arXiv subjects

Hervé Aubin

Publications and source records attributed to Hervé Aubin.

9 recordsLinked to original sources

arXiv2023

Local dynamics and detection of topology in spin-1 chains

Antiferromagnetic spin-1 chains host the celebrated symmetry protected topological Haldane phase, whose spin-1/2 edge states were evidenced in bulk by, e.g., Electron Spin Resonance (ESR). Recent success in assembling effective spin-1 antiferromagnetic chains from nanographene and porphyrin molecules opens the possibility of local, site-by-site, characterization. The nascent technique of combined ESR-STM is able to measure the spin dynamics with atomic real-space resolution, and could fully reveal and manipulate the spin-1/2 degree of freedom. In this work, we combine exact diagonalization and DMRG to investigate the local dynamic spin structure factor of the different phases of the bilinear-biquadratic Hamiltonian with single-ion anisotropy in presence of an external magnetic field. We find that the signature of the Haldane phase is a low-energy peak created by singlet-triplet transitions in the edge-state manifold. We predict that the signature peak is experimentally observable, although for chains of length above N = 30 its energy should be first tuned by application of external magnetic field. We fully characterize the peak in real-space and energy, and further show its robustness to weak anisotropy and a relevant range of temperatures.

cond-mat.str-el↗

arXiv2023

Hydrogenic Spin-Valley states of the Bromine donor in 2H-MoTe$_2$

In semiconductors, the identification of doping atomic elements allowing to encode a qubit within spin states is of intense interest for quantum technologies. In transition metal dichalcogenides semiconductors, the strong spin-orbit coupling produces locked spin-valley states with expected long coherence time. Here we study the substitutional Bromine Br\textsubscript{Te} dopant in 2H-MoTe$_2$. Electron spin resonance measurements show that this dopant carries a spin with long-lived nanoseconds coherence time. Using scanning tunneling spectroscopy, we find that the hydrogenic wavefunctions associated with the dopant levels have characteristics spatial modulations that result from their hybridization to the \textbf{Q}-valleys of the conduction band. From a Fourier analysis of the conductance maps, we find that the amplitude and phase of the Fourier components change with energy according to the different irreducible representations of the impurity-site point-group symmetry. These results demonstrate that a dopant can inherit the locked spin-valley properties of the semiconductor and so exhibit long spin-coherence time.

cond-mat.mes-hall↗

arXiv2023

STM observation of the hinge-states of bismuth nanocrystals

The recent application of topological quantum chemistry to rhombohedral bismuth established the non-trivial band structure of this material. This is a 2$^{nd}$order topological insulator characterized by the presence of topology-imposed hinge-states. The spatial distribution of hinge-states and the possible presence of additional symmetry-protected surface-states is expected to depend on the crystal shape and symmetries. To explore this issue, we have grown bismuth nanocrystals in the tens of nanometers on the $(110)$ surface of InAs. By scanning tunneling spectroscopy, we mapped the local density of states on all facets and identified the presence of the hinge-states at the intersection of all facets. Our study confirm the classification of bulk bismuth as a 2$^{nd}$order topological insulator. We propose that the ubiquitous presence of the hinge-states result from their tunnel-coupling across the nanometer-sized facets.

cond-mat.mes-hall↗

arXiv2020

Wide critical fluctuations of the field-induced phase transition in graphite

In the immediate vicinity of the critical temperature (T$_c$) of a phase transition, there are fluctuations of the order parameter, which reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second-order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20 percent) reduction in the amplitude of the magnetic field (from 33 T to 27 T) leads to a threefold decrease of T$_c$ and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T$_c$. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the $3DXY$ universality class, similar to the case of $^4$He superfluid transition.

cond-mat.str-el↗

arXiv2018

Quantum confinement effects in Pb Nanocrystals grown on InAs

In the recent work of Ref.\cite{Vlaic2017-bs}, it has been shown that Pb nanocrystals grown on the electron accumulation layer at the (110) surface of InAs are in the regime of Coulomb blockade. This enabled the first scanning tunneling spectroscopy study of the superconducting parity effect across the Anderson limit. The nature of the tunnel barrier between the nanocrystals and the substrate has been attributed to a quantum constriction of the electronic wave-function at the interface due to the large Fermi wavelength of the electron accumulation layer in InAs. In this manuscript, we detail and review the arguments leading to this conclusion. Furthermore, we show that, thanks to this highly clean tunnel barrier, this system is remarkably suited for the study of discrete electronic levels induced by quantum confinement effects in the Pb nanocrystals. We identified three distinct regimes of quantum confinement. For the largest nanocrystals, quantum confinement effects appear through the formation of quantum well states regularly organized in energy and in space. For the smallest nanocrystals, only atomic-like electronic levels separated by a large energy scale are observed. Finally, in the intermediate size regime, discrete electronic levels associated to electronic wave-functions with a random spatial structure are observed, as expected from Random Matrix Theory.

