arXiv subjects

S. O. Valenzuela

Publications and source records attributed to S. O. Valenzuela.

At least 19 recordsLinked to original sources

arXiv2023

Large-area synthesis of ferromagnetic Fe$_{5-x}$GeTe$_{2}$/graphene van der Waals heterostructures with Curie temperature above room temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other two-dimensional (2D) crystals are promising building blocks for the realization of ultra-compact devices with integrated magnetic, electronic and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing to realize highly uniform heterostructures with well-defined interfaces between different 2D layered materials. It also requires that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, we demonstrate large-area growth of Fe$_{5-x}$GeTe$_{2}$/graphene heterostructures achieved by vdW epitaxy of Fe$_{5-x}$GeTe$_{2}$ on epitaxial graphene. Structural characterization confirmed the realization of a continuous vdW heterostructure film with a sharp interface between Fe$_{5-x}$GeTe$_{2}$ and graphene. Magnetic and transport studies revealed that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond non-scalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

cond-mat.mtrl-sci↗

arXiv2021

Unraveling heat transport and dissipation in suspended MoSe$_2$ crystals from bulk to monolayer

Understanding thermal transport in layered transition metal dichalcogenide (TMD) crystals is crucial for a myriad of applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood. Here, we present a combined experimental-theoretical study of the intrinsic lattice thermal conductivity of the representative TMD MoSe$_2$, focusing on the effect of material thickness and the material's environment. We use Raman thermometry measurements on suspended crystals, where we identify and eliminate crucial artefacts, and perform $ab$ $initio$ simulations with phonons at finite, rather than zero, temperature. We find that phonon dispersions and lifetimes change strongly with thickness, yet (sub)nanometer thin TMD films exhibit a similar in-plane thermal conductivity ($\sim$20~Wm$^{-1}$K$^{-1}$) as bulk crystals ($\sim$40~Wm$^{-1}$K$^{-1}$). This is the result of compensating phonon contributions, in particular low-frequency modes with a surprisingly long mean free path of several micrometers that contribute significantly to thermal transport for monolayers. We furthermore demonstrate that out-of-plane heat dissipation to air is remarkably efficient, in particular for the thinnest crystals. These results are crucial for the design of TMD-based applications in thermal management, thermoelectrics and (opto)electronics.

cond-mat.mtrl-sci↗

arXiv2020

Absence of magnetic-proximity effect at the interface of Bi$_2$Se$_3$ and (Bi,Sb)$_2$Te$_3$ with EuS

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)$_2$(Se,Te)$_3$ family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of $\sim\!10^{-3}$ $μ_\mathrm{B}$/at.

cond-mat.mes-hall↗

arXiv2020

Ferromagnetic resonance assisted optomechanical magnetometer

The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this work we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity better than 900 pT Hz$^{-1/2}$ at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field we demonstrate sensitivity values at resonance around a few nT Hz$^{-1/2}$ up to the GHz range. Our results show that our hybrid system can be used to build high-speed sensor of oscillating magnetic fields.

cond-mat.mes-hall↗

arXiv2019

Spin communication over 30 $μ$m long channels of chemical vapor deposited graphene on SiO$_2$

We demonstrate a high-yield fabrication of non-local spin valve devices with room-temperature spin lifetimes of up to 3 ns and spin relaxation lengths as long as 9 $μ$m in platinum-based chemical vapor deposition (Pt-CVD) synthesized single-layer graphene on SiO$_2$/Si substrates. The spin-lifetime systematically presents a marked minimum at the charge neutrality point, as typically observed in pristine exfoliated graphene. However, by studying the carrier density dependence beyond n ~ 5 x 10$^{12}$ cm$^{-2}$, via electrostatic gating, it is found that the spin lifetime reaches a maximum and then starts decreasing, a behavior that is reminiscent of that predicted when the spin-relaxation is driven by spin-orbit interaction. The spin lifetimes and relaxation lengths compare well with state-of-the-art results using exfoliated graphene on SiO$_2$/Si, being a factor two-to-three larger than the best values reported at room temperature using the same substrate. As a result, the spin signal can be readily measured across 30 $μ$m long graphene channels. These observations indicate that Pt-CVD graphene is a promising material for large-scale spin-based logic-in-memory applications.

cond-mat.mes-hall↗

arXiv2018

Spin precession in anisotropic media

We generalize the diffusive model for spin injection and detection in nonlocal spin structures to account for spin precession under an applied magnetic field in an anisotropic medium, for which the spin lifetime is not unique and depends on the spin orientation.We demonstrate that the spin precession (Hanle) line shape is strongly dependent on the degree of anisotropy and on the orientation of the magnetic field. In particular, we show that the anisotropy of the spin lifetime can be extracted from the measured spin signal, after dephasing in an oblique magnetic field, by using an analytical formula with a single fitting parameter. Alternatively, after identifying the fingerprints associated with the anisotropy, we propose a simple scaling of the Hanle line shapes at specific magnetic field orientations that results in a universal curve only in the isotropic case. The deviation from the universal curve can be used as a complementary means of quantifying the anisotropy by direct comparison with the solution of our generalized model. Finally, we applied our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution.

