Charge and spin transport in graphene-based devices

Ph.DDOCTOR OF PHILOSOPH

Optospintronics in graphene via proximity coupling

The observation of micron size spin relaxation makes graphene a promising material for applications in spintronics requiring long distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphenes intrinsically low spin orbit coupling strength and optical absorption place an obstacle in their realization. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of a circularly polarized light activates the spin polarized charge carriers in the WSe2 layer due to its spin coupled valley selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 um distance, and are finally detected electrically using BN/Co contacts in a non local geometry. Polarization dependent measurements confirm the spin origin of the non local signal

Control of interlayer excitons in two-dimensional van der Waals heterostructures

Long-lived interlayer excitons with distinct spin-valley physics in van der Waals heterostructures based on transition metal dichalcogenides make them promising for information processing in next-generation devices. While the emission characteristics of interlayer excitons in different types of hetero stacks have been extensively studied, the manipulation of these excitons required to alter the valley-state or tune the emission energy and intensity is still lacking. Here, we demonstrate such control over interlayer excitons in MoSe2/WSe2 heterostructures. The encapsulation of our stack with h-BN ensures ultraclean interfaces, allowing us to resolve four separate narrow interlayer emission peaks. We observe two main interlayer transitions with opposite helicities under circularly polarized excitation, either conserving or inverting the polarization of incoming light. We further demonstrate control over the wavelength, intensity, and polarization of exciton emission by electrical and magnetic fields. Such ability to manipulate the interlayer excitons and their polarization could pave the way for novel excitonic and valleytronic device applications

van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices

Due to the chemical inertness of 2D hexagonal-Boron Nitride (h-BN), few atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such as Black Phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping and contact resistance are not well known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (~ 4.5 k-ohm), and are Schottky barrier free. This allows us to probe high electron mobilities (4,200 cm2/Vs) and observe insulator-metal transitions even under two-terminal measurement geometry

Resolving the spin splitting in the conduction band of monolayer MoS2

Time-reversal symmetry and broken spin degeneracy enable the exploration of spin and valley quantum degrees of freedom in monolayer transition-metal dichalcogenides. While the strength of the large spin splitting in the valance band of these materials is now well-known, probing the 10-100 times smaller splitting in the conduction band poses significant challenges. Since it is easier to achieve n-type conduction in most of them, resolving the energy levels in the conduction band is crucial for the prospect of developing new spintronic and valleytronic devices. Here, we study quantum transport in high mobility monolayer MoS2 devices where we observe well-developed quantized conductance in multiples of e(2)/h in zero magnetic field. We extract a sub-band spacing energy of 0.8 meV. The application of a magnetic field gradually increases the interband spacing due to the valley-Zeeman effect. Here, we extract a g-factor of similar to 2.16 in the conduction band of monolayer MoS2

Prevalence of Subclinical Hypothyroidism among Patients with Acute Myocardial Infarction

Introduction. Subclinical hypothyroidism (SCH) is defined as a serum thyroid-stimulating hormone (TSH) level above the upper limit of normal despite normal levels of serum free thyroxine. There is growing evidence that SCH is associated with increased cardiovascular risk. We tried to investigate prevalence of SCH in acute myocardial infarction patients. Methods and Results. We evaluate free T3, free T4, and TSH levels of 604 patients (age 58.4) retrospectively, who have been admitted to the coronary intensive care unit between years 2004–2009 with the diagnosis of ST elevation (STEMI) or non-ST elevation acute myocardial infarction (NSTEMI). Mild subclinical hypothyroidism (TSH 4.5 to 9.9 mU/l) was present in 54 (8.94%) participants and severe subclinical hypothyroidism (TSH 10.0 to 19.9 mU/l) in 11 (1.82%). So 65 patients (10.76%) had TSH levels between 4.5 and 20. Conclusions. In conclusion, 65 patients (10.76%) had TSH levels between 4.5 and 20 in our study, and it is a considerable amount. Large-scale studies are needed to clarify the effects of SCH on myocardial infarction both on etiologic and prognostic grounds