arXiv subjects

M. Rosticher

Publications and source records attributed to M. Rosticher.

16 recordsLinked to original sources

arXiv2023

Electroluminescence and Energy Transfer Mediated by Hyperbolic Polaritons

Under high electrical current, some materials can emit electromagnetic radiation beyond incandescence. This phenomenon, referred to as electroluminescence, leads to the efficient emission of visible photons and is the basis of domestic lighting devices (e.g., light-emitting diodes). In principle, electroluminescence can lead to mid-infrared (mid-IR) emission of confined light-matter excitations called phonon-polaritons, resulting from the coupling of photons with crystal lattice vibrations (optical phonons). In particular, phonon-polaritons arising in the van der Waals crystal hexagonal boron nitride (hBN) exhibit hyperbolic dispersion, which enhances light-matter coupling. For this reason, electroluminescence of hyperbolic phonon-polaritons (HPhPs) has been proposed as an explanation for the peculiar radiative energy transfer within hBN-encapsulated graphene transistors. However, since HPhPs are confined, they are inaccessible in the far-field, so that any hint of electroluminescence is only based on indirect electronic signatures and needs to be confirmed by direct observation. Here, we demonstrate far-field mid-IR ({\lambda} = 6.5 {\mu}m) electroluminescence of HPhPs excited by strongly biased high-mobility graphene within a van der Waals heterostructure, and we quantify the associated radiative energy transfer through the material. The presence of HPhPs is revealed via far-field mid-IR spectroscopy due to their elastic scattering at discontinuities in the heterostructure. The associated radiative flux is quantified by mid-IR pyrometry of the substrate receiving the energy. This radiative energy transfer is shown to be reduced in hBN with nanoscale inhomogeneities, demonstrating the central role of the electromagnetic environment in this process.

cond-mat.mes-hall↗

arXiv2023

High-sensitivity AC-charge detection with a MHz-frequency fluxonium qubit

Owing to their strong dipole moment and long coherence times, superconducting qubits have demonstrated remarkable success in hybrid quantum circuits. However, most qubit architectures are limited to the GHz frequency range, severely constraining the class of systems they can interact with. The fluxonium qubit, on the other hand, can be biased to very low frequency while being manipulated and read out with standard microwave techniques. Here, we design and operate a heavy fluxonium with an unprecedentedly low transition frequency of $1.8~\mathrm{MHz}$. We demonstrate resolved sideband cooling of the ``hot'' qubit transition with a final ground state population of $97.7~\%$, corresponding to an effective temperature of $23~\mu\mathrm{K}$. We further demonstrate coherent manipulation with coherence times $T_1=34~\mu\mathrm{s}$, $T_2^*=39~\mu\mathrm{s}$, and single-shot readout of the qubit state. Importantly, by directly addressing the qubit transition with a capacitively coupled waveguide, we showcase its high sensitivity to a radio-frequency field. Through cyclic qubit preparation and interrogation, we transform this low-frequency fluxonium qubit into a frequency-resolved charge sensor. This method results in a charge sensitivity of $33~\mu\mathrm{e}/\sqrt{\mathrm{Hz}}$, or an energy sensitivity (in joules per hertz) of $2.8~\hbar$. This method rivals state-of-the-art transport-based devices, while maintaining inherent insensitivity to DC charge noise. The high charge sensitivity combined with large capacitive shunt unlocks new avenues for exploring quantum phenomena in the $1-10~\mathrm{MHz}$ range, such as the strong-coupling regime with a resonant macroscopic mechanical resonator.

quant-ph↗

arXiv2023

Electroluminescence of the graphene 2D semi-metal

Electroluminescence, a non-thermal radiative process, is ubiquitous in semi-conductors and insulators but fundamentally precluded in metals. We show here that this restriction can be circumvented in high-quality graphene. By investigating the radiative emission of semi-metallic graphene field-effect transistors over a broad spectral range, spanning the near- and mid-infrared, we demonstrate direct far-field electroluminescence from hBN-encapsulated graphene in the mid-infrared under large bias in ambient conditions. Through a series of test experiments ruling out its incandescence origin, we determine that the electroluminescent signal results from the electrical pumping produced by interband tunneling. We show that the mid-infrared electroluminescence is spectrally shaped by a natural quarter-wave resonance of the heterostructure. This work invites a reassessment of the use of metals and semi-metals as non-equilibrium light emitters, and the exploration of their intriguing specificities in terms of carrier injection and relaxation, as well as emission tunability and switching speed.

