arXiv subjects

T. T. Heikkila

Publications and source records attributed to T. T. Heikkila.

At least 19 recordsLinked to original sources

arXiv2025

Probing the flat-band limit of the superconducting proximity effect in Twisted Bilayer Graphene Josephson junctions

While extensively studied in normal metals, semimetals and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here the TBG acts as a highly tunable topological flat-band system, which due to its twist-angle dependent bandwidth, allows to probe the SC proximity effect at the crossover from the dispersive to the flat-band limit. Contrary to our original expectations, we find that the SC remains strong even in the flat-band limit, and gives rise to broad, dome shaped SC regions, in the filling dependent phase diagram. In addition, we find that unlike in conventional JJs, the critical current Ic strongly deviates from a scaling with the normal state conductance GN. We attribute these findings to the onset of strong electron interactions, which can give rise to an excess critical current, and also work out the potential importance of quantum geometric terms as well as multiband pairing mechanisms. Our results present the first detailed study of the SC proximity effect in the flat-band limit and shed new light on the mechanisms that drive the formation of SC domes in flat-band systems.

cond-mat.supr-con↗

arXiv2016

Momentum-space structure of surface states in a topological semimetal with a nexus point of Dirac lines

Three-dimensional topological semimetals come in different variants, either containing Weyl points or Dirac lines. Here we describe a more complicated momentum-space topological defect where several separate Dirac lines connect with each other, forming a momentum-space equivalent of the real-space nexus considered before for Helium-3. Close to the nexus the Dirac lines exhibit a transition from type I to type II lines. We consider a general model of stacked honeycomb lattices with the symmetry of Bernal (AB) stacked graphite and show that the structural mirror symmetries in such systems protect the presence of the Dirac lines, and also naturally lead to the formation of the nexus. By the bulk-boundary correspondence of topological media, the presence of Dirac lines lead to the formation of drumhead surface states at the side surfaces of the system. We calculate the surface state spectrum, and especially illustrate the effect of the nexus on these states.

cond-mat.mes-hall↗

arXiv2015

Nexus and Dirac lines in topological materials

We consider the $Z_2$ topology of the Dirac lines, i.e., lines of band contacts, on an example of graphite. Four lines --- three with topological charge $N_1=1$ each and one with $N_1=-1$ --- merge together near the H-point and annihilate due to summation law $1+1+1-1=0$. The merging point is similar to the real-space nexus, an analog of the Dirac monopole at which the $Z_2$ strings terminate.

cond-mat.mes-hall↗

arXiv2014

Single-photon cavity optomechanics mediated by a quantum two-level system

Coupling electromagnetic waves in a cavity and mechanical vibrations via the radiation pressure of the photons [1,2] is a promising platform for investigations of quantum mechanical properties of motion of macroscopic bodies and thereby the limits of quantum mechanics [3,4]. A drawback is that the effect of one photon tends to be tiny, and hence one of the pressing challenges is to substantially increase the interaction strength towards the scale of the cavity damping rate. A novel scenario is to introduce into the setup a quantum two-level system (qubit), which, besides strengthening the coupling, allows for rich physics via strongly enhanced nonlinearities [5-8]. Addressing these issues, here we present a design of cavity optomechanics in the microwave frequency regime involving a Josephson junction qubit. We demonstrate boosting of the radiation pressure interaction energy by six orders of magnitude, allowing to approach the strong coupling regime, where a single quantum of vibrations shifts the cavity frequency by more than its linewidth. We observe nonlinear phenomena at single-photon energies, such as an enhanced damping due to the two-level system. This work opens up nonlinear cavity optomechanics as a plausible tool for the study of quantum properties of motion.

cond-mat.mes-hall↗

arXiv2013

Non-universal shot noise in quasiequilibrium spin valves

We show that the breakdown of the Wiedemann-Franz law due to electron--electron scattering in diffusive spin valves may result in a strong suppression of the Fano factor that describes the ratio between shot noise and average current. In the parallel configuration of magnetizations, we find the universal value $\sqrt{3}/4$ in the absence of a normal-metal spacer layer, but including the spacer leads to a non-monotonous suppression of this value before reaching back to the universal value for large spacer lengths. On the other hand, in the case of an antiparallel configuration with a negligibly small spacer, the Fano factor is $\sqrt{3 (1-P^2)}/4$, where $P$ denotes the polarization of the conductivities. For $P\rightarrow \pm 1$, the current through the system is almost noiseless.

