Marius Costache | University of Barcelona (original) (raw)

Papers by Marius Costache

Physical Review Letters, Feb 25, 2014

We report on electric-field and temperature dependent transport measurements in exfoliated thin c... more 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 Ω ≈ 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.

Physical Review B, Jul 31, 2006

We study spin accumulation in an aluminium island, in which the injection of a spin current and t... more We study spin accumulation in an aluminium island, in which the injection of a spin current and the detection of the spin accumulation are done by means of four cobalt electrodes that connect to the island through transparent tunnel barriers. Although the four electrodes are designed as two electrode pairs of the same shape, they nonetheless all exhibit distinct switching fields. As a result the device can have several different magnetic configurations. From the measurements of the amplitude of the spin accumulation, we can identify these configurations, and using the diffusion equation for the spin imbalance, we extract the spin relaxation length λ sf = 400 ± 50 nm and an interface spin current polarization P = (10 ± 1)% at low temperature and λ sf = 350 ± 50 nm, P = (8 ± 1)% at room temperature.

Applied Physics Letters, 2002

We have fabricated a multiterminal lateral mesoscopic metallic spin valve demonstrating spin prec... more We have fabricated a multiterminal lateral mesoscopic metallic spin valve demonstrating spin precession at room temperature (RT), using tunnel barriers in combination with metallic ferromagnetic electrodes as a spin injector and detector. The observed modulation of the output signal due to the spin precession is discussed and explained in terms of a time-of-flight experiment of electrons in a diffusive conductor. The obtained spin relaxation length λsf=500 nm in an aluminum strip will make detailed studies of spin dependent transport phenomena possible and allow one to explore the possibilities of the electron spin for new electronic applications at RT.

Physical review, Feb 3, 2017

We generalize the diffusive model for spin injection and detection in nonlocal spin structures to... more 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) lineshape 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 lineshapes 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 apply our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution.

2D Materials

The ability to control the generation of spins in arbitrary directions is a long-sought goal in s... more The ability to control the generation of spins in arbitrary directions is a long-sought goal in spintronics. Charge to spin interconversion (CSI) phenomena depend strongly on symmetry. Systems with reduced crystal symmetry allow anisotropic CSI with unconventional components, where charge and spin currents and the spin polarization are not mutually perpendicular to each other. Here, we demonstrate experimentally that the CSI in graphene-WTe2induces spins with components in all three spatial directions. By performing multi-terminal nonlocal spin precession experiments, with specific magnetic fields orientations, we discuss how to disentangle the CSI from the spin Hall and inverse spin galvanic effects.

One of the main challenges of the field of spintronics [1] is the controlled injection of spin po... more One of the main challenges of the field of spintronics [1] is the controlled injection of spin polarized currents from ferromagnetic into nonmagnetic materials by means of an insulating tunnel barrier. In order to get a better understanding of the involved tunnelling process, novel devices have been developed that, in contrast to the widely studied magnetic tunnel junctions, allow one to distinguish between tunnelling out-of or into a ferromagnetic electrode by inverting the applied voltage bias [2]. The obtained data with such devices show a strong asymmetry about zero bias, which we analyze with a theoretical approach based on an analytical free-electron model. Our simple model is unable to render all of the complexity inherent to nonideal interfaces, scattering, or complex band structures. However, it qualitatively explains the experimental observations and shows that complex behaviour of the polarization as a function of voltage is intrinsic to spin tunnelling and is highly sens...

Understanding the heating, energy flow and relaxation of charge carriers in nanostructures is ess... more Understanding the heating, energy flow and relaxation of charge carriers in nanostructures is essential for the management of heat in next generation devices, where the ever decreasing dimensions lead to increasing leakage currents, and Joule dissipation occurs in smaller and smaller volumes. The generated heat, which has to be efficiently driven away from the electronically active region, could also be used for energy harvesting by taking advantage of thermoelectric phenomena. In this talk, I will discuss our current research on hot-carrier propagation across monolayer graphene. The device consists of a monolayer graphene flake contacted by multiple metal leads. Using two remote leads for electrical heating, a carrier temperature gradient is generated 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 r...

Journal of Physics: Materials, 2021

In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept acr... more In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moiré materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to...

2D Materials, 2019

We demonstrate a high-yield fabrication of non-local spin valve devices with roomtemperature spin... more We demonstrate a high-yield fabrication of non-local spin valve devices with roomtemperature spin lifetimes of up to 3 ns and spin relaxation lengths as long as 9 µm 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 spinrelaxation 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-µm long graphene channels. These observations indicate that Pt-CVD graphene is a promising material for large-scale spin-based logic-in-memory applications.

Nature Physics, 2017

A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it... more A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it with semiconducting transition metal dichalcogenides. Signatures of such an enhancement have been reported but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and tungsten or molybdenum disulphide (WS 2 , MoS 2). We observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, being largest when the spins point out of the graphene plane. This indicates that the strong spin-valley coupling in the transition metal dichalcogenide is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials.

