Dr. Watchara Liewrian

♠เบอร์โทรศัพท์ - (+66)

E-mail - watchara.lie@mail.kmutt.ac.th


Spin Transport in Graphene Nanostructure: From Electronics to Spintronics

       Spintronics is a new field of electronics in which the spin degree of freedom is also used to convey or store information [1]. This has opened up new ways for creating a new generation of electronic devices which are smaller, faster and consumes less electric power.  This makes them more versatile than today’s charge-based electronic devices. Spintronics devices are now used as magnetic field sensors, such as a read/write heads of the hard disks, as magnetic random access memories (MRAM). To accomplish this, one needs to create a non-equilibrium spin population in certain parts of the device.  This can be done by including a ferromagnetic metal or semiconductor in the device in order to Zeeman split the current into two separate spin currents and then find a way to make the conductance for the two spin currents to be unequal.  This would allow for the accumulation of spins to occur. At present, most applications of spintronics are based on the highly spin polarization of the current which is crucial to the operation of a spintronics device that operates through the manipulation of half-metallic nature of ferromagnetic layers in the device.  Future generations of spintronics devices are expected to be based on magnetic tunnel junctions. 

Graphene is a two-dimensional honeycomb lattice. It was quickly established that the mobile low excitation energy carriers in graphene mimic the behavior of massless relativistic Dirac particles having an effective speed of light ceff = vF (the Fermi velocity) ~ 106 m/s. The discovery of graphene can bridge the gap between quantum electrodynamics and condensed matter physics, because of the unique features of its band structure.  Band structure calculations showed that the E versus k surface consists of two cones intersecting at the two Dirac points K and K' (these being the points in the reciprocal momentum space which correspond to the points A and B in the honeycomb structure of graphene). The inverted cone at the bottom is the spectrum for the negatively charged carriers (electrons) while the upright cone on the top is the spectrum for the positively charge carriers (holes).  This type of band structure means that the graphene is a zero gap semiconductor in which the carriers are relativistic particles whose behaviors are governed by the Weyl-Dirac Equations.

Graphene is an attractive material for spintronics due to the low intrinsic spin-orbit coupling and the carriers in graphene have long spin flip length lsf ~ 1 µm at room temperature which should lead to excellent spin transport properties. The half-metallicity in a zigzag graphene nanoribbon can be induced by an external transverse electric field.  This pointed to the way that graphene-based spintronics could operate. For spintronics applications, ferromagnetic insulators deposited on graphene can induce ferromagnetic state which causes the charged carriers with spin passing through the region to split into two sub bands. For combining graphene with ferromagnetic material such as nickel, a spin-orbit interaction in ferromagnetism suppressed mobility of carriers in graphene. The development of graphene-based spintronic relies on the ability to manipulate the spin information of charge carriers in gapless semiconductor through metallic voltage gate. This is an attractive pathway for designing spintronic devices because this is easy to generate and control of spin orientation by using the electrical fields. The densities of the carriers and the type of doping in graphene could be controlled by applying an external electric field across the graphene sheet. A positive gate on top of the graphene would create an excess amount of electrons (an n-region) in the graphene below the gate while a negative gate would create an excess amount of holes (a p-region) in the graphene below the gate. 

1 - Sikarin Yoo-Kong and Watchara Liewrian, 2015, Double path-integral method for obtaining the mobility of the one-dimensional charge transport in molecular chain, The Europian Journal of Physics E, 38(12), 135.2015Watchara Liewrian
2 - Chisanupong Puttaprom, Sikarin Yoo-Kong, Monsit Tanasittikosol and Watchara Liewrian, 2014,
Entanglement entropy for a particle coupled with its surrounding, Bulgarian journal of physics; 41, pp.1-9
2014Watchara Liewrian

DoctoralPhD (Physics)Mahidol University (Thailand)2011
MasterMSc (Physics) Mahidol University (Thailand)
BachelorBSc (Physics) King Mongkut's University of Technology Thonburi (Thailand)