Research

My research is focused in systems of strongly correlated electrons among them are the superconducting High Tc Cuprates, disordered superconductors, Heavy Fermions, doped Mott insulators, Kondo insulators, