arXiv subjects

Hyok-Jon Kwon

Publications and source records attributed to Hyok-Jon Kwon.

14 recordsLinked to original sources

arXiv2004

Fractional ac Josephson effect in unconventional superconductors

For certain orientations of Josephson junctions between two p_x-wave or two d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic relation between the Josephson current I and the phase difference phi: I ~ sin(phi/2). Consequently, the ac Josephson current has the fractional frequency eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction.

cond-mat.supr-con↗

arXiv2004

Fractional ac Josephson effect in p- and d-wave superconductors

For certain orientations of Josephson junctions between two p_x-wave or two d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic relation between the Josephson current I and the phase difference phi: I sin(phi/2). Consequently, the ac Josephson current has the fractional frequency eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction. We also study junctions between singlet s-wave and triplet p_x-wave, as well as between chiral p_x + ip_y-wave superconductors.

cond-mat.supr-con↗

arXiv2002

Spontaneous formation of a pi soliton in a superconducting wire with an odd number of electrons

We consider a one-dimensional superconducting wire where the total number of electrons can be controlled in the Coulomb blockade regime. We predict that a pi soliton (kink) will spontaneously form in the system when the number of electron is odd, because this configuration has a lower energy. If the wire with an odd number of electrons is closed in a ring, the phase difference on the two sides of the soliton will generate a supercurrent detectable by SQUID. The two degenerate states with the current flowing clockwise or counterclockwise can be utilized as a qubit.

cond-mat.mes-hall↗

arXiv2002

Edge states and determination of pairing symmetry in superconducting Sr2RuO4

We calculate the energy dispersion of the surface Andreev states and their contribution to tunneling conductance for the order parameters with horizontal and vertical lines of nodes proposed for superconducting Sr2RuO4. For vertical lines, we find double peaks in tunneling spectra reflecting the van Hove singularities in the density of surface states originating from the turning points in their energy dispersion. For horizontal lines, we find a single cusp-like peak at zero bias, which agrees very well with the experimental data on tunneling in Sr2RuO4.

cond-mat.supr-con↗

arXiv2001

How to detect edge electron states in (TMTSF)2X and Sr2RuO4 experimentally

We discuss a number of experiments that could detect the electron edge states in the organic quasi-one-dimensional conductors (TMTSF)2X and the inorganic quasi-two-dimensional perovskites Sr2RuO4. We consider the chiral edges states in the magnetic-field-induced spin-density-wave (FISDW) phase of (TMTSF)2X and in the time-reversal-symmetry-breaking triplet superconducting phase of Sr2RuO4, as well as the nonchiral midgap edge states in the triplet superconducting phase of $(TMTSF)2X. The most realistic experiment appears to be an observation of spontaneous magnetic flux at the edges of Sr2RuO4 by a scanning SQUID microscope.

cond-mat.supr-con↗

arXiv2001

Dual proximity effect near superconductor-insulator transition

We show that quantum vortex-loop proliferation may be one possible explanation of the super-long-range proximity effect observed in an insulating underdoped cuprate, using the dual theory of quantum vortices. As a test of this scenario, we propose that a dual proximity effect experiment can confirm the superfluid motion of the quantum vortices in the vortex-proliferated insulator and can measure the divergent correlation lengths near the superconductor-insulator transition.

cond-mat.supr-con↗

arXiv2001

Midgap edge states and pairing symmetry of quasi-one-dimensional organic superconductors

The singlet s-, d- and triplet p-wave pairing symmetries in quasi-one-dimensional organic superconductors can be experimentally discriminated by probing the Andreev bound states at the sample edges. These states have the energy in the middle of the superconducting gap and manifest themselves as a zero-bias peak in tunneling conductance into the corresponding edge. Their existence is related to the sign change of the pairing potential around the Fermi surface. We present an exact self-consistent solution of the edge problem showing the presence of the midgap states for p_x-wave superconductivity. The spins of the edge state respond paramagnetically to a magnetic field parallel to the vector d that characterizes triplet pairing.

cond-mat.supr-con↗

arXiv2001

Edge electron states for quasi-one-dimensional organic conductors in the magnetic-field-induced spin-density-wave phases

