arXiv subjects

Adam Rycerz

Publications and source records attributed to Adam Rycerz.

At least 19 recordsLinked to original sources

arXiv2026

Josephson network as a model for high-temperature superconductor and beyond: Macroscopic quantum coherence probed by microwave absorption

Since the seminal experiment by Stankowski et al. [Phys. Rev. B 36, 7126 (1987)] on YBa$_2$Cu$_3$O$_{7-x}$, Magnetically-Modulated Microwave Absorption (MMMA) has become an important technique for detecting the superconducting transition in inhomogeneous ceramic compounds. The rise of microwave absorption below $T_c$ is accompanied by several low-field anomalies, usually attributed to the weak links between superconducting grains showing the Josephson effect, or to the dynamics of Abrikosov-Josephson vortices. In this article, we briefly review selected theoretical models rationalizing MMMA spectra for high-temperature superconductors, focusing on the $3$-dimensional array of Josephson junctions with random parameters including the resistivity, capacity and inductance of each junction, showing characteristic absorption anomalies observed in the experiment. The implications for recently studied microwave absorption in Josephson junction qubits are also discussed.

cond-mat.supr-con↗

arXiv2026

Mesoscopic Josephson effect in graphene disk at magnetic field

Unlike for tunneling Josephson junctions, for which the current-phase relation is given by the sine function, with the critical current ($I_c$) and normal-state resistance ($R_N$) following the relation $I_cR_N=(\pi/2)\,\Delta_0/e$ (where $\Delta_0$ is the superconducting gap and electron charge is $-e$), mesoscopic Josephson junctions show more complex current-phase relations, with the skewness $S>0$, what is related to the presence -- in case the leads are in the normal state -- of transmission probabilities taking the values comparable to $1$. Here, we show that these features also appear for a superconductor-graphene-superconductor (S-g-S) junction in the disk-shaped (Corbino) geometry, when the magnetic field is adjusted such that $I_c\rightarrow{}0$ and $R_N\rightarrow{}\infty$. In such a case, the product $I_cR_N\approx{}1.85\,\Delta_0/e$, and the skewness $S\approx{}0.14$. The results obtained from quantum-mechanical mode-matching analysis for the Dirac-Bogoliubov-De-Gennes equation are compared with simpler model assuming incoherent scattering between two circular interfaces separating the sample and the leads.

cond-mat.mes-hall↗

arXiv2026

Josephson effect in graphene Corbino disks

Peculiar features of the Josephson effect in graphene were described theoretically by Titov and Beenakker [Phys. Rev. B 74, 041401(R) (2006)], who solved the Dirac-Bogoliubov-de-Gennes equation for a superconductor-graphene-superconductor junction with rectangular geometry. Here, we adopt the analysis for graphene Corbino disks, finding out that -- for the outer to inner radii ratio $r_2/r_1\gtrsim{}5$ -- such systems may demonstrate, when varying the electrochemical potential and the spatial profile of the electrostatic barrier, crossover from standard Josephson tunneling (SJT), via graphene-specific multimode Dirac-Josephson tunneling (MDJT), towards the ballistic Josephson effect (BJE). Signatures of SJT appear only near the Dirac point when the barrier shape is close to rectangular, MDJT appears in the tripolar range and is very robust against varying the barrier shape, and BJE is restored in the unipolar range when smoothing the barrier shape. A comparison with the results of a numerical simulation of quantum transport on the honeycomb lattice is also given.

cond-mat.mes-hall↗

arXiv2026

Sub-Sharvin conductance and Josephson effect in graphene

Titov and Beenakker [Phys. Rev. B 74, 041401(R) (2006)] found, by solving the Dirac-Bogoliubov-De-Gennes equation, that the product of critical current and normal-state resistance for superconductor-graphene-superconductor (S-g-S) Josephson junction takes values (for a short junction and zero temperature) between $I_cR_N\approx{}2.1$ and $I_cR_N\approx{}2.4$ in units of $e/\Delta_0$, where $\Delta_0$ is the superconducting gap. These values are notably higher than the tunnelling bound ($\pi/2$), but lower than the ballistic bound ($\pi$). Here we analyze numerically the tunneling of Cooper pairs through S-g-S junctions in which the longitudinal electrostatic potential profile is tuned, within gates electrodes, from a rectangular to a parabolic one. In the unipolar regime (i.e., when the chemical potential is above the top of a barrier, $\mu>0$), it is found that $I_cR_N$ gradually evolves from the graphene-specific to the ballistic value. At the same time, the normal-state conductance increases from the sub-Sharvin value of $1/R_N\approx(\pi/4)\,G_{\rm Sharvin}$ towards to the Sharvin value $G_{\rm Sharvin}=g_0|\mu|W/(\pi\hbar{}v_F)$, with the conductance quantum $g_0=4e^2/h$, the junction width $W$, and the Fermi velocity in graphene $v_F$. In contrast, in the tripolar regime ($\mu<0$), both normal-state conductance and the critical current are suppressed when smoothing the potential; however, $I_c{}R_N$ remains close to the graphene-specific range, even for a parabolic potential. The skewness of the current-phase relation is also discussed.