cond-mat.mes-hall↗

arXiv2012

Surface superconductivity in rhombohedral graphite

We show that rhombohedral graphite may support surface superconductivity with an unusual relation between the BCS coupling constant and the order parameter. This feature results from the properties of the states localized on the graphite surfaces. In a description including only the nearest neighbour coupling of the graphene layers, the surface states are topologically protected and have a flat band dispersion. We show that including higher order couplings destroys this flat band character and leads to a particle-hole symmetry breaking quadratic dispersion with a large effective mass. Employing this dispersion, we then show its effect on superconductivity and find two regimes of parameters, depending on the relation between the strength of the coupling constant and the details of the quadratic dispersion. For low coupling strengths, superconductivity is localized on the surfaces, but the order parameter is exponentially suppressed as in a conventional BCS superconductor, whereas for large coupling strengths we obtain surface superconductivity with a linear relation between the order parameter and the coupling constant. Our results may explain the recent findings of graphite superconductivity with a relatively high transition temperature.

cond-mat.supr-con↗

arXiv2011

Flat bands in topological media

Topological media are systems whose properties are protected by topology and thus are robust to deformations of the system. In topological insulators and superconductors the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac or Majorana fermions on the surface of the system and inside vortex cores. Here we show that in gapless topological media, the bulk-surface and bulk-vortex correspondence is more effective: it produces topologically protected gapless fermions without dispersion -- the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and we show that this property may give rise in particular to surface superconductivity which could exist even at room temperature.

cond-mat.str-el↗

arXiv2009

Phase sensitive electron-phonon coupling in a superconducting proximity structure

We study the role of the superconducting proximity effect on the electron-phonon energy exchange in diffusive normal metals (N) attached to superconductors (S). The proximity effect modifies the spectral response of the normal metal, in particular the local density of states. This leads to a weakening of the electron-phonon energy relaxation. We show that the effect is easily observable with modern thermometry methods, and predict that it can be tuned in structures connected to multiple superconductors by adjusting the phase difference between superconducting order parameters at the two NS interfaces.

cond-mat.mes-hall↗

arXiv2008

Effective capacitance in a single-electron transistor

Starting from the Kubo formula for conductance, we calculate the frequency-dependent response of a single-electron transistor (SET) driven by an ac signal. Treating tunneling processes within the lowest order approximation, valid for a wide range of parameters, we discover a finite reactive part even under Coulomb blockade due to virtual processes. At low frequencies this can be described by an effective capacitance. This effect can be probed with microwave reflection measurements in radio-frequency (rf) SET provided that the capacitance of the surroundings does not completely mask that of the SET.

cond-mat.mes-hall↗

arXiv2008

Ultrasensitive Proximity Josephson Sensor with Kinetic Inductance Read-Out

We propose a mesoscopic kinetic-inductance radiation detector based on a long superconductor--normal metal--superconductor Josephson junction. The operation of this proximity Josephson sensor (PJS) relies on large kinetic inductance variations under irradiation due to the exponential temperature dependence of the critical current. Coupled with a dc SQUID readout, the PJS is able to provide a signal to noise (S/N) ratio up to ~10^3 in the THz regime if operated as calorimeter, while electrical noise equivalent power (NEP) as low as ~7x10^{-20} W(Hz)^(-1/2) at 200 mK can be achieved in the bolometer operation. The high performance together with the ease of fabrication make this structure attractive as an ultrasensitive cryogenic detector of THz electromagnetic radiation.

cond-mat.mes-hall↗

arXiv2006

Wide-band detection of the third moment of shot noise by a hysteretic Josephson junction

We use a hysteretic Josephson junction as an on-chip detector of the third moment of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the present experiment. The third moment of shot noise results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We analyze the observed asymmetry assuming adiabatic response of the detector.