We develop a microscopic picture of the electron states localized at the edges perpendicular to the chains in the Bechgaard salts in the quantum Hall regime. In a magnetic-field-induced spin-density-wave state (FISDW) characterized by an integer N, there exist N branches of chiral gapless edge excitations. Localization length is much longer and velocity much lower for these states than for the edge states parallel to the chains. We calculate the contribution of these states to the specific heat and propose a time-of-flight experiment to probe the propagating edge modes directly.

cond-mat.mes-hall↗

arXiv2001

Quantum vortex fluctuations in cuprate superconductors

We study the effects of quantum vortex fluctuations in two-dimensional superconductors using a dual theory of vortices, and investigate the relevance to underdoped cuprates where the superconductor-insulator transition (SIT) is possibly driven by quantum vortex proliferation. We find that a broad enough phase fluctuation regime may exist for experimental observation of the quantum vortex fluctuations near SIT in underdoped cuprates. We propose that this scenario can be tested via pair-tunneling experiments which measure the characteristic resonances in the zero-temperature pair-field susceptibility in the vortex-proliferated insulating phase.

cond-mat.supr-con↗

arXiv2001

Observability of quantum phase fluctuations in cuprate superconductors

We study the order parameter phase fluctuation effects in cuprate superconductors near T=0, using a quasi-two-dimensional d-wave BCS model. An effective phason theory is obtained which is used to estimate the strength of the fluctuations, the fluctuation correction to the in-plane penetration depth, and the pair-field susceptibility. We find that while the phase fluctuation effects are difficult to observe in the renormalization of the superfluid phase stiffness, they may be observed in a pair tunneling experiment which measures the pair-field susceptibility.

cond-mat.supr-con↗

arXiv1999

Novel antiferromagnetic quantum phase transition in underdoped cuprates

We investigate a zero-temperature itinerant antiferromagnetic transition where the fermions possess a d-wave gap. This problem pertains to both the nodal liquid insulating phase and the d-wave superconducting phase of the underdoped cuprates. We find that a non-trivial quantum phase transition exists, and that the quantum critical point is dominated by a long-ranged interaction ($|x-y|^{-2d}$) of the Néel order parameter, which is induced by the Dirac-like fermions near gap nodes. We formulate a Ginzburg-Landau functional and estimate the critical exponents via the large-N expansion method.

cond-mat.supr-con↗

arXiv1999

Order parameter fluctuation effects in d-wave BCS superconductors

We study order parameter fluctuation effects in the superconducting state as a possible precursor to the pseudogap phenomena. Using a low-energy effective theory in the d-wave BCS model, we self-consistently calculate the single-particle properties. We find that the fluctuations reduce the spectral gap, and the shape of the gap is deformed, showing reduced slope of the gap at the nodes. We also show the angle-dependence of the quasiparticle lifetime due to the fluctuations.

cond-mat.supr-con↗

arXiv1998

The effect of phase fluctuations on the single-particle properties of the underdoped cuprates

We study the effect of order parameter phase fluctuations on the single-particle properties of fermions in the underdoped cuprate superconductors using a phenomenological low-energy theory. We identify the fermion-phase field coupling as the Doppler-shift of the quasiparticle spectrum induced by the fluctuating superfluid velocity and we calculate the effect of these fluctuations on the fermion self-energy. We show that the vortex pair unbinding near the superconducting transition causes a significant broadening in the fermion spectral function, producing a pseudogap-like feature. We also discuss the specific heat and show that the phase fluctuation effect is visible due to the short coherence length.

cond-mat.supr-con↗

arXiv1997

Kohn-Luttinger effect in nested Fermion liquids

We study the Kohn-Luttinger effect in a two-dimensional (2D) nested Fermion liquid with a repulsive interaction via the renormalization group method and identify the resulting order parameter symmetry. Using the band structure of the 2D Hubbard model close to half-filling as a prototype, we construct an effective low-energy theory. We use multidimensional bosonization to incorporate the zero-sound channel and find marginal Fermi liquid behavior in the absence of any instability. We show an analog of the Landau theorem in nested Fermion liquids, which serves as the criterion of the BCS instability. Including repulsive or antiferromagnetic exchange interactions in the low-energy theory, we show that the $d_{x^2-y^2}$-wave BCS channel is renormalized to be the most attractive. Below half-filling, when the nesting is not perfect, there is competition between the spin-density-wave (SDW) and the BCS channels; when the SDW coupling is small enough, there occurs a $d_{x^2-y^2}$-wave superconducting instability at sufficiently low temperatures.

cond-mat.str-el↗