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- Ilderton, Antony, 1978, et al.
(författare)
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Nonperturbative pair production in interpolating fields
- 2015
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Ingår i: Physical Review D - Particles, Fields, Gravitation and Cosmology. - 2470-0010 .- 2470-0029. ; 92:6, s. artikel nr 065001-
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Tidskriftsartikel (refereegranskat)abstract
- We compare the effects of timelike, lightlike and spacelike one-dimensional inhomogeneities on the probability of nonperturbative pair production in strong fields. Using interpolating coordinates we give a unifying picture in which the effect of the inhomogeneity is encoded in branch cuts and poles circulated by complex worldline instantons. For spacelike inhomogeneities the length of the cut is related to the existence of critical points, while for lightlike inhomogeneities the cut contracts to a pole and the instantons become contractible to points, leading to simplifications particular to the lightlike case. We calculate the effective action in fields with up to three nonzero components, and investigate its behavior under changes in field dependence.
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| 2. |
- Ilderton, Antony, 1978, et al.
(författare)
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Pair production from residues of complex worldline instantons
- 2015
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Ingår i: Physical Review D - Particles, Fields, Gravitation and Cosmology. - 2470-0010 .- 2470-0029. ; 92:2
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Tidskriftsartikel (refereegranskat)abstract
- We study nonperturbative pair production in electric fields with lightlike inhomogeneities, using complex worldline instantons. We show that the instanton contribution to the pair production probability is a complex contour integral over the instanton itself, and that pair production in the considered fields can be recast in terms of Cauchy's residue theorem. The instantons contribute residues from the poles they circulate (i.e. give local contributions), and the invariance of complex integrals under contour deformation manifests in the instanton contributions as invariance under a set of generalized, complex, reparametrizations.
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| 3. |
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| 4. |
- Wårdh, Jonatan, 1990, et al.
(författare)
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Effective model for a supercurrent in a pair-density wave
- 2017
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Ingår i: Physical Review B Condensed Matter. - 0163-1829. ; 96:22
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Tidskriftsartikel (refereegranskat)abstract
- We extend the standard effective model of d-wave superconductivity of a single band tight-binding Hamiltonian with a nearest-neighbor attraction to include finite range periodically modulated pair hopping. The pair hopping is characterized by a fixed wave number Q=Qx̂ breaking lattice rotational symmetry. Within self-consistent BCS theory we study the general variational state consisting of two incommensurate singlet pair amplitudes ΔQ1 and ΔQ2 and find two types of ground states; one of the Larkin-Ovchnnikov (LO) or pair-density wave (PDW) type with ΔQ1=ΔQ2 and Q1=−Q2≈Q, and one of the Fulde-Ferrell (FF) type with ΔQ2=0 and Q1≈±Q. An anomalous term in the static current operator arising from the pair hopping ensures that Bloch's theorem on ground state current is enforced also for the time-reversal and parity breaking FF state, despite no spin-population imbalance. We also consider a supercurrent by exploring the space of pair momenta Q1 and Q2 and identify characteristics of a state with multiple finite momentum order parameters. This includes the possibility of phase separation of current densities and spontaneous mirror-symmetry breaking manifested in the directional dependence of the depairing current.
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| 5. |
- Wårdh, Jonatan, 1990
(författare)
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Fluctuating superconductivity and pair-density wave order in the cuprate superconductors
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-temperature superconductors are some of nature’s most enigmatic materials. Besides carrying a supercurrent, these materials manifest a range of electronic and structural orders. A state of modulated superconductivity, called a pair-density wave (PDW), has been suggested to occur in copper-based (cuprate) high-temperature superconductors, with the possibility of explaining these various orders, and perhaps even superconductivity itself. This thesis is based upon four appended papers and concerns the nature of the PDWstate and the cuprate superconductors. In the first two papers, we consider a so-called pair-hopping interaction that can stabilize a (mean-field) PDWstate. In the first paper, we use this interaction to study the supercurrent carried by a PDW state, which, due to it being a multiplecomponent order, can lead to phase-separation and additional symmetry breaking. In the second paper, we study the competition between a PDWstate and an ordinary uniform superconducting state in the context of a BCS-BEC crossover. We find a suppressed superfluid stiffness in the vicinity of a PDWinstability, with implications on the nature of the underdoped cuprates. The third paper includes an experimental study on thin films of La_2-x Sr_xCuO_4, which above Tc develops a highly anisotropic resistive response, especially pronounced for underdoped samples, pointing towards an exotic pseudogap phase in the underdoped cuprates with quasi-1D phase superfluid stiffness. We interpret these results in terms of nematic order manifested in the superconducting fluctuations. In the last paper of this thesis, we consider a scenario where the cuprate pseudogap phase consists of a thermally disorder PDW state with vestigial order. We show that a vestigial PDWnematic order coexisting with a uniformsuperconducting order yields an anisotropic superconductor on a formconsistent with the fluctuations seen in La_2-x Sr_xCuO_4. Finally, in addition to providing background for the appended papers, this thesis contains an introduction to the general phenomenology of the cuprate superconductors.
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| 7. |
- Wårdh, Jonatan, 1990, et al.
(författare)
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Suppression of superfluid stiffness near a Lifshitz-point instability to finite-momentum superconductivity
- 2018
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Ingår i: Physical Review B Condensed Matter. - 0163-1829. ; 98
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Tidskriftsartikel (refereegranskat)abstract
- We derive the effective Ginzburg-Landau (GL) theory for finite-momentum [Fulde-Ferrell-Larkin-Ovchinnikov/pair-density wave (FFLO/PDW)] superconductivity without spin population imbalance from a model with local attraction and repulsive pair hopping. We find that the GL free energy must include up to sixth-order derivatives of the order parameter, providing a unified description of the interdependency of zero and finite-momentum superconductivity. For weak pair hopping the phase diagram contains a line of Lifshitz points where vanishing superfluid stiffness induces a continuous change to a long wavelength FF state. For larger pair hopping there is a bicritical region where the pair momentum changes discontinuously. Here the FF type state is near degenerate with the LO or PDW type state. At the intersection of these two regimes there is a “super-Lifshitz” point with extra soft fluctuations. The instability to finite-momentum superconductivity occurs for arbitrarily weak pair hopping for sufficiently large attraction suggesting that even a small repulsive pair hopping may be significant in a microscopic model of strongly correlated superconductivity. Several generic features of the model may have bearing on the cuprate superconductors, including the suppression of superfluid stiffness in proximity to a Lifshitz point as well as the existence of subleading FFLO order (or vice versa) in the bicritical regime.
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