cond-mat.mes-hall↗

arXiv2024

Quantum transport of Dirac fermions in selected graphene nanosystems away from the charge-neutrality point

Peculiar electronic properties of graphene, including the universal dc conductivity and the pseudodiffusive shot noise, are usually attributed to a small vicinity of the charge-neutrality point, away from which electron's effective mass raises, and nanostructures in graphene start to behave similarly to familiar Sharvin contacts in semiconducting heterostructures hosting two-dimensional electron gas. Using the effective Dirac equation for low-energy excitations it can be shown that, as long as abrupt potential steps separate the sample area from the leads, graphene-specific features can be identified even relatively far from the charge-neutrality point. Namely, the conductance is reduced, comparing to the standard Sharvin value, whereas the shot noise is amplified. Here, we confront the results of earlier analytic considerations with numerical simulations of quantum transport on the honeycomb lattice, for selected systems for which considerations starting from the Dirac equation cannot be directly adapted. For a wedge-shape constriction with zigzag edges and approximately square shape of the narrowest section, the transport characteristics can be tuned from graphene-specific sub-Sharvin values to standard Sharvin values, depending on whether the electrostatic potential profile in the narrowest section is rectangular or smooth. The half-Corbino disk with rectangular potential profile exhibits both the conductance and the noise close to the sub-Sharvin values. For a circular quantum dot with two narrow openings and irregular edges, the conductance is close to the Sharvin value, and the Fano factor approaches the value of $F=1/4$. This suggests that, in experimental attempt to verify the predictions for sub-Sharvin transport regime, one should focus rather on nanosystems, for which the scatterings on edges are insignificant next to the scatterings on sample-lead interfaces.

cond-mat.mes-hall↗

arXiv2024

Sub-Sharvin conductance and incoherent shot-noise in graphene disks at magnetic field

Highly-doped graphene samples show the conductance reduced and the shot-noise power enhanced compared to standard ballistic systems in two-dimensional electron gas. These features can be understood within a model assuming incoherent scattering of Dirac electrons between two interfaces separating the sample and the leads. Here we find, by adopting the above-mentioned model for the edge-free (Corbino) geometry and by means of the computer simulation of quantum transport, that another graphene-specific feature should be observable when the current flow through a doped disk is blocked by high magnetic field. In case the conductance drops to zero, the Fano factor approaches the value of $F\approx{}0.56$, with a very weak dependence on the disk radii ratio. The role of finite source-drain voltages and the system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape are also discussed.

cond-mat.mes-hall↗

arXiv2023

Strain-induced Aharonov-Bohm effect at nanoscale and ground state of a carbon nanotube with zigzag edges

Magnetic flux piercing a carbon nanotube induce periodic gap oscillations which represent the Aharonov-Bohm effect at nanoscale. Here we point out, by analyzing numerically the anisotropic Hubbard model on a honeycomb lattice, that similar oscillations should be observable when uniaxial strain is applied to a nanotube. In both cases, a vector potential (magnetic- or strain-induced) may affect the measurable quantities at zero field. The analysis, carried out within the Gutzwiller Approximation, shows that for small semiconducting nanotube with zigzag edges and realistic value of the Hubbard repulsion ($U/t_0=1.6$, with $t_0\approx{}2.5\,$eV being the equilibrium hopping integral) energy gap can be reduced by a factor of more than $100$ due to the strain.