cond-mat.mes-hall↗

arXiv2023

Magneto-exciton limit of quantum Hall breakdown in graphene

One of the intrinsic drift velocity limit of the quantum Hall effect is the collective magneto-exciton (ME) instability. It has been demonstrated in bilayer graphene (BLG) using noise measurements. We reproduce this experiment in monolayer graphene (MLG), and show that the same mechanism carries a direct relativistic signature on the breakdown velocity. Based on theoretical calculations of MLG- and BLG-ME spectra, we show that Doppler-induced instabilities manifest for a ME phase velocity determined by a universal value of the ME conductivity, set by the Hall conductance.

cond-mat.mes-hall↗

arXiv2023

High-field 1/f noise in hBN-encapsulated graphene transistors

Low-frequency 1/f noise in electronics is a conductance fluctuation, that has been expressed in terms of a mobility "$\alpha$-noise" by Hooge and Kleinpenning. Understanding this noise in graphene is a key towards high-performance electronics. Early investigations in diffusive graphene have pointed out a deviation from the standard Hooge formula, with a modified expression where the free-carrier density is substituted by a constant density $n_\Delta\sim10^{12}\;\mathrm{cm^{-2}}$. We investigate hBN-encapsulated graphene transistors where high mobility gives rise to the non-linear velocity-saturation regime. In this regime, the $\alpha$-noise is accounted for by substituting conductance by differential conductance $G$, ressulting in a bell-shape dependence of flicker noise with bias voltage $V$. The same analysis holds at larger bias in the Zener regime, with two main differences: the first one is a strong enhancement of the Hooge parameter reflecting the hundred-times larger coupling of interband excitations to the hyperbolic phonon-polariton (HPhP) modes of the mid-infrared Reststrahlen (RS) bands of hBN. The second is an exponential suppression of this coupling at large fields, which we attribute to decoherence effects. We also show that the HPhP bands control the amplitude of flicker noise according to the graphene-hBN thermal coupling estimated with microwave noise thermometry. The phenomenology of $\alpha$-noise in graphene supports a quantum-coherent bremsstrahlung interpretation of flicker noise.

cond-mat.mes-hall↗

arXiv2022

Mesoscopic Klein-Schwinger effect in graphene

Strong electric field annihilation by particle-antiparticle pair creation, also known as the Schwinger effect, is a non-perturbative prediction of quantum electrodynamics. Its experimental demonstration remains elusive, as threshold electric fields are extremely strong and beyond current reach. Here, we propose a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac fermions with an approximate electron-hole symmetry. Using transport measurements, we report on universal 1d-Schwinger conductance at the pinchoff of ballistic graphene transistors. Strong pinchoff electric fields are concentrated within approximately 1 $\mu$m of the transistor's drain, and induce Schwinger electron-hole pair creation at saturation. This effect precedes a collective instability toward an ohmic Zener regime, which is rejected at twice the pinchoff voltage in long devices. These observations advance our understanding of current saturation limits in ballistic graphene and provide a direction for further quantum electrodynamic experiments in the laboratory.

cond-mat.mes-hall↗

arXiv2022

One hundred second bit-flip time in a two-photon dissipative oscillator

Current implementations of quantum bits (qubits) continue to undergo too many errors to be scaled into useful quantum machines. An emerging strategy is to encode quantum information in the two meta-stable pointer states of an oscillator exchanging pairs of photons with its environment, a mechanism shown to provide stability without inducing decoherence. Adding photons in these states increases their separation, and macroscopic bit-flip times are expected even for a handful of photons, a range suitable to implement a qubit. However, previous experimental realizations have saturated in the millisecond range. In this work, we aim for the maximum bit-flip time we could achieve in a two-photon dissipative oscillator. To this end, we design a Josephson circuit in a regime that circumvents all suspected dynamical instabilities, and employ a minimally invasive fluorescence detection tool, at the cost of a two-photon exchange rate dominated by single-photon loss. We attain bit-flip times of the order of 100 seconds for states pinned by two-photon dissipation and containing about 40 photons. This experiment lays a solid foundation from which the two-photon exchange rate can be gradually increased, thus gaining access to the preparation and measurement of quantum superposition states, and pursuing the route towards a logical qubit with built-in bit-flip protection.

quant-ph↗

arXiv2022

Dielectric permittivity, conductivity and breakdown field of hexagonal boron nitride

