Research

We are in a constant search of yet unexplored quantum physical phenomena. Goal of our microscopic studies is to offer valuable input for development of novel theoretical models and guidance in the design of materials with targeted physical properties.

Why NMR?

Many novel quantum phases of matter have been revealed in strongly correlated electron systems. A complex array of these often competing phases leads to emergent phenomena. As many of the phases are associated with the breaking of the same symmetry, they are difficult to discern in bulk measurements of the thermodynamic properties. Consequently, little is known about microscopic nature of these emergent phases and theoretical models for their description, that should involve complex interplay between the charge/orbital, spin, and lattice degrees of freedom, are lacking. Therefore, understanding of these complex phenomena requires implementation of microscopic experimental tools that are simultaneously sensitive to all of the above degrees of freedom. NMR is perfectly suited for this study thanks to its unique sensitivity to both magnetic and charge/orbital excitations at the relevant low energy scales. These low energy excitations are visible signatures of the underlying ground state and their study allows quantitative comparison with theory.

Superconductivity

Unconventional superconductivity, magnetism, and vortex states

The discovery of the CeMIn5 family of materials almost ten years ago sparked a strong research interest since they provide the ground for the study of many big puzzles of strong correlated electron physics, such as quantum criticality, unconventional superconductivity and quantum magnetism. We are particularly interesting in studying nature of coexistence of inherently competing orders, magnetism and superconductivity. NMR provides an exceptional experimental tool for the investigation of such problems since it allows us to separately probe superconducting and magnetic degrees of freedom.

Modulated Superconductivity - (FFLO)

Applied magnetic fields tend to align the spins, whereas the spins in a Cooper pair, in its simplest form, are antiparallel. Those antagonistic tendencies lead to pair breaking. When the pair breaking effect (magnetic field) is sufficiently large, a Cooper pair has two options for "survival". It can become a triplet pair, in which the two spins point in the same direction with the magnetic field, or it can remain in the singlet state with the spins pointing in opposite directions and acquire instead a finite momentum. This momentum leads to a spatially modulated state consisting of periodically alternating "normal" and "superconducting" regions. We are studying nature of superconducting states in high magnetic fields, and how such states interact with vortices.

This work is carried out in collaboration with the NMR group at the French National High Magnetic Field Laboratory in Grenoble, France, lead by Dr. C. Berthier & Dr. M. Horvatic.

Quantum Magnetism

Broadly speaking, this project represents the fundamental study of the quantum mechanics of the spin in strongly correlated electron systems.

2D spin liquids

We deploy NMR to determine the microscopic spin texture and the nature of the low energy excitations in various magnetic field-induced phases in 2D frustrated quantum magnets. These results allow direct quantitative comparison with theory. We adopted and developed NMR probes to allow us to study such field induced quantum phase transitions at the NHMFL. Furthermore, we use the NMR rate (two-spin correlation function) to infer nature of the spin liquid phases, distinct by the flux. Specifically, our results permit to deduce the underlying statistics (fermionic vs. bosonic) of low energy excitations and whether they are fractional in nature. Strong motivation for the study of such systems comes from the observation of superconductivity, possibly mediated by spin fluctuations, in doped materials with the same underlying lattice.

This project is carried out in collaboration with R. Coldea (Oxford, UK), A. Reyes (NHMFL), and theory group of B. Marston (Brown).

Materials with strong spin-orbit coupling (SOC)

In Mott insulators with strong SOC coupling, implying that spin itself is not a good quantum number, various exotic quantum phases (for example, spin nematic, orbital and/or quadrupole orders) are predicted to emerge. Most of the phases are distinguishable by either local point symmetry breaking or local spin expectation values. In principle, these quantities are measurable by a local probe such as NMR (but not discernible in bulk magnetization).

Moreover, we explore effects of giant spin-orbit coupling and quantum confinement in functional and enabling nanoscale structures. These include studies of surface induced magnetic anisotropy and its enhancement via nanoscale and composite structures. The goal of our fundamental research part is to understand giant spin-orbit coupling, surface induced anisotropy, and electron interactions.

2D systems and topological states of matter

The recently discovered topological insulator (TI) is a novel state of quantum matter and is characterized by time-reversal-symmetric gapless topological surface states, which appear within the bulk energy gap. The TI with tunable surface states are necessary for applications in spintronics and quantum computation. We are developing resonance techniques to directly probe spin degrees of freedom and role that spin-momentum-locking plays on transport properties of these surface states.

Semiconductors

Transparent oxyde - semiconductors

We are studying microscopic properties of Sn doped In-oxydes. This project is carried out in collaboration with Prof. David Paine, whose group synthesizes these materials and performs characterization of bulk properties. Our combining microscopic and bulk findings allow us to determine a role oxygen-vacancy dynamics, valence fluctuations, and impurity sub-gap states play in determining bulk transport properties.

Nanoparticles

Particles, with physical size that is small enough, display properties which differ significantly from the bulk material due to both surface effects and quantum confinement effects. The characterization of electronic states that dictate their physical properties requires sensitive techniques to separately probe the metallicity of the surface and bulk of material. In addition, such technique should be able to probe both charge and spin degrees of freedom. NMR offers such sensitive measurement of the electronic wavefunction, correlations, and dynamics. We are developing resonance techniques to study quantum size, surface, and spin-orbit coupling effects on the electronic properties of multimetallic, semimetalic, and magnetic nanoparticles, nanowires, and their thin film structures.

Technical Developments

Surface coils

We are exploring ways in which to use surface coils to efficiently probe 2D-like samples. For example, the meander-line coil is a serpentine array of parallel conductors. The effective RF field is confined to a region adjacent to the coil and its penetration depth is determined only by the spacing between parallel conductors. Since the signal-to-noise ratio in an NMR experiment is proportional to the filling fraction (ratio of the volume of the effective RF field to the sample volume), the meander-line coils are ideally suited for probing thin 2D-like samples or for probing sizable surface area to a limited depth.

Piezo resonance

We are developing piezoelectric resonance technique to allow us to efficiently probe anisotropy of magnetic nanoparticles.

Industrial Collaborations

  • Millikelvin Technilogies

  • Sofar Acoustics