Research

SQUID On Tip sensors

Our lab has developed novel nano-SQUID magnetometers, which enable high sensitivity and high resolution imaging of nano-scale magnetic phenomena.

Nano-SQUIDs thermal imaging of dissipation - a novel tool for directly probing quantum states of matter

In our lab we developed a novel nano-thermometer based on a superconducting quantum interference device (tSOT: SQUID on Tip thermometer) with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette. This tool provides scanning cryogenic thermal sensing that is 4 orders of magnitude more sensitive than previous devices allowing the detection of a sub 1 μK temperature difference. Furthermore, it is non-contact and non-invasive and allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin.

Single vortex pinning

The scanning SOT microscope was used to investigate the dynamics of quantized magnetic vortices and their pinning by materials defects in lead films with unprecedented sub-Angstrom sensitivity to vortex displacement.

Superparamagnetism in Magnetic Topological Insulators

Scanning SQUID microscopy measurements of magnetic topological insulator thin films surprisingly reveal a far more complex picture than commonly assumed: we find that the magnetism clearly originates in superparamagnetism, dominated by nanoscale regions of highly localized magnetic domains rather than long-ranged homogeneous magnetic order.

Emergent nanoscale superparamagnetism at oxide interfaces

We report an emergent phenomenon at the LaMnO3/SrTiO3 interface where an antiferromagnetic Mott insulator abruptly transforms into a nanoscale inhomogeneous magnetic state. Upon increasing the thickness of LaMnO3, our scanning nanoSQUID-on-tip microscopy shows spontaneous formation of isolated magnetic nanoislands, which display thermally activated moment reversals in response to an in-plane magnetic field.

Suppression of geometrical barrier in BSCCO crystals by Josephson vortex stacks

We studied the effect of in-plane field (Hx) on the flux penetration and hysteretic magnetization in BSCCO crystals at elevated temperatures. At these temperatures the exit and entrance of vortices into the sample is governed by geometrical barrier that cause a nonuniform vortex distribution inside the sample in a form of a dome in the center of the sample.

Dynamical aspects of the vortex phase diagram in BSCCO

We studied the dynamical aspects of the vortex matter phase diagram. The sample edges and extreme anisotropy of BSCCO render naive transport measurements inadequate due to a highly nonuniform current distribution across the width and the thickness of the sample.

Therefore, we devised an alternative indirect approach using array of Hall sensors, which combines measurement of the current distribution across the sample with a simple model of the sample edges and bulk in terms of resistors and inductors. As a result, we unveiled the underlying electrodynamic edge mechanism that drives the previously unresolved Tx transition.

Thermodynamic vortex matter phase diagram

In order to investigate the thermodynamic vortex matter phase diagram the vortex lattice has to be in an equilibrium state. At low temperatures below the irreversibility line the lattice is usually not in thermal equilibrium due to strong pinning and large magnetic hysteresis. In order to resolve the thermodynamic properties, we relaxed the hysteretic magnetization in BSCCO crystals by ‘vortex shaking’ using an in-plane ac field.

Influence of spatial variations in the lower critical field on the equilibrium field penetration into superconductors

The thermodynamic lower critical field Hc1 is one of the fundamental parameters of the mixed state in type II superconductors. At Hc1 the formation of vortices becomes energetically favorable and the superconductor undergoes a transition from the Meissner state to the mixed state. The value of Hc1 and the way the field penetrates into the sample are directly related to the free energy of a vortex and to essential mixed state parameters such as the penetration depth and the Ginzburg-Landau parameter.

Melting of 'porous' vortex matter

Different phases of vortex matter emerge as a result of competition between the elasticity of the vortex lattice, which tries to maintain long-range order, and thermal fluctuations and pinning which try to disorder the vortex matter. One of the ways of artificially controlling the extent of disorder in superconductors is by introducing columnar defects by bombarding the samples with high-energy heavy ions.

Shear-induced Vortex Decoupling

Local magnetization studies in BSCCO crystals have been carried out in the presence of elevated transport currents using Hall sensors arrays, and the corresponding transport and magnetization properties were measured simultaneously. These measurements reveal an apparently paradoxical situation in which a finite resistivity is observed well below the magnetic irreversibility line.

Instabilities and Memory Effects in Vortex Matter

In recent years a number of very surprising vortex dynamics phenomena were observed in both low and high temperature superconductors with no evident explanation. These include memory effects, slow voltage oscillations, unconventional frequency dependence, extreme sensitivity of the ac response to a dc bias, instabilities, noise, slow voltage oscillations, and more.
We have proposed an edge contamination model which accounts for the observed phenomena and have developed a special experimental approach to test its predictions.

Disorder-driven transition and vortex instabilities in Corbino disk geometry

The nature of the disorder-driven solid-solid transition in the vortex matter in superconductors, and the associated instabilities, have recently attracted wide attention. A closely related question is the origin of the commonly observed peak-effect, at which the critical current increases significantly in the vicinity of Hc2 in low-Tc superconductors. Since its discovery more than twenty years ago the nature of the peak-effect has been controversial. In this work we present evidence that the peak-effect reflects a first-order transition from a quasi-ordered lattice (Bragg glass) into a strongly pinned disordered phase.