cond-mat.mes-hall↗

arXiv2017

Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures

Spin Hall effects have surged as promising phenomena for spin logics operations without ferromagnets. However, the magnitude of the detected electric signals at room temperature in metallic systems has been so far underwhelming. Here, we demonstrate a two-order of magnitude enhancement of the signal in monolayer graphene/Pt devices when compared to their fully metallic counterparts. The enhancement stems in part from efficient spin injection and the large resistivity of graphene but we also observe 100% spin absorption in Pt and find an unusually large effective spin Hall angle of up to 0.15. The large spin-to-charge conversion allows us to characterise spin precession in graphene under the presence of a magnetic field. Furthermore, by developing an analytical model based on the 1D diffusive spin-transport, we demonstrate that the effective spin-relaxation time in graphene can be accurately determined using the (inverse) spin Hall effect as a means of detection. This is a necessary step to gather full understanding of the consequences of spin absorption in spin Hall devices, which is known to suppress effective spin lifetimes in both metallic and graphene systems.

cond-mat.mes-hall↗

arXiv2015

Hot-Carrier Seebeck Effect: Diffusion and Remote Detection of Hot Carriers in Graphene

We investigate hot carrier propagation across graphene using an electrical nonlocal injection/detection method. The device consists of a monolayer graphene flake contacted by multiple metal leads. Using two remote leads for electrical heating, we generate a carrier temperature gradient that results in a measurable thermoelectric voltage VNL across the remaining (detector) leads. Due to the nonlocal character of the measurement, VNL is exclusively due to the Seebeck effect. Remarkably, a departure from the ordinary relationship between Joule power P and VNL, VNL ~ P, becomes readily apparent at low temperatures, representing a fingerprint of hot-carrier dominated thermoelectricity. By studying VNL as a function of bias, we directly determine the carrier temperature and the characteristic cooling length for hot-carrier propagation, which are key parameters for a variety of new applications that rely on hot-carrier transport.

cond-mat.mes-hall↗

arXiv2015

Enhanced spin accumulation at room temperature in graphene spin valves with amorphous carbon interfacial layers

We demonstrate a large enhancement of the spin accumulation in monolayer graphene following electron-beam induced deposition of an amorphous carbon layer at the ferromagnet-graphene interface. The enhancement is 10^4-fold when graphene is deposited onto poly(methyl metacrylate) (PMMA) and exposed with sufficient electron-beam dose to cross-link the PMMA, and 10^3-fold when graphene is deposited directly onto SiO2 and exposed with identical dose. We attribute the difference to a more efficient carbon deposition in the former case due to an increase in the presence of compounds containing carbon, which are released by the PMMA. The amorphous carbon interface can sustain very large current densities without degrading, which leads to very large spin accumulations exceeding 500 microeVs at room temperature.

cond-mat.mes-hall↗

arXiv2014

Fingerprints of Inelastic Transport at the Surface of the Topological Insulator Bi2Se3: Role of Electron-Phonon Coupling

We report on electric-field and temperature dependent transport measurements in exfoliated thin crystals of Bi$_{2}$Se$_{3}$ topological insulator. At low temperatures ($< 50$ K) and when the chemical potential lies inside the bulk gap, the crystal resistivity is strongly temperature dependent, reflecting inelastic scattering due to the thermal activation of optical phonons. A linear increase of the current with voltage is obtained up to a threshold value at which current saturation takes place. We show that the activated behavior, the voltage threshold and the saturation current can all be quantitatively explained by considering a single optical phonon mode with energy $\hbar Ω\approx 8$ meV. This phonon mode strongly interacts with the surface states of the material and represents the dominant source of scattering at the surface at high electric fields.

cond-mat.mes-hall↗

arXiv2008

Quantum Phase Tomography of a Strongly Driven Qubit

The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stueckelberg oscillations in qubit magnetization. The resulting oscillatory patterns are a hallmark of the coherent strongly-driven regime in qubits, quantum dots and other two-level systems. The two-dimensional Fourier transforms of these patterns are found to exhibit a family of one-dimensional curves in Fourier space, in agreement with recent observations in a superconducting qubit. We interpret these images in terms of time evolution of the quantum phase of qubit state and show that they can be used to probe dephasing mechanisms in the qubit.