cond-mat.mes-hall↗

arXiv2006

Nonequilibrium characteristics in all-superconducting tunnel structures

We study the nonequilibrium characteristics of superconducting tunnel structures in the case when one of the superconductors is a small island confined between large superconductors. The state of this island can be probed for example via the supercurrent flowing through it. We study both the far-from-equilibrium limit when the rate of injection for the electrons into the island exceeds the energy relaxation inside it, and the quasiequilibrium limit when the electrons equilibrate between themselves. We also address the crossover between these limits employing the collision integral derived for the superconducting case. The clearest signatures of the nonequilibrium limit are the anomalous heating effects seen as a supercurrent suppression at low voltages, and the hysteresis at voltages close to the gap edge $2Δ/e$, resulting from the peculiar form of the nonequilibrium distribution function.

cond-mat.supr-con↗

arXiv2006

Influence of Supercurrents on Low-Temperature Thermopower in Mesoscopic N/S Structures

The thermopower of mesoscopic normal metal/superconductor structures has been measured at low temperatures. Effect of supercurrent present in normal part of the structure was studied in two cases: when it was created by applied external magnetic field and when it was applied directly using extra superconducting electrodes. Temperature and magnetic field dependencies of thermopower are compared to the numerical simulations based on the quasiclassical theory of the superconducting proximity effect.

cond-mat.supr-con↗

arXiv2005

Quantum transitions induced by the third cumulant of current fluctuations

We investigate the transitions induced by external current fluctuations on a small probe quantum system. The rates for the transitions between the energy states are calculated using the real-time Keldysh formalism for the density matrix evolution. We especially detail the effects of the third cumulant of current fluctuations inductively coupled to a quantum bit and propose a setup for detecting the frequency-dependent third cumulant through the transitions it induces.

cond-mat.mes-hall↗

arXiv2005

Circuit theory for noise in incoherent normal-superconducting dot structures

We consider the current fluctuations in a mesoscopic circuit consisting of nodes connected by arbitrary connectors, in a setup with multiple normal or superconducting terminals. In the limit of weak superconducting proximity effect, simplified equations for the second-order cross-correlators can be derived from the general counting field theory, and the result coincides with the semiclassical principle of minimal correlations. We discuss the derivation of this result in a multi-dot case.

cond-mat.supr-con↗

arXiv2005

Nonequilibrium phenomena in multiple normal-superconducting tunnel heterostructures

Using the nonequilibrium theory of superconductivity with the tunnel Hamiltonian, we consider a mesoscopic NISINISIN heterostructure, i.e., a structure consisting of five intermittent normal-metal (N) and superconducting (S) regions separated by insulating tunnel barriers (I). Applying the bias voltage between the outer normal electrodes one can drive the central N island very far from equilibrium. Depending on the resistance ratio of outer and inner tunnel junctions, one can realize either effective electron cooling in the central N island or create highly nonequilibrium energy distributions of electrons in both S and N islands. These distributions exhibit multiple peaks at a distance of integer multiples of the superconducting chemical potential. In the latter case the superconducting gap in the S islands is strongly suppressed as compared to its equilibrium value.

cond-mat.supr-con↗

arXiv2004

Observation of shot-noise-induced asymmetry in the Coulomb blockaded Josephson junction

We have investigated the influence of shot noise on the IV-curves of a single mesoscopic Josephson junction. We observe a linear enhancement of zero-bias conductance of the Josephson junction with increasing shot noise power. Moreover, the IV-curves become increasingly asymmetric. Our analysis on the asymmetry shows that the Coulomb blockade of Cooper pairs is strongly influenced by the non-Gaussian character of the shot noise.

cond-mat.mes-hall↗

arXiv2004

Mesoscopic supercurrent transistor controlled by nonequilibrium cooling

The distinctive quasiparticle distribution existing under nonequilibrium in a superconductor-insulator-normal metal-insulator-superconductor (SINIS) mesoscopic line is proposed as a novel tool to control the supercurrent intensity in a long Josephson weak link. We present a description of this system in the framework of the diffusive-limit quasiclassical Green-function theory and take into account the effects of inelastic scattering with arbitrary strength. Supercurrent enhancement and suppression, including a marked transition to a $π$-junction are striking features leading to a fully tunable structure. The role of the degree of nonequilibrium, temperature, and materials choice as well as features like noise, switching time, and current and power gain are also addressed.

cond-mat.mes-hall↗