Gotowa bibliografia na temat „Vortex-charge in superconductors”

The interplay between charge order and d-wave superconductivity in high-Tc cuprates remains an open question. While mounting evidence from spectroscopic probes indicates that charge order competes with superconductivity, to date little is known about the impact of charge order on charge transport in the mixed state, when vortices are present. Here we study the low-temperature electrical resistivity of three distinctly different cuprate families under intense magnetic fields, over a broad range of hole doping and current excitations. We find that the electronic transport in the doping regime where long-range charge order is known to be present is characterized by a nonohmic resistivity, the identifying feature of an anomalous vortex liquid. The field and temperature range in which this nonohmic behavior occurs indicates that the presence of long-range charge order is closely related to the emergence of this anomalous vortex liquid, near a vortex solid boundary that is defined by the excitation current in the T→ 0 limit. Our findings further suggest that this anomalous vortex liquid, a manifestation of fragile superconductivity with a suppressed critical current density, is ubiquitous in the high-field state of charge-ordered cuprates.

We have investigated the vortex in chiral superconductors, especially in p-wave case. In chiral superconductors the Cooper pair has orbital angular momentum hence U(1), parity (P) and time reversal symmetry (T) are broken simultaneously. We have found that the vortex has fractional charge and fractional angular momentum which comes from P- and T-violation. The fractionalization of the angular momentum suggests that the vortex could be an anyon which obeys the fractional statistics. We have also pointed out that the electric field is induced near the vortex core and non-trivial electromagnetic phenomena are expected to occur.

From high resolution measurements of the nuclear quadrupole frequencies we obtain experimental evidence that a vortex in high Tc superconductors (HTSC) traps a finite electric charge. In slightly overdoped YBa2Cu3O7 the vortex is negatively charged by trapping electrons, while in underdoped YBa2Cu4O8 it is positively charged by expelling electrons. The sign of the trapped charge is opposite to the sign predicted by the conventional BCS theory. Moreover, in both materials the deviation of the magnitude of the charge from the theory is significant. These features can be attributed to the novel electronic structure of the vortex in HTSC

I review the concept of statistics transmutation in two dimensions and apply it to the understanding of Fractional quantum-Hall effect and anyon superconductivity. The relevance of the anyon model to copper-oxide superconductors is also discussed.

The nature of vortex structure in the mixed state of high-temperature superconductors (HTS) is investigated by solving the Bogoliubov-de Gennes equations with consideration of competition between antiferromagnetic (AF) and d-wave superconductivity (DSC) orders. By varying the applied magnetic field and temperature, the geometry of vortex structure can take two different forms: conventional vortex lattice (triangular or square), or vortex stripe phases where all the order parameters including spin density wave, charge density wave and superconducting order exhibit stripe-like behavior. This novel vortex stripe phases may show up at low temperature and adjacent to upper critical field H c2 Phase diagram of temperature dependence of H c2 will be presented. Our results may shed light on the understanding of the low-temperature H c2 anomalies in some HTS. New experiments are proposed to test our predictions.

The penetration of magnetic fields in the form of quantized vortices in type-II superconductors is well known. However, it is not well known that such vortices can be electrically charged. The effect is quite subtle and originates from the particle-hole symmetry in a superconductor. The high Tc superconductors (HTS) are predicted to be better candidates for vortex-charge detection due to their large superconducting gap. Thus far, a direct measurement of charged cores has remained challenging due to their small value and electrostatic screening by the surrounding opposite charges. In recent years, cavity-optomechanical techniques have emerged as an attractive method to improve the sensitivity of various measurements. Such methods have shown exquisite force sensitivities down to the standard quantum limit and control over the quantum states of the motion. Recently, such techniques have also drawn attention to probe the thermodynamic properties of atomically thin two-dimensional (2D) materials. The 2D crystals are particularly attractive for developing mechanical resonators and their integration in optomechanical device due to their low mass, and hence larger coupling with light field. Motivated from these aspects, we develop a device to directly detect the charges in the flux-vortices by measuring the electromechanical response. Here the electrostatic effect of vortex-charge is transduced to the mechanical response. To study the vortex charge, a few UC thick crystals of high-transition temperature superconductor Bi2Sr2CaCu2O8+δ (BSCCO) is used for the mechanical resonator. One important parameter of the mechanical resonator is its resonant frequency. However, estimating the resonant frequency requires elastic modulus like Young's modulus and pre-tension in the flake. While the elastic coefficients of the bulk crystals of BSCCO have been observed with large variations, there is no investigation into the elastic properties of a few UC thick nanoscale samples. Further, the mechanical properties of a few unit cells (UC) thick exfoliated crystals could be significantly different from their bulk counterpart. To begin with, we present systematic measurements of the mechanical properties of a few unit cells (UC) thick exfoliated crystals of a high-Tc cuprate superconductor BSCCO. We determine the elastic properties of these crystals by deformation using an atomic force microscope (AFM) at room temperature. With the spatial measurements of local compliance and their detailed modelling, we determine Young's modulus of rigidity and the pre-stress. Young's modulus of rigidity is found to be in the range of 22 GPa to 30 GPa for flakes with thickness from 5 UC to 18 UC. The pre-stress spreads over the range of 5 MPa - 46 MPa, indicating a run-to-run variation during the exfoliation process. The determination of Young's modulus of rigidity for thin flakes is further verified from the recently reported buckling technique [1]. In the next chapter, we present nanoelectromechanical resonators fabricated with thin exfoliated crystals of BSCCO. The mechanical r= eadout is performed by capacitively coupling their motion to a coplanar waveguide microwave cavity fabricated with a superconducting alloy of molybdenum-rhenium (MoRe). We demonstrate mechanical frequency tunability with external dc-bias voltage and quality factors up to ~36600. Our spectroscopic and time-domain measurements show that mechanical dissipation in these systems is limited by the contact resistance arising from resistive outer layers. The temperature dependence of dissipation indicates the presence of tunnelling states, further suggesting that their intrinsic performance could be as good as other two-dimensional atomic crystals such as grap= hene [2]. Learning from these two experiments, we improve the performance of the device and carry out the mechanical exfoliation in inert atmosphere. We integrate a mechanical resonator made of a thin flake of HTS BSCCO into a microwave circuit to realize a cavity-electromechanical device. In the final chapter, we studied the electromechanical response of the mechanical resonator when a magnetic field perpendicular to the CuO2 plane is applied. As the magnetic field penetrates the surface of a superconductor, it results in the formation of flux-vortices. These flux-vortices will have charged vortex core and create a dipolelike electric field. Due to the exquisite sensitivity of cavity-based devices to the external forces, we directly detect the charges in the flux vortices by measuring the electromechanical response of the mechanical resonator [3]. Our measurements reveal the strength of surface electric dipole moment due to a single vortex core to be approximately 30 |e|aB, where aB is the Bohr radius and e is the electron charge. Further, using the value of surface dipole moment, we have estimated the vortex line charge to be +4.9 × 10-2|e|/nm, which is equivalent to a charge per CuO_2 layer to be +3.7 × 10-2|e|.