cond-mat.mes-hall↗

arXiv2023

Thermoelectric properties of the Corbino disk in graphene

Thermopower and the Lorentz number for an edge-free (Corbino) graphene disk in the quantum Hall regime is calculated within the Landauer-B\"{u}ttiker formalism. We find, by varying the electrochemical potential, that amplitude of the Seebeck coefficient follows a modified Goldsmid-Sharp relation, in which energy gap is identified with the interval between zero-th and first Landau level in bulk graphene. Analogous relation for the Lorentz number is also determined. Therefore, these thermoelectric properties are solely defined by the magnetic field, temperature, the Fermi velocity in graphene, and fundamental constants including the electrons charge, the Planck and Boltzmann constants, being independent on the system geometric dimensions. This suggests that the Corbino disk in graphene may operate as a thermoelectric thermometer, allowing to determine small temperature difference between two reservoirs, if mean temperature and magnetic field are known.

cond-mat.mes-hall↗

arXiv2022

Mott transition in the Hubbard model on anisotropic honeycomb lattice with implications for strained graphene: Gutzwiller variational study

Modification of interatomic distances due to high pressure leads to exotic phenomena, including metallicity, superconductivity and magnetism, observed in materials not showing such properties in normal conditions. In two-dimensional crystals, such as graphene, atomic bond lengths can be modified by more that 10 percent by applying in-plane strain, i.e., without generating high pressure in the bulk. In this work, we study the strain-induced Mott transition on a honeycomb lattice by using computationally inexpensive techniques, including Gutzwiller Wave Function (GWF) and different variants of Gutzwiller Approximation (GA), obtaining the lower and upper bounds for critical Hubbard repulsion ($U$) of electrons. For uniaxial strain in the armchair direction the band gap is absent, and electron correlations play a dominant role. A significant reduction of the critical Hubbard $U$ is predicted. Model considerations are mapped onto tight-binding Hamiltonian for monolayer graphene by the auxiliary Su-Schrieffer-Heeger model for acoustic phonons, assuming zero stress in the direction perpendicular to the strain applied. Our results suggest that graphene, although staying in semimetallic phase even for extremely high uniaxial strains, may show measurable signatures of electron correlations, such as the band narrowing and the reduction of double occupancies.

cond-mat.str-el↗

arXiv2022

Theory of the sub-Sharvin charge transport in graphene disks

Ballistic graphene samples in a multimode regime show the sub-Sharvin charge transport, characterized by the conductance reduced by a factor of $\pi/4$ comparing to standard Sharvin contacts in two-dimensional electron gas, and the shot-noise power enhanced up to $F\approx{}1/8$ (with $F$ the Fano factor) [Phys. Rev. B 104, 165413 (2021)]. Here we consider the disk-shaped (Corbino) setup in graphene, with inner radius $r_1$ and outer radius $r_2$, finding that the multimode conductance is slightly enhanced for any $r_1<r_2$, reaching $(4\!-\!\pi)\approx{}0.8684$ of the Sharvin value for $r_1\ll{}r_2$. At the same limit, the Fano factor is reduced, approaching $(9\pi-28)/(12-3\pi)\approx{}0.1065<1/8$. Closed-form approximating expressions for any $r_1/r_2$ ratio are derived supposing incoherent scattering of Dirac fermions on asymmetric double barrier and compared with exact numerical results following from the mode-matching method. Sub-Sharvin values are restored in the narrow-disk limit $r_1/r_2\rightarrow{}1$. For experimentally-accessible radii ratios $0.5\leqslant{}r_1/r_2\leqslant{}0.8$ both the conductance and the Fano factor are noticeably closer to the values predicted for the $r_1\ll{}r_2$ limit, yet still differ from standard Sharvin transport characteristics. The system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape is studied numerically, and the crossover from the sub-Sharvin to standard Sharvin transport regime is demonstrated. Implications for a finite section of the disk are also discussed.

