Saroj Prasad Dash

Associate Professor at the Department of Microtechnology and Nanoscience, Quantum Device Physics Laboratory

Saroj Dash is leading a research group on Quantum device Physics, Nanoelectronics and Spintronics research at Chalmers. He holds a PhD degree in Physics from Max Planck Institute (2007, Stuttgart, Germany). His previous positions include postdocs at Uni. of Twente and Uni. of Groningen in Netherlands for three years. He was appointed at Chalmers in November 2010, where his group focus is on electronic charge and spin transport in graphene, semiconductor nanostructures, other two-dimensional materials and topological insulators. His group develops novel approaches for nanofabrication and design new measurement techniques that lead to fundamental physics experiments. The goal is to exploit spin degree of freedom of electrons for integration of memory and logic functionalities in nanoelectronic devices.

Research interests
1. Graphene nanoelectronics and spintronics
2. van der Waals heterostructures of 2D materials
3. 2D semiconductors for electronics and spintronics
4. Spin transport in silicon devices
5. Topological Insulators for spintronic devices

Read about the different master thesis projects (30 or 60 credits) available in the Saroj Dash Group at the Teaching tab.

Spin Hall Effect in Weyl Semimetal WTe2

Weyl semimetals with large Berry curvature, strong spin-orbit coupling, novel spin texture, and broken inversion symmetry are predicted to exhibit a large spin Hall effect that can efficiently convert the charge current to a spin current. Here for the first time, we report the direct experimental observation of a large and gate-controlled spin Hall and inverse spin Hall effects in a layered semimetal WTe2 at room temperature obeying Onsager reciprocity relation. We demonstrate the creation and detection of the pure spin current generated by spin Hall phenomenon in WTe2 by making van der Waals heterostructures with graphene, taking advantage of its long spin coherence length and a large spin transmission efficiency at the heterostructure interface. These experimental findings can pave the way for utilization of gate tunable spin-orbit induced phenomena in Weyl semimetals for spin-based device architectures. 

B Zhao, D Khokhriakov, Y Zhang, H Fu, B Karpiak, AM Hoque, X Xu, Y Jiang, B Yan, SP Dash 
arXiv:1812.02113 (2018). 
https://arxiv.org/abs/1812.02113

Spin Texture in Graphene - Topological Insulator Heterostructures

Dirac materials such as graphene and topological insulators (TIs) are known to have unique electronic and spintronic properties. We combine graphene with TIs in van der Waals heterostructures to demonstrate the emergence of a strong proximity-induced spin-orbit coupling in graphene. By performing spin transport and precession measurements supported by ab initio simulations, we discover a strong tunability and suppression of the spin signal and spin lifetime due to the hybridization of graphene and TI electronic bands. The enhanced spin-orbit coupling strength is estimated to be nearly an order of magnitude higher than in pristine graphene. These findings in graphene-TI heterostructures could open interesting opportunities for exploring exotic physical phenomena and new device functionalities governed by topological proximity effects.
D Khokhriakov, AW Cummings, K Song, M Vila, B Karpiak, A Dankert, S Roche, SP Dash












Graphene

Graphene is an ideal medium for long-distance spin communication in future spintronic technologies. Here we demonstrate a high spintronic performance in CVD graphene on SiO2/Si substrate at room temperature. Our detailed investigation reinforces the observed performance in CVD graphene over wafer scale and opens up new prospects for the development of lateral spin-based memory and logic applications.


Long distance spin communication in chemical vapour deposited graphene.
M.V. Kamalakar, G. Chris, A. Dankert, S.P. Dash;
Nature Communications, 6, 6766 (2015).
News - https://phys.org/news/2015-04-graphene-future-spintronic-devices.html








2D materials heterostructures

Graphene/MoS2 - Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin–orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS2). Here we combine graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature. Our findings demonstrate an all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures.

Electrical gate control of spin current in van der Waals heterostructures at room temperature.
A. Dankert, S.P. Dash;
Nature Communications 8, 16093 (2017).
News - https://graphene-flagship.eu/field-effect-transistor-using-graphenes-electron-spin

Graphene/h-BN - Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.



Inversion of spin signal and spin filtering in ferromagnet| hexagonal boron nitride-graphene van der Waals heterostructures; MV Kamalakar, A Dankert, PJ Kelly, SP Dash;
Scientific reports 6, 21168 (2016)

Enhanced Tunnel Spin Injection into Graphene using CVD Hexagonal Boron Nitride;
MV Kamalakar, A Dankert, J Bergsten, T Ive, SP Dash;
Scientific Reports, 4: 61446 (2014)


Topological Insulators

Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking of massless Dirac Fermions in their surface states. Here we report the room temperature electrical detection of the spin polarization on the surface of Bi2Se3 by employing spin sensitive ferromagnetic tunnel contacts. These findings provide further information about the electrical detection of current-induced spin polarizations in 3D TIs at ambient temperatures and could lead to innovative spin-based technologies.


Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators
A. Dankert, J. Geurs, M.V. Kamalakar, S. Charpentier, S.P. Dash;
Nano Letters, 15, 12, 7976 (2015).
News - https://www.sciencedaily.com/releases/2015/12/151207081831.htm

2D Semiconductor Transistor

Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nanoelectronic, optoelectronic, and spintronic applications. Here, we investigate the field-effect transistor behavior of MoS2 and elucidate that the presence of a large Schottky barrier at the MoS2/ferromagnet interface is a major obstacle for electronic devices. We circumvent this problem by a reduction in the Schottky barrier height with the introduction of a thin TiO2 tunnel barrier between the ferromagnet and MoS2. This results in an enhancement of the transistor on-state current and an increment in the field-effect mobility.


High Performance Molybdenum Disulfide Field Effect Transistors with Spin Tunnel Contacts
A. Dankert, L. Langouche, V.K. Mutta, S.P. Dash;
ACS Nano 8 (1), 476 (2014).


Black phosphorus (BP) has been recently unveiled as a promising 2D 

direct bandgap semiconducting material. Here, ambipolar field-effect transistor behavior of nanolayers of BP with ferromagnetic tunnel contacts is reported. Using TiO2/Co contacts, a reduced Schottky barrier <50 meV, which can be tuned further by the gate voltage, is obtained. 

​Low Schottky Barrier Black Phosphorus Field-Effect Devices with Ferromagnetic Tunnel Contacts

MV Kamalakar, BN Madhushankar, A Dankert, SP Dash
Small 11 (18), 2209-2216 (2015).


Silicon Spintronics

The control and manipulation of the electron spin in semiconductors is central to spintronics, which aims to represent digital information using spin orientation rather than electron charge. Here we demonstrate room-temperature electrical injection of spin polarization into n-type and p-type silicon from a ferromagnetic tunnel contact, spin manipulation using the Hanle effect and the electrical detection of the induced spin accumulation. These results open the way to the implementation of spin functionality in complementary silicon devices and electronic circuits operating at ambient temperature.


Electrical creation of spin polarization in silicon at room temperature
S.P. Dash, S. Sharma, R.S. Patel, M.P. de Jong, R. Jansen;

Nature 462, 491 (2009).

Most significant breakthroughs in 2009 (3rd on the list) by Physics world.
News - http://www.nature.com/news/2009/091125/full/news.2009.1107.html




Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin–orbit interaction.

Oscillatory spin-polarized tunneling from silicon quantum wells controlled by electric field
R. Jansen, B.C. Min, S.P. Dash;

Nature Materials 9, 133 (2010).

Magnetic Tunnel Junctions with 2D materials

The two-dimensional (2D) atomically thin insulator h-BN and semiconducting MoS2 constitutes a new paradigm in tunnel based devices. A large band gap, along with its atomically flat nature without dangling bonds or interface trap states, makes it an ideal candidate for tunnel spin transport in magnetic tunnel junctions. Here, we demonstrate the tunneling of spin-polarized electrons through monolayer h-BN and multi-layer MoS2 prepared by chemical vapor deposition, in magnetic tunnel junctions. In ferromagnet/2D material/ferromagnet heterostructures fabricated on a chip scale, we show tunnel magnetoresistance corresponding to spin polarization of 5–10% persists up to room temperature. These results open the way for integration of 2D spin tunnel barriers in active spintronic devices and circuits operating at ambient temperature.

Spin-Polarized Tunneling through Chemical Vapor Deposited
Multilayer Molybdenum Disulfide. 
A. Dankert, P. Pashaei, M. V. Kamalakar, A.P.S. Gaur, S. Sahoo, S.P. Dash
ACS Nano, 11(6), 6389–6395 (2017)

Tunnel magnetoresistance with atomically thin two-dimensional hexagonal boron nitride barriers.
A. Dankert, M. V. Kamalakar, A. Wajid, R. S. Patel, S. P. Dash
Nano Research, 8(4), 1357–1364 (2015)


Read more about the Saroj Dash Group


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Full list of Publications, see Google Scholar here

Link to Chalmers Research,  here​

Published: Thu 25 Apr 2019.