cond-mat.supr-con↗

arXiv2017

Transport in a Single Self-Doped Nanocrystal

Addressing the optical properties of a single nanoparticle in the infrared is particularly challenging, thus alternative methods for characterizing the conductance spectrum of nanoparticles in this spectral range need to be developed. Here we describe an efficient method of fabricating single nanoparticle tunnel junctions on a chip circuit. We apply this method to narrow band gap nanoparticles of HgSe, which band structure combine the inverted character of the bulk semimetal with quantum confinement and self-doping. Upon tuning the gate bias, measurement reveals the presence of two energy gaps in the spectrum. The wider gap results from the interband gap, while the narrower gap results from intraband transitions. The observation of the latter near zero gate voltage confirms the doped character of the nanoparticle at the single particle level, which is in full agreement with the ensemble optical and transport measurements. Finally we probe the phototransport within a single quantum dot and demonstrate a large photogain mechanism resulting from photogating.

cond-mat.mes-hall↗

arXiv2017

Superconducting parity effect across the Anderson limit

How small superconductors can be? For isolated nanoparticles subject to quantum size effects, P.W. Anderson conjectured in 1959 that superconductivity could only exist when the electronic level spacing $δ$ is smaller than the superconducting gap energy $Δ$. Here, we report a scanning tunneling spectroscopy study of superconducting lead (Pb) nanocrystals grown on the (110) surface of InAs. We find that for nanocrystals of lateral size smaller than the Fermi wavelength of the 2D electron gas at the surface of InAs, the electronic transmission of the interface is weak; this leads to Coulomb blockade and enables the extraction of the electron addition energy of the nanocrystals. For large nanocrystals, the addition energy displays superconducting parity effect, a direct consequence of Cooper pairing. Studying this parity effect as function of nanocrystal volume, we find the suppression of Cooper pairing when the mean electronic level spacing overcomes the superconducting gap energy, thus demonstrating unambiguously the validity of the Anderson criterion.

cond-mat.supr-con↗

arXiv2017

Shiba Bound States across the mobility edge in doped InAs nanowires

We present a study of Andreev Quantum Dots (QDots) fabricated with small-diameter (30 nm) Si-doped InAs nanowires where the Fermi level can be tuned across a mobility edge separating localized states from delocalized states. The transition to the insulating phase is identified by a drop in the amplitude and width of the excited levels and is found to have remarkable consequences on the spectrum of superconducting SubGap Resonances (SGRs). While at deeply localized levels, only quasiparticles co-tunneling is observed, for slightly delocalized levels, Shiba bound states form and a parity changing quantum phase transition is identified by a crossing of the bound states at zero energy. Finally, in the metallic regime, single Andreev resonances are observed.

cond-mat.supr-con↗

arXiv2016

Nernst effect in metals and superconductors: a review of concepts and experiments

The Nernst effect is the transverse electric field produced by a longitudinal thermal gradient in presence of magnetic field. In the beginning of this century, Nernst experiments on cuprates were analyzed assuming that: i) The contribution of quasi-particles to the Nernst signal is negligible; and ii) Gaussian superconducting fluctuations cannot produce a Nernst signal well above the critical temperature. Both these assumptions were contradicted by subsequent experiments. This paper reviews experiments documenting multiple sources of a Nernst signal, which, according to the Brigman relation, measures the flow of transverse entropy caused by a longitudinal particle flow. Along the lines of Landauer's approach to transport phenomena, the magnitude of the transverse magneto-thermoelectric response is linked to the quantum of thermoelectric conductance and a number of material-dependent length scales: the mean-free-path, the Fermi wavelength, the de Broglie thermal wavelength and the superconducting coherence length. Extremely mobile quasi-particles in dilute metals generate a widely-documented Nernst signal. Fluctuating Cooper pairs in the normal state of superconductors have been found to produce a detectable Nernst signal with an amplitude conform to the Gaussian theory, first conceived by Ussishkin, Sondhi and Huse. In addition to these microscopic sources, mobile Abrikosov vortices, mesoscopic objects carrying simultaneously entropy and magnetic flux, can produce a sizeable Nernst response. Finally, in metals subject to a magnetic field strong enough to truncate the Fermi surface to a few Landau tubes, each exiting tube generates a peak in the Nernst response. The survey of these well-established sources of the Nernst signal is a helpful guide to identify the origin of the Nernst signal in other controversial cases.

cond-mat.str-el↗