Beena Kalisky

Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface

LaAlO{sub 3} and SrTiO{sub 3} are insulating, nonmagnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility, superconductivity at low temperatures, and electric-field-tuned metal-insulator and superconductor-insulator phase transitions. Bulk magnetization and magnetoresistance measurements also suggest some form of magnetism depending on preparation conditions and suggest a tendency towards nanoscale electronic phase separation. Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism, and superconductivity. We find submicron patches of ferromagnetism in a uniform background of paramagnetism, with a nonuniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as suggested by theoretical predictions based on nearly degenerate interface subbands associated with the Ti orbitals. The magnitude and temperature dependence of the paramagnetic response suggests that the vast majority of the electrons at the interface are localized, and do not contribute to transport measurements. In addition to the implications for magnetism, the existence of a 2D superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background suggests the potential for exotic superconducting phenomena.

Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures

In LaAlO(3)/SrTiO(3) heterointerfaces, charge carriers migrate from the LaAlO(3) to the interface in an electronic reconstruction. Magnetism has been observed in LaAlO(3)/SrTiO(3), but its relationship to the interface conductivity is unknown. Here we show that reconstruction is necessary, but not sufficient, for the formation of magnetism. Using scanning superconducting quantum interference device microscopy we find that magnetism appears only above a critical LaAlO(3) thickness, similar to the conductivity. We observe no change in ferromagnetism with gate voltage, and detect ferromagnetism in a non-conducting p-type sample. These observations indicate that the carriers at the interface do not need to be itinerant to generate magnetism. The ferromagnetism appears in isolated patches whose density varies greatly between samples. This inhomogeneity strongly suggests that disorder or local strain generates magnetism in a population of the interface carriers.

Imaging currents in HgTe quantum wells in the quantum spin Hall regime

The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels. The QSH state was predicted and experimentally demonstrated to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices. Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells. These images distinguish between current that passes through each edge and the bulk. On tuning the bulk conductivity by gating or raising the temperature, we observe a regime in which the edge channels clearly coexist with the conducting bulk, providing input to the question of how ballistic transport may be limited in the edge channels. Our results represent a versatile method for characterization of new QSH materials systems.

Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces.

The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO; refs 1, 2). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

Local measurement of the penetration depth in the pnictide superconductor Ba(Fe0.95Co0.05)2As2

We use magnetic force microscopy (MFM) to measure the local penetration depth

Behavior of vortices near twin boundaries in underdoped Ba(Fe 1 − x Co x ) 2 As 2

\ensuremath{\lambda}

Scanning probe manipulation of magnetism at the LaAlO3/SrTiO3 heterointerface.

in

Mechanical Control of Individual Superconducting Vortices

\text{Ba}{({\text{Fe}}_{0.95}{\text{Co}}_{0.05})}_{2}{\text{As}}_{2}

Scanning SQUID susceptometry of a paramagnetic superconductor

single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density

Stripes of increased diamagnetic susceptibility in underdoped superconducting Ba ( Fe 1 − x Co x ) 2 As 2 single crystals: Evidence for an enhanced superfluid density at twin boundaries

{\ensuremath{\rho}}_{s}