cond-mat.mes-hall↗

arXiv2021

Sub-Sharvin conductance and enhanced shot noise in doped graphene

Ideal Sharvin contact in a multimode regime shows the conductance $G\approx{}G_{\rm Sharvin}=g_0k_F{}W/\pi$ (with $g_0$ the conductance quantum, $k_F$ the Fermi momentum, and $W$ the contact width) accompanied by strongly suppressed shot-noise quantified by small Fano factor $F\approx{}0$. For ballistic graphene away from the charge-neutrality point the sub-Sharvin transport occurs, characterised by suppressed conductance $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ and enhanced shot noise $F\approx{}1/8$. All these results can be derived from a basic model of quantum scattering, involving assumptions of infinite height and perfectly rectangular shape of the potential barrier in the sample. Here we have carried out the numerical analysis of the scattering on a family of smooth barriers of finite height interpolating between parabollic and rectangular shapes. We find that tuning the barrier shape one can modify the asymmetry between electron- and hole-doped systems. For electronic dopings, the system crosses from Sharvin to sub-Sharvin transport regime (indicated by both the conductance and the Fano factor) as the potential becomes closer to the rectangular shape. In contrast, for hole dopings, the conductivity is strongly suppressed when the barrier is parabolic and slowly converges to $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ as the potential evolves towards rectangular shape. In such a case the Sharvin transport regime is inaccessible, shot noise is generically enhanced (with much slower convergence to $F\approx{}1/8$) comparing to the electron-doped case, and aperiodic oscillations of both $G$ and $F$ are prominent due to the formation of quasibound states.

cond-mat.mes-hall↗

arXiv2021

Wiedemann-Franz law for massless Dirac fermions with implications for graphene

In the 2016 experiment by Crossno et al. [Science 351, 1058 (2016)], electronic contribution to the thermal conductivity of graphene was found to violate the well-known Wiedemann-Franz (WF) law for metals. At liquid nitrogen temperatures, the thermal to electrical conductivity ratio of charge-neutral samples was more than 10 times higher than predicted by the WF law, what was attributed to interactions between particles leading to collective behavior described by hydrodynamics. Here we show, by adapting the handbook derivation of the WF law to the case of massless Dirac fermions, that significantly enhanced thermal conductivity should appear also in few- or even sub-kelvin temperatures, where the role of interactions can be neglected. The comparison with numerical results obtained within the Landauer-B\"uttiker formalism for rectangular and disk-shaped (Corbino) devices in ballistic graphene is also provided.

cond-mat.mes-hall↗

arXiv2020

Adiabatic pumping driven by moving kink and quantum standard ampere in buckled graphene nanoribbon

A quantum pump in buckled graphene ribbon with armchair edges is discussed numerically. By solving the Su-Schrieffer-Heeger model and performing the computer simulation of quantum transport we find that a kink adiabatically moving along the metallic ribbon results in highly-efficient pumping, with a charge per kink transition close to the maximal value determined by the Fermi velocity in graphene. Remarkably, insulating nanoribbon show the quantized value of a charge per kink ($2e$) in relatively wide range of the system parameters, providing a candidate for the quantum standard ampere. We attribute it to the presence of a localized electronic state, moving together with a kink, whose energy lies within the ribbon energy gap.

cond-mat.mes-hall↗

arXiv2020

Graphene disk in a solenoid magnetic potential: Aharonov-Bohm effect without a two-slit-like setup

The Aharonov-Bohm effect allows one to demonstrate the physical meaningfulness of magnetic vector potential by passing the current in zero magnetic field regions. In the standard (a {\em two-slit-like}) setup a conducting ring is pierced by magnetic flux and the quantum interference for an electron passing simultaneously the two ring arms is observed. Here we show, by analyzing the transport via evanescent waves, that the ballistic Corbino disk in graphene subjected to a solenoid magnetic potential may exhibit the conductance oscillations of the Aharonov-Bohm kind although the current flows through a single conducting element only.

cond-mat.mes-hall↗

arXiv2020

Adiabatic quantum pumping in buckled graphene nanoribbon driven by a kink

We propose a new type of quantum pump in buckled graphene nanoribbon, adiabatically driven by a kink moving along the ribbon. From a practical point of view, pumps with moving scatterers present advantages as compared to gate-driven pumps, like enhanced charge transfer per cycle per channel. The kink geometry is simplified by truncating the spatial arrangement of carbon atoms with the classical $\phi^4$ model solution, including a width renormalization following from the Su-Schrieffer-Heeger model for carbon nanostructures. We demonstrate numerically, as a proof of concept, that for moderate deformations a stationary kink at the ribbon center may block the current driven by the external voltage bias. In the absence of a bias, a moving kink leads to highly-effective pump effect, with a charge per unit cycle dependent on the gate voltage.

cond-mat.mes-hall↗

arXiv2019

Conductivity scaling and the effects of symmetry-breaking terms in bilayer graphene Hamiltonian

