Stephan Roche

@article{ISI:000739656004687,
title = {Two-dimensional materials prospects for non-volatile spintronic memories},
author = {Hyunsoo Yang and Sergio O. Valenzuela and Mairbek Chshiev and Sébastien Couet and Bernard Dieny and Bruno Dlubak and Albert Fert and Kevin Garello and Matthieu Jamet and Dae-Eun Jeong and Kangho Lee and Taeyoung Lee and Marie-Blandine Martin and Gouri Sankar Kar and Pierre Sénéor and Hyeon-Jin Shin and Stephan Roche},
doi = {10.1038/s41586-022-04768-0},
times_cited = {0},
issn = {1476-4687},
year = {2022},
date = {2022-06-22},
journal = {NATURE},
volume = {606},
pages = {663–673},
abstract = {Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000739656000004,
title = {Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers },
author = {Marc Vila and Chuang-Han Hsu and Jose H Garcia and Antonio L Benitez and Xavier Waintal and Sergio O Valenzuela and Vitor M Pereira and Stephan Roche},
doi = {10.1103/PhysRevResearch.3.043230},
times_cited = {5},
year = {2021},
date = {2021-12-30},
journal = {PHYSICAL REVIEW RESEARCH },
volume = {3},
number = {4},
publisher = {AMER PHYSICAL SOC },
address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA },
abstract = {The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit lambda(s)theta(xy) approximate to 1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (lambda(s)) and charge-to-spin interconversion efficiency as large as theta(xy) approximate to 80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000655306100001,
title = {Room-temperature tunnel magnetoresistance across biomolecular tunnel junctions based on ferritin},
author = {Senthil Kumar Karuppannan and Rupali Reddy Pasula and Tun Seng Herng and Jun Ding and Xiao Chi and Enrique del Barco and Stephan Roche and Xiaojiang Yu and Nikolai Yakovlev and Sierin Lim and Christian A Nijhuis},
doi = {10.1088/2515-7639/abfa79},
times_cited = {3},
year = {2021},
date = {2021-07-01},
journal = {JOURNAL OF PHYSICS-MATERIALS},
volume = {4},
number = {3},
publisher = {IOP Publishing Ltd},
address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND},
abstract = {We report exceptionally large tunnel magnetoresistance (TMR) for biomolecular tunnel junctions based on ferritins immobilized between Ni and EGaIn electrodes. Ferritin stores iron in the form of ferrihydrite nanoparticles (NPs) and fulfills the following roles: (a) it dictates the tunnel barrier, (b) it magnetically decouples the NPs from the ferromagnetic (FM) electrode, (c) it stabilizes the NPs, and (d) it acts as a spin filter reducing the complexity of the tunnel junctions since only one FM electrode is required. The mechanism of charge transport is long-range tunneling which results in TMR of 60 +/- 10% at 200 K and 25 +/- 5% at room temperature. We propose a magnon-assisted transmission to explain the substantially larger TMR switching fields (up to 1 Tesla) than the characteristic coercive fields (a few Gauss) of ferritin ferrihydrite particles at T < 20 K. These results highlight the genuine potential of biomolecular tunnel junctions in designing functional nanoscale spintronic devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000600286000017,
title = {Canted Persistent Spin Texture and Quantum Spin Hall Effect in WTe_{2}},
author = {Jose H Garcia and Marc Vila and Chuang-Han Hsu and Xavier Waintal and Vitor M Pereira and Stephan Roche},
doi = {10.1103/PhysRevLett.125.256603},
times_cited = {0},
issn = {0031-9007},
year = {2020},
date = {2020-12-18},
journal = {PHYSICAL REVIEW LETTERS},
volume = {125},
number = {25},
publisher = {AMER PHYSICAL SOC},
address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA},
abstract = {We report an unconventional quantum spin Hall phase in the monolayer WTe2, which exhibits hitherto unknown features in other topological materials. The low symmetry of the structure induces a canted spin texture in the yz plane, which dictates the spin polarization of topologically protected boundary states. Additionally, the spin Hall conductivity gets quantized (2e(2)/h) with a spin quantization axis parallel to the canting direction. These findings are based on large-scale quantum simulations of the spin Hall conductivity tensor and nonlocal resistances in multiprobe geometries using a realistic tight-binding model elaborated from first-principle methods. The observation of this canted quantum spin Hall effect, related to the formation of topological edge states with nontrivial spin polarization, demands for specific experimental design and suggests interesting alternatives for manipulating spin information in topological materials.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000379651600002,
title = {Spin dynamics in bilayer graphene: Role of electron-hole puddles and Dyakonov-Perel mechanism},
author = {Dinh Van Tuan and Shaffique Adam and Stephan Roche},
doi = {10.1103/PhysRevB.94.041405},
times_cited = {0},
issn = {2469-9950},
year = {2016},
date = {2016-07-15},
journal = {PHYSICAL REVIEW B},
volume = {94},
number = {4},
publisher = {AMER PHYSICAL SOC},
address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA},
abstract = {We report on spin transport features which are unique to high quality bilayer graphene, in the absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of a propagating wave packet is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy dependence of the spin lifetime is similar to monolayer graphene with an M-shaped profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hole puddles (characteristics of supported graphene either on SiO2 or hBN) surprisingly results in a large enhancement of the low-energy spin lifetime and a lowering of its high-energy values. Such a feature, unique to the bilayer, is explained in terms of a reinforced Dyakonov-Perel mechanism at the Dirac point, whereas spin relaxation at higher energies is driven by pure dephasing effects. This suggests further electrostatic control of the spin transport length scales in graphene devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000302957000011,
title = {Quenching of the Quantum Hall Effect in Graphene with Scrolled Edges},
author = {Alessandro Cresti and Michael M Fogler and Francisco Guinea and Castro A H Neto and Stephan Roche},
doi = {10.1103/PhysRevLett.108.166602},
times_cited = {1},
issn = {0031-9007},
year = {2012},
date = {2012-04-18},
journal = {PHYSICAL REVIEW LETTERS},
volume = {108},
number = {16},
publisher = {AMER PHYSICAL SOC},
address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA},
abstract = {Edge nanoscrolls are shown to strongly influence transport properties of suspended graphene in the quantum Hall regime. The relatively long arclength of the scrolls in combination with their compact transverse size results in formation of many nonchiral transport channels in the scrolls. They short circuit the bulk current paths and inhibit the observation of the quantized two-terminal resistance. Unlike competing theoretical proposals, this mechanism of disrupting the Hall quantization in suspended graphene is not caused by ill-chosen placement of the contacts, singular elastic strains, or a small sample size.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}