In view of the extensive use of hexagonal boron nitride (hBN) in 2D material electronics, it becomes important to refine its dielectric characterization in terms of low-field permittivity and high-field strength and conductivity up to the breakdown voltage. The present study aims at filling this gap using DC and RF transport in two Au-hBN-Au capacitor series of variable thickness in the 10--100 nm range, made of large high-pressure, high-temperature (HPHT) crystals and a polymer derivative ceramics (PDC) crystals. We deduce an out-of-plane low field dielectric constant $\epsilon_\parallel=3.4\pm0.2$ consistent with the theoretical prediction of Ohba et al., that narrows down the generally accepted window $\epsilon_\parallel=3$--$4$. The DC-current leakage at high-field is found to obey the Frenkel-Pool law for thermally-activated trap-assisted electron transport with a dynamic dielectric constant $\epsilon_\parallel\simeq3.1$ and a trap energy $\Phi_B\simeq1.3\;\mathrm{eV}$, that is comparable with standard technologically relevant dielectrics.

cond-mat.mtrl-sci↗

arXiv2020

Edge mode engineering for optimal ultracoherent silicon nitride membranes

Due to their high force sensitivity, mechanical resonators combining low mechanical dissipation with a small motional mass are highly demanded in fields as diverse as resonant force imaging, mass sensing, or cavity optomechanics. "Soft-clamping" is a phononic engineering technique by which mechanical modes of highly-stressed membranes or strings are localized away from lossy regions, thereby enabling ultrahigh-Q for ng-scale devices. Here, we report on parasitic modes arising from the finite size of the structure which can significantly degrade the performance of these membranes. Through interferometric measurements and finite-element simulations, we show that these parasitic modes can hybridize with the localized modes of our structures, reducing the quality factors by up to one order of magnitude. To circumvent this problem, we engineer the spectral profile of the parasitic modes in order to avoid their overlap with the high-Q defect mode. We verify via a statistical analysis that this modal engineering reproducibly yields higher quality factors in fabricated devices, consistent with theoretically predicted values.

physics.app-ph↗

arXiv2020

Ultrasensitive Photoresponse of Graphene Quantum Dot in the Coulomb Blockade Regime to THz Radiation

Graphene quantum dots (GQDs) have recently attracted considerable attention, with appealing properties for terahertz (THz) technology. This includes the demonstration of large thermal bolometric effects in GQDs when illuminated by THz radiation. However, the interaction of THz photons with GQDs in the Coulomb blockade regime - single electron transport regime - remains unexplored. Here, we demonstrate the ultrasensitive photoresponse to THz radiation (from <0.1 to 10 THz) of a hBN-encapsulated GQD in the Coulomb blockade regime at low temperature (170 mK). We show that THz radiation of $\sim$10 pW provides a photocurrent response in the nanoampere range, resulting from a renormalization of the chemical potential of the GQD of $\sim$0.15 meV. We attribute this photoresponse to an interfacial photogating effect. Furthermore, our analysis reveals the absence of thermal effects, opening new directions in the study of coherent quantum effects at THz frequencies in GQDs.

cond-mat.mes-hall↗

arXiv2019

A corner reflector of graphene Dirac fermions as a phonon-scattering sensor

Dirac fermion optics exploits the refraction of chiral fermions across optics-inspired Klein-tunneling barriers defined by high-transparency p-n junctions. We consider the corner reflector (CR) geometry introduced in optics or radars. We fabricate Dirac fermion CRs using bottom-gate-defined barriers in hBN-encapsulated graphene. By suppressing transmission upon multiple internal reflections, CRs are sensitive to minute phonon scattering rates. We report on doping-independent CR transmission in quantitative agreement with a simple scattering model including thermal phonon scattering. As a new signature of CRs, we observe Fabry-Pérot oscillations at low temperature, consistent with single-path reflections. Finally, we demonstrate high-frequency operation which promotes CRs as fast phonon detectors. Our work establishes the relevance of Dirac fermion optics in graphene and opens a route for its implementation in topological Dirac matter.