We study the ballistic conductivity of bilayer graphene in the presence of symmetry-breaking terms in effective Hamiltonian for low-energy excitations, such as the trigonal-warping term ($\gamma_3$), the electron-hole symmetry breaking interlayer hopping ($\gamma_4$), and the staggered potential ($\delta_{AB}$). Earlier, it was shown that for $\gamma_3\neq{}0$, in the absence of remaining symmetry-breaking terms (i.e., $\gamma_4=\delta_{AB}=0$), the conductivity ($\sigma$) approaches the value of $3\sigma_0$ for the system size $L\rightarrow{}\infty$ (with $\sigma_0=8e^2/(\pi{}h)$ being the result in the absence of trigonal warping, $\gamma_3=0$). We demonstrate that $\gamma_4\neq{}0$ leads to the divergent conductivity if $\gamma_3\neq{}0$, or to the vanishing conductivity if $\gamma_3=0$. For realistic values of the tight-binding model parameters, $\gamma_3=0.3\,$eV, $\gamma_4=0.15\,$eV (and $\delta_{AB}=0$), the conductivity values are in the range of $\sigma/\sigma_0\approx{}4-5$ for $100\,$nm$\ <L<1\,\mu$m, in agreement with existing experimental results. The staggered potential ($\delta_{AB}\neq{}0$) suppresses zero-temperature transport, leading to $\sigma\rightarrow{}0$ for $L\rightarrow{}\infty$. Although $\sigma=\sigma(L)$ is no longer universal, the Fano factor approaches the pseudodiffusive value ($F\rightarrow{}1/3$ for $L\rightarrow{}\infty$) in any case with non-vanishing $\sigma$ (otherwise, $F\rightarrow{}1$) signaling the transport is ruled by evanescent waves. Temperature effects are briefly discussed in terms of a phenomenological model for staggered potential $\delta_{AB}=\delta_{AB}(T)$ showing that, for $0<T\leqslant{}T_c\approx{}12\,$K and $\delta_{AB}(0)=1.5\,$meV, $\sigma(L)$ is noticeably affected by $T$ for $L\gtrsim{}100\,$nm.

cond-mat.mes-hall↗

arXiv2019

Mesoscopic valley filter in graphene Corbino disk containing a p-n junction

The Corbino geometry allows one to investigate the propagation of electric current along a p-n interface in ballistic graphene in the absence of edge states appearing for the familiar Hall-bar geometry. Using the transfer matrix in the angular-momentum space we find that for sufficiently strong magnetic fields the current propagates only in one direction, determined by the magnetic field direction and the interface orientation, and the two valleys, K and K', are equally occupied. Spatially-anisotropic effective mass may suppress one of the valley currents, selected by the external electric field, transforming the system into a mesoscopic version of the valley filter. The filtering mechanism can be fully understood within the effective Dirac theory, without referring to atomic-scale effects which are significant in proposals operating on localized edge states.

cond-mat.mes-hall↗

arXiv2018

Thermoelectric properties of gapped bilayer graphene

Unlike in conventional semiconductors, both the chemical potential and the band gap in bilayer graphene (BLG) can be tuned via application of external electric field. Among numerous device implications, this property also designates BLG as a candidate for high-performance thermoelectric material. In this theoretical study we have calculated the Seebeck coefficients for abrupt interface separating weakly- and heavily-doped areas in BLG, and for a more realistic rectangular sample of mesoscopic size, contacted by two electrodes. For a given band gap ($\Delta$) and temperature ($T$) the maximal Seebeck coefficient is close to the Goldsmid-Sharp value $|S|_{\rm max}^{\rm GS}=\Delta/(2eT)$, the deviations can be approximated by the asymptotic expression $|S|_{\rm max}^{\rm GS}-|S|_{\rm max}=(k_B/e)\times\left[\frac{1}{2}\ln{u}+\ln{}2-\frac{1}{2}+{\cal O}(u^{-1})\right]$, with the electron charge $-e$, the Boltzmann constant $k_B$, and $u = \Delta/(2k_BT)\gg{}1$. Surprisingly, the effects of trigonal warping term in the BLG low-energy Hamiltonian are clearly visible at few-Kelvin temperatures, for all accessible values of $\Delta\leqslant{}300\,$meV. We also show that thermoelectric figure of merit is noticeably enhanced ($ZT>3$) when a rigid substrate suppresses out-of-plane vibrations, reducing the contribution from $ZA$ phonons to the thermal conductivity.

cond-mat.mes-hall↗