cond-mat.mes-hall↗

arXiv2007

Microwave-Induced Cooling of a Superconducting Qubit

We demonstrated microwave-induced cooling in a superconducting flux qubit. The thermal population in the first-excited state of the qubit is driven to a higher-excited state by way of a sideband transition. Subsequent relaxation into the ground state results in cooling. Effective temperatures as low as Teff~ 3 millikelvin are achieved for bath temperatures Tbath = 30 - 400 millikelvin, a cooling factor between 10 and 100. This demonstration provides an analog to optical cooling of trapped ions and atoms and is generalizable to other solid-state quantum systems. Active cooling of qubits, applied to quantum information science, provides a means for qubit-state preparation with improved fidelity and for suppressing decoherence in multi-qubit systems.

cond-mat.supr-con↗

arXiv2006

Coherent Quasiclassical Dynamics of a Persistent Current Qubit

A new regime of coherent quantum dynamics of a qubit is realized at low driving frequencies in the strong driving limit. Coherent transitions between qubit states occur via the Landau-Zener process when the system is swept through an energy-level avoided crossing. The quantum interference mediated by repeated transitions gives rise to an oscillatory dependence of the qubit population on the driving field amplitude and flux detuning. These interference fringes, which at high frequencies consist of individual multiphoton resonances, persist even for driving frequencies smaller than the decoherence rate, where individual resonances are no longer distinguishable. A theoretical model that incorporates dephasing agrees well with the observations.

cond-mat.supr-con↗

arXiv2005

Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields

In this work we study by ac susceptibility measurements the evolution of the solid vortex lattice mobility under oscillating forces. Previous work had already shown that in YBCO single crystals, below the melting transition, a temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular) can heal the vortex lattice (VL) and increase its mobility, but a temporarily asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into a more pinned disordered state. In this work we present evidence that the mobility of the VL is reduced for large vortex displacements, in agreement with predictions of recent simulations. We show that with large symmetric oscillating fields both an initially ordered or an initially disordered VL configuration evolve towards a less mobile lattice, supporting the scenario of plastic flow.

cond-mat.supr-con↗

arXiv2004

Voltage Dependence of Spin Polarized Tunneling

A mesoscopic spin valve is used to determine the effective spin polarization of electrons tunneling from and into ferromagnetic transition metals at finite voltages. The tunneling spin polarization from the ferromagnet (FM) slowly decreases with bias, but drops faster and even inverts with voltage when electrons tunnel into it. A bias-dependent free electron model shows that in the former case electrons originate near the Fermi level of the FM with large polarization whereas in the latter, electrons tunnel into hot electron states for which the polarization is significantly reduced. The change in sign is ascribed to the detailed matching of the electron wave function through the tunnel barrier.

cond-mat.mes-hall↗

arXiv2003

DC measurements of macroscopic quantum levels in a superconducting qubit structure with a time-ordered meter

DC measurements are made in a superconducting, persistent current qubit structure with a time-ordered meter. The persistent-current qubit has a double-well potential, with the two minima corresponding to magnetization states of opposite sign. Macroscopic resonant tunneling between the two wells is observed at values of energy bias that correspond to the positions of the calculated quantum levels. The magnetometer, a Superconducting Quantum Interference Device (SQUID), detects the state of the qubit in a time-ordered fashion, measuring one state before the other. This results in a different meter output depending on the initial state, providing different signatures of the energy levels for each tunneling direction. From these measurements, the intrawell relaxation time is found to be about 50 microseconds.

cond-mat.supr-con↗

arXiv2003

Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure

Measurements of thermal activation are made in a superconducting, niobium Persistent-Current (PC) qubit structure, which has two stable classical states of equal and opposite circulating current. The magnetization signal is read out by ramping the bias current of a DC SQUID. This ramping causes time-ordered measurements of the two states, where measurement of one state occurs before the other. This time-ordering results in an effective measurement time, which can be used to probe the thermal activation rate between the two states. Fitting the magnetization signal as a function of temperature and ramp time allows one to estimate a quality factor of 10^6 for our devices, a value favorable for the observation of long quantum coherence times at lower temperatures.

cond-mat.mes-hall↗

arXiv2001

Oscillatory dynamics and organization of the vortex solid in YBa2Cu3O7 single crystals

We report on the degree of order of the vortex solid in YBa2Cu3O7 single crystals observed in ac susceptibility measurements. We show that when vortices are "shaken" by a temporarily symmetric ac field they are driven into an easy-to-move, ordered structure but, on the contrary, when the ac field is temporarily asymmetric, they are driven into a more pinned disordered state. This is characteristic of tearing of the vortex lattice and shows that ordering due to symmetric ac fields is essentially different from an equilibration process or a dynamical crystallization that is expected to occur at high driving currents.

cond-mat.supr-con↗