaustive analysis of the the interplay between the superconducting and magnetic proximity effects and the conditions under which the structure undergoes transitions to a non-trivial topological phase. We address the 2D bound state problem using a general transfer matrix approach that reduces the problem to an effective 1D Hamiltonian. This allows for straightforward study of topological properties in different symmetry classes. As an example we consider a narrow channel coupled with multiple ferromagnetic superconducting fingers, and discuss how the Majorana bound states can be spatially controlled by tuning the superconducting phases. Following our approach we also show the energy spectrum, the free energy and finally the multiterminal Josephson current of the setup.Comment: 8 pages; 5 figure

A superconducting absolute spin valve

A superconductor with a spin-split excitation spectrum behaves as an ideal ferromagnetic spin-injector in a tunneling junction. It was theoretical predicted that the combination of two such spin-split superconductors with independently tunable magnetizations, may be used as an ideal $absolute$ spin-valve. Here we report on the first switchable superconducting spin-valve based on two EuS/Al bilayers coupled through an aluminum oxide tunnel barrier. The spin-valve shows a relative resistance change between the parallel and antiparallel configuration of the EuS layers up to 900% that demonstrates a highly spin-polarized currents through the junction. Our device may be pivotal for realization of thermoelectric radiation detectors, logical element for a memory cell in cryogenics superconductor-based computers and superconducting spintronics in general.Comment: 6 pages, 4 color figures, 1 tabl

Color tuning of light-emitting-diodes by modulating the concentration of red-emitting silicon nanocrystal phosphors

Luminescent forms of nanostructured silicon have received significant attention in the context of quantum-confined light-emitting devices thanks to size-tunable emission wavelength and high-intensity photoluminescence, as well as natural abundance, low cost, and non-toxicity. Here, we show that red-emitting silicon nanocrystal (SiN) phosphors, obtained by electrochemical erosion of silicon, allow for effectively tuning the color of commercial light-emitting-diodes (LEDs) from blue to violet, magenta, and red, by coating the LED with polydimethylsiloxane encapsulating different SiN concentrations. High reliability of the tuning process, with respect to SiN fabrication and concentration, and excellent stability of the tuning color, with respect to LED bias current, is demonstrated through simultaneous electrical/optical characterization of SiN-modified commercial LEDs, thus envisaging exciting perspectives for silicon nanocrystals in the field of light-emitting applications