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RSC advances, Jan 22, 2018

Exposing graphene to a hydrogen post-etching process yields dendritic graphene shapes. Here, we d... more Exposing graphene to a hydrogen post-etching process yields dendritic graphene shapes. Here, we demonstrate that similar dendritic structures can be achieved at long growth times without adding hydrogen externally. These shapes are not a result of a surface diffusion controlled growth but of the competing backward reaction (etching), which dominates the growth dynamics at long times due to an rise in the hydrogen partial pressure. We have performed a systematic study on the growth of graphene as a function of time to identify the onset and gradual evolution of graphene shapes caused by etching and then demonstrated that the etching can be stopped by reducing the flow of hydrogen from the feed. In addition, we have found that the etching rate due to the rise in hydrogen is strongly dependent on the confinement (geometrical confinement) of copper foil. Highly etched graphene with dendritic shapes was observed in unconfined copper foil regions while no etching was found in graphene gro...

Science (New York, N.Y.), Apr 13, 2018

Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, i... more Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecul...

Applied Physics Letters, Nov 6, 2006

We measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-... more We measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-chip microwave transmission line device. The method used is based on detection of the oscillating magnetic flux due to the magnetization dynamics, with an inductive pickup loop. The dependence of the resonance frequency on applied static magnetic field agrees very well with the Kittel formula, demonstrating that the uniform magnetization precession mode is being driven.

Science, 2010

Spin Control Controlling and manipulating the spin of an electron is a central requirement for ap... more Spin Control Controlling and manipulating the spin of an electron is a central requirement for applications in spintronics. Some of the challenges researchers are facing include efficient creation of spin currents, minimization of Joule heating, and extending the lifetime of electronic spins, which is especially important for quantum information applications. Costache and Valenzuela (p. 1645 ) address the first challenge by designing and fabricating an efficient and simple superconducting-based single-electron transistor that can produce spin current with controlled flow. Key to the design is asymmetric tunneling, which leads to a ratchet effect (or diode-like behavior), allowing the separation of up and down spins. Jonietz et al. (p. 1648 ) use electric currents five orders of magnitude smaller than those used previously in nanostructures to manipulate magnetization in a bulk material, MnSi, pointing the way toward decreased Joule heating in spintronic devices. This so-called spin-...

APL Materials, 2021

Paper published as part of the special topic on Emerging Materials for Spin-Charge Interconversio... more Paper published as part of the special topic on Emerging Materials for Spin-Charge Interconversion ARTICLES YOU MAY BE INTERESTED IN Chirality-selective easily adjustable spin current from uniaxial antiferromagnets

Nano Letters, 2020

Bi 1−x Sb x) 2 Te 3 topological insulators (TIs) are gathering increasing attention owing to thei... more Bi 1−x Sb x) 2 Te 3 topological insulators (TIs) are gathering increasing attention owing to their large charge-to-spin conversion efficiency and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the magnetization of a ferromagnet (FM). The origin of the torques, however, remains elusive, while the implications of hybridized 1 arXiv:2009.08215v1 [cond-mat.mes-hall] 17 Sep 2020 states and the strong material intermixing at the TI/FM interface are essentially unexplored. By combining interface chemical analysis and spin-transfer ferromagnetic resonance (ST-FMR) measurements, we demonstrate that intermixing plays a critical role in the generation of SOTs. By inserting a suitable normal metal spacer, material intermixing is reduced and the TI properties at the interface are largely improved, resulting in strong variations in the nature of the SOTs. A dramatic enhancement of a field-like torque, opposing and surpassing the Oersted-field torque, is observed, which can be attributed to the non-equilibrium spin density in Rashba-split surface bands and to the suppression of spin memory loss.

Spintronics XIV

Topological insulators (TI) have gained much interest in the field of spintronics for the generat... more Topological insulators (TI) have gained much interest in the field of spintronics for the generation of pure spin currents. Indeed, three-dimensional TIs are predicted to host exotic properties like topologically protected surface states (TSS), which show Dirac-like band dispersion and spin-momentum locking [1]. One of the main strategies is to take advantages of the spin polarization of the TSSs to manipulate the magnetization of an adjacent ferromagnetic thin film (FM) using the spin-orbit torque (SOT) mechanism [2]. In the past few years, the community attempted to replace the traditional heavy metals by a TI in order to enhance the SOT efficiency with limited success. It now appears that the interface sharpness and the high chemical affinity between Bi-based TIs and classical 3d FMs is a major hurdle to reach the predicted breakthrough in magnetization switching power-efficiency [3]. The emergence of ferromagnetism in two dimensions in 2017, which started a new field in condensed matter physics, could bring a solution to this issue.The van der Waals (vdW) nature of the interaction between the TI and the 2D-FM should limit chemical reactions, interface intermixing and hybridization of state between the two layers.

Nature nanotechnology, Jan 4, 2017

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimula... more In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents 1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect 3-5 , in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect 6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport 9-11 , energy-dependent carrier mobility and unique density of states 12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current ...

Aps Meeting Abstracts, Mar 1, 2008

The high transition temperature and simple AlB2 structure make the MgB2 superconductor a promisin... more The high transition temperature and simple AlB2 structure make the MgB2 superconductor a promising new material for application in superconducting electronic devices. In order to reach this goal, in addition to the development of MgB2 Josephson Junctions (JJ), we have explored the superconducting properties of lithographically patterned micron sized bridges in high quality epitaxial MgB2 films. These thin film micron