cond-mat.mes-hall↗

arXiv2018

Ultra-long wavelength Dirac plasmons in graphene capacitors

Graphene is a valuable 2D platform for plasmonics as illustrated in recent THz and mid-infrared optics experiments. These high-energy plasmons however, couple to the dielectric surface modes giving rise to hybrid plasmon-polariton excitations. Ultra-long-wavelengthes address the low energy end of the plasmon spectrum, in the GHz-THz electronic domain, where intrinsic graphene Dirac plasmons are essentially decoupled from their environment. However experiments are elusive due to the damping by ohmic losses at low frequencies. We demonstrate here a plasma resonance capacitor (PRC) using hexagonal boron-nitride (hBN) encapsulated graphene at cryogenic temperatures in the near ballistic regime. We report on a $100\;\mathrm{μm}$ quarter-wave plasmon mode, at $40\;\mathrm{GHz}$, with a quality factor $Q\simeq2$. The accuracy of the resonant technique yields a precise determination of the electronic compressibility and kinetic inductance, allowing to assess residual deviations from intrinsic Dirac plasmonics. Our capacitor GHz experiment constitutes a first step toward the demonstration of plasma resonance transistors for microwave detection in the sub-THz domain for wireless communications and sensing. It also paves the way to the realization of doping modulated superlattices where plasmon propagation is controlled by Klein tunneling.

cond-mat.mes-hall↗

arXiv2018

Contact gating at GHz frequency in graphene

The paradigm of graphene transistors is based on the gate modulation of the channel carrier density by means of a local channel gate. This standard architecture is subject to the scaling limit of the channel length and further restrictions due to access and contact resistances impeding the device performance. We propose a novel design, overcoming these issues by implementing additional local gates underneath the contact region which allow a full control of the Klein barrier taking place at the contact edge. In particular, our work demonstrates the GHz operation of transistors driven by independent contact gates. We benchmark the standard channel and novel contact gating and report for the later dynamical transconductance levels at the state of the art. Our finding may find applications in electronics and optoelectronics whenever there is need to control independently the Fermi level and the electrostatic potential of electronic sources or to get rid of cumbersome local channel gates.

cond-mat.mes-hall↗

arXiv2018

A graphene Zener-Klein transistor cooled by a hyperbolic substrate

Engineering of cooling mechanisms is a bottleneck in nanoelectronics. Whereas thermal exchanges in diffusive graphene are mostly driven by defect assisted acoustic phonon scattering, the case of high-mobility graphene on hexagonal Boron Nitride (hBN) is radically different with a prominent contribution of remote phonons from the substrate. A bi-layer graphene on hBN transistor with local gate is driven in a regime where almost perfect current saturation is achieved by compensation of the decrease of the carrier density and Zener-Klein tunneling (ZKT) at high bias. Using noise thermometry, we show that this Zener-Klein tunneling triggers a new cooling pathway due to the emission of hyperbolic phonon polaritons (HPP) in hBN by out-of-equilibrium electron-hole pairs beyond the super-Planckian regime. The combination of ZKT-transport and HPP-cooling promotes graphene on BN transistors as a valuable nanotechnology for power devices and RF electronics.

cond-mat.mes-hall↗

arXiv2011

Electron cotunneling transport in gold nanocrystal arrays

We describe current-voltage characteristics I(V) of alkyl-ligated gold nanocrystals $\sim 5 nm$ arrays in long screening length limit. Arrays with different alkyl ligand lengths have been prepared to tune the electronic tunnel coupling between the nanocrystals. For long ligands, electronic diffusion occurs through sequential tunneling and follows activated laws, as function of temperature $σ\propto e^{-T_0/T}$ and as function of electric field $I \propto e^{-\mathcal{E}_0/\mathcal{E}}$. For better conducting arrays, i.e. with small ligands, the transport properties crossover to the cotunneling regime and follows Efros-Shklovskii laws as function of temperature $σ\propto e^{-(T_{ES}/T)^{1/2}}$ and as function of electric field $I \propto e^{-(\mathcal{E}_{ES}/\mathcal{E})^{1/2}}$. The data shows that electronic transport in nanocrystal arrays can be tuned from the sequential tunneling to the cotunneling regime by increasing the tunnel barrier transparency.

cond-mat.str-el↗

arXiv2010

High-resolution spatial mapping of a superconducting NbN wire using single-electron detection

Superconducting NbN wires have recently received attention as detectors for visible and infrared photons. We present experiments in which we use a NbN wire for high-efficiency (40 %) detection of single electrons with keV energy. We use the beam of a scanning electron microscope as a focussed, stable, and calibrated electron source. Scanning the beam over the surface of the wire provides a map of the detection efficiency. This map shows features as small as 150 nm, revealing wire inhomogeneities. The intrinsic resolution of this mapping method, superior to optical methods, provides the basis of a characterization tool relevant for photon detectors.

cond-mat.supr-con↗