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We study the Hall effect in square, planar type-II superconductors using numerical simulations of time dependent Ginzburg-Landau (TDGL) equations. The Hall field in some type-II superconductors displays sign-change behavior at some magnetic fields due to the induced field of vortex flow, when its contribution is strong enough to reverse the field direction. In this work, we use modified TDGL equations which couple an externally applied current, and also incorporate normal-state and flux-flow Hall effects. We obtain the profile of Hall angle as a function of applied magnetic field for four different sizes (l\times l) of the superconductor: l/ \xi belongs to {3, 5, 15, 20}. We obtain vastly different profiles for each size, proving that size is an important parameter that determines Hall behavior. We find that electric field dynamics provides an insight into several anomalous features including signchange of Hall angle, and leads us to the precise transient behavior of order parameter responsible for them., Comment: 8 Pages, 8 Figures

We have probed the superconducting proximity effect through long high-quality monocrystalline Ag nanowires, by realizing Josephson junctions of different lengths, with different superconducting materials. Thanks to the high number of junctions probed, both the contact resistance and electron diffusion constant could be determined, enabling a comparison of the measured switching current to theoretical prediction, over the entire regime from short to long diffusive junctions. Although the length dependence of the switching current is as expected, the amplitude of the ${R}_{N}{I}_{c}$ product is smaller than predicted by theory. We also address the magnetic field dependence of the switching current. The quasi-Gaussian decay of the switching current with field expected in a long narrow junction is observed for all superconducting contacts we used except for aluminum. We present the striking nonmonotonous effect of field on the switching current of junctions with aluminum contacts when probed in a four-wire configuration and analyze it in terms of improved quasiparticle thermalization by a magnetic field.

Physical review / B 101(8), 085121 (1-10) (2020). doi:10.1103/PhysRevB.101.085121, We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator SnO$_2$ and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions—an extraordinary fact recalling the well known catalytic properties SnO$_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of n-type conductivity in SnO$_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band., Published by Inst., Woodbury, NY

In the version of this Article originally published, B. Andrei Bernevig was missing the following two additional affiliations: Physics Department, Freie Universitat Berlin, Berlin, Germany, and Max Planck Institute of Microstructure Physics, Halle, Germany. This has been corrected in the online versions of the Article.

The mathematical field of topology has become a framework to describe the low-energy electronic structure of crystalline solids. A typical feature of a bulk insulating three-dimensional topological crystal are conducting two-dimensional surface states. This constitutes the topological bulk-boundary correspondence. Here, we establish that the electronic structure of bismuth, an element consistently described as bulk topologically trivial, is in fact topological and follows a generalized bulk-boundary correspondence of higher-order: not the surfaces of the crystal, but its hinges host topologically protected conducting modes. These hinge modes are protected against localization by time-reversal symmetry locally, and globally by the three-fold rotational symmetry and inversion symmetry of the bismuth crystal. We support our claim theoretically and experimentally. Our theoretical analysis is based on symmetry arguments, topological indices, first-principle calculations, and the recently introduced framework of topological quantum chemistry. We provide supporting evidence from two complementary experimental techniques. With scanning-tunneling spectroscopy, we probe the unique signatures of the rotational symmetry of the one-dimensional states located at step edges of the crystal surface. With Josephson interferometry, we demonstrate their universal topological contribution to the electronic transport. Our work establishes bismuth as a higher-order topological insulator., Comment: 16 pages, 6 figures, accepted manuscript

Electronic properties of low dimensional superconductors are determined by many-body-effects. This physics has been studied traditionally with superconducting thin films, and in recent times with two-dimensional electron gases (2DEGs) at oxide interfaces. In this work, we show that a superconducting 2DEG can be generated by simply evaporating a thin layer of metallic Al under ultra-high vacuum on a SrTiO3 crystal, whereby Al oxidizes into amorphous insulating alumina, doping the SrTiO3 surface with oxygen vacancies. The superconducting critical temperature of the resulting 2DEG is found to be tunable with a gate voltage with a maximum value of 360 mK. A gate-induced switching between superconducting and resistive states is demonstrated. Compared to conventionally-used pulsed-laser deposition (PLD), our work simplifies to a large extent the process of fabricating oxide-based superconducting 2DEGs. It will make such systems accessible to a broad range of experimental techniques useful to understand low-dimensional phase transitions and complex many-body-phenomena in electronic systems., Comment: 15 pages, 5 figures

The protection against backscattering provided by topology is a striking property. In two-dimensional insulators, a consequence of this topological protection is the ballistic nature of the one-dimensional helical edge states. One demonstration of ballisticity is the quantized Hall conductance. Here we provide another demonstration of ballistic transport, in the way the edge states carry a supercurrent. The system we have investigated is a micrometre-long monocrystalline bismuth nanowire with topological surfaces, that we connect to two superconducting electrodes. We have measured the relation between the Josephson current flowing through the nanowire and the superconducting phase difference at its ends, the current–phase relation. The sharp sawtooth-shaped phase-modulated current–phase relation we find demonstrates that transport occurs selectively along two ballistic edges of the nanowire. In addition, we show that a magnetic field induces 0–π transitions and φ0-junction behaviour, providing a way to manipulate the phase of the supercurrent-carrying edge states and generate spin supercurrents., Demonstration of ballistic conduction and spin polarization of edge state currents in two dimensional topological insulators remains a challenge. Here, Murani et al. report a direct signature of ballistic one dimensional transport along the topological surfaces of a bismuth nanowire connected to superconducting electrodes.

