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Victor Vakaryuk

Screen Name: vakaryuk
Email: vakaryuk@gmail.com
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Branch Affiliation: Johns Hopkins University
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Research Area: Pnictides: These are multi-band superconductors which, some argue, could represent a new quantum state of matter. I am interested in the possibility of dynamics of the relative phase between the bands. Previously known examples of such dynamics include Leggett mode. We are looking at the so-called phase soliton - topological defect in which phase windings in different bands are different. This is complicated by the presence of the interband (Josephson) coupling which in pnictides is believed to be quite strong. This penalizes deviations of the relative phase from the equilibrium value which could be 0 or pi depending on the sign of the interband coupling. That is why a general attitude towards existence of phase solitons in pnictides is skeptical (even for metastable configurations). In cond-mat/1203.4554 (now accepted to PRL) I and my collaborators have proposed to bring the phase soliton to life using proximity effect. Let’s consider a two-band material with originally pi-phase difference between the bands (so-called s+- pairing). Let’s then make a proximity sandwich which consists of this material and a strong single-band s-wave SC. It is intuitively clear and could be confirmed by a calculation that, because of the proximity, originally pi-shifted phases will cant toward each other. Vaguely speaking, for a very good proximity, phases of the two-band material align with a phase standard impose by the s-wave SC. Now let’s take a ring of a pi-shifted material and place an s-wave segment or, as we call it, proximity patch on it. In this way under the patch the phases of the bands will be aligned while away from the patch they will be anti-aligned. This situation, we argue, is much more favorable for the existence of the phase soliton. Loosely speaking, in this structure half of the soliton is already present. This is not the case for the originally 0-shifted (or s++) material. We are suggesting to utilize this observation to test pairing symmetry in pnictides. Chiral superconductors : These unconventional superconductors support the existence of new type of topological defects - chiral domain walls. While this has been known for quite a while, criteria for thermodynamic stability and equilibrium configuration of such domains were missing in the literature. Using London theory I considered an entry of a domain wall or a vortex into the sample. I showed that preferred shape of the domain wall is a loop. The formation of a straight domain wall is never preferred in equilibrium. Values of the entry (critical) fields for both types of defects, as well as the equilibrium size of the domain-wall loop, are calculated. I also considered a mesoscopic chiral sample and calculated its zero-field magnetization, susceptibility, and a change in the magnetic moment due to a vortex or a domain wall entry. I showed that in the case of a soft domain wall whose energetics is dominated by the chiral current (and not by the surface tension) its behavior in mesoscopic samples is substantially different from that in the bulk case and can be used for a controllable transfer of edge excitations. The applicability of these results to Sr2RuO4 - a tentative chiral superconductor -  was also discussed. Charge transport in nanostructures: I have worked on transport properties of several systems:  quasi -1D nanowires, patterned superconducting networks and graphene proximity junctions. I will briefly describe the latter.  Graphene proximity junctions are unique in a sense the their critical current can be tuned by a gate voltage while keeping the temperature constant. We were able to explain experimentally observed scaling of switching current distribution, in particular, of the temperature and gate voltage dependencies of the switching current dispersion. This was done by generalizing the Kurkijarvi theory of the field-escape distribution for escape from a metastable potential well. Half-quantum vortices:  I have pointed out to a novel effect which implies that a half-quantum vortex, even in the absence of the Zeeman coupling, should be accompanied by a non-zero spin polarization of kinematic origin. This effect was proposed to explain the observed in-plane field coupling of a half-quantum vortex in cantilever magnetometry measurements in Sr2RuO4 done by Raffi Budakian in Urbana. I have also worked on a variety of topics related to half-quantum vortices such as their stability in different geometries and patterned microstructures. Mesoscopic superconductors: Violation of the conventional flux quantization in mesoscopic superconductors. I was able to point out that the expected hc/2e periodicity of the free energy is modified as the sample gets smaller. This happens regardless the nature of superconducting state i.e.~even in the absence of low-energy quasiparticle excitations (whose presence is required for other mechanisms). The reason for the break down is that the center-of-mass and internal degrees of freedom of Cooper pairs cannot be separated. This implies, in particular, that standard Ginzburg-Landau description is no longer applicable. Vortex pinning:  Extended structure defects such as edge and screw dislocations serve as effective pinning centers for vortices. This could be essential for technological applications as a way to improve current-carrying abilities of superconductors. I, in collaboration, studied complicated pinning profiles caused by the presence of networks of edge and screw dislocations.  This study was later applied to explain angular dependences of critical current in thin YBCO films. Other: Ongoing projects which include fluctuating response of of d-wave superconducting rings above Tc, interference of phase slips in double wire systems and the Kerr effect in pnictide-s-wave proximity structures which break time reversal symmetry.
Research Description:
Thrust Area: Quantum Matter


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