International audience; 2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3-based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

Superconducting nanowires can be fabricated by decomposition of an organometallic gas using a focused beam of Ga ions. However, physical damage and unintentional doping often results from the exposure to the ion beam, motivating the search for a means to achieve similar structures with a beam of electrons instead of ions. This has so far remained an experimental challenge. We report the fabrication of superconducting tungsten nanowires by electron-beam-induced-deposition, with critical temperature of 2.0 K and critical magnetic field of 3.7 T, and compare them with superconducting wires made with ions. This work opens up new possibilities for the realization of nanoscale superconducting devices, without the requirement of an ion beam column., Comment: 5 pages, 4 figures

Using graphene resonator, we perform electromechanical measurements in quantum Hall regime to probe the coupling between a quantum Hall (QH) system and its mechanical motion. Mechanically perturbing the QH state through resonance modifies the DC resistance of the system and results in a Fano-lineshape due to electronic interference. Magnetization of the system modifies the resonator's equilibrium position and effective stiffness leading to changes in resonant frequency. Our experiments show that there is an intimate coupling between the quantum Hall state and mechanics - electron transport is affected by physical motion and in turn the magnetization modifies the electromechanical response., For supplemental material please go to http://www.tifr.res.in/~nano/publications.htm

The elastic response of suspended NbSe3 nanowires is studied across the charge density wave phase transition. The nanoscale dimensions of the resonator lead to a large resonant frequency (10-100 MHz), bringing the excited phonon frequency in close proximity of the plasmon mode of the electronic condensate - a parameter window not accessible in bulk systems. The interaction between the phonon and plasmon modes strongly modifies the elastic properties at high frequencies. This is manifested in the nanomechanics of the system as a sharp peak in the temperature dependence of the elastic modulus (relative change of 12.8%) in the charge density wave phase., Comment: 6 pages, 3 figures

We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins., 4 pages, 3 figures

We measure the thermal conductivity ($\kappa$) of individual InAs nanowires (NWs), and find that it is 3 orders of magnitude smaller than the bulk value in the temperature range of 10 to 50 K. We argue that the low $\kappa$ arises from the strong localization of phonons in the random superlattice of twin-defects oriented perpendicular to the axis of the NW. We observe significant electronic contribution arising from the surface accumulation layer which gives rise to tunability of $\kappa$ with the application of electrostatic gate and magnetic field. Our devices and measurements of $\kappa$ at different carrier concentrations and magnetic field without introducing structural defects, offer a means to study new aspects of nanoscale thermal transport., Comment: 5 pages, 4 figures, URL for supplementary material included in references

(thickness∼30-50 nm) to probe charge density wave (CDW) physics at nanoscale. Variation ofelastic and electronic properties accompanying the CDW phase transition (around 30 K) are in-vestigated simultaneously using the devices as self-sensing heterodyne mixers. Elastic modulus isobserved to change by 10 per cent, an amount significantly larger than what had been reported ear-lier in the case of bulk crystals. We also study the modulation of conductance by electric field effect,and examine its relation to the order parameter and the CDW energy gap at the Fermi surface.

We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 K to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300K and 30K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With lowering of temperature, we find that the positively dispersing electromechanical modes evolve to negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature compensated sensors., For supplementary information and high resolution figures please go to http://www.tifr.res.in/~deshmukh/publication.html

We probe electro-mechanical properties of InAs nanowire (diameter ~ 100 nm) resonators where the suspended nanowire (NW) is also the active channel of a field effect transistor (FET). We observe and explain the non-monotonic dispersion of the resonant frequency with DC gate voltage (VgDC). The effect of electronic screening on the properties of the resonator can be seen in the amplitude. We observe the mixing of mechanical modes with VgDC. We also experimentally probe and quantitatively explain the hysteretic non-linear properties, as a function of VgDC, of the resonator using the Duffing equation., Comment: For supporting information and high resolution figures please go to the following link http://www.tifr.res.in/~deshmukh/publication.html

In this report, we study electromechanical properties of a suspended InAs nanowire (NW) resonator. At low temperatures, the NW acts as the island of a single electron transistor (SET) and we observe a strong coupling between electrons and mechanical modes at resonance; the rate of electron tunneling is approximately 10 times the resonant frequency. Above and below the mechanical resonance, the magnitude of Coulomb peaks is different and we observe Fano resonance in conductance due to the interference between two contributions to potential of the SET. The quality factor ($Q$) of these devices is observed $\sim10^5$ at 100 mK., Comment: 4 pages. Supplementary material at http://www.tifr.res.in/~nano

We present a simple fabrication technique for lateral nanowire wrap-gate devices with high capacitive coupling and field-effect mobility. Our process uses e-beam lithography with a single resist-spinning step, and does not require chemical etching. We measure, in the temperature range 1.5-250 K, a subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 $cm^2/Vs$ -- significantly larger than previously reported lateral wrap-gate devices. At depletion, the barrier height due to the gated region is proportional to applied wrap-gate voltage., Comment: 3 pages, 3 figures

We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300 and 30 K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With a lowering of temperature, we find that the positively dispersing electromechanical modes evolve into negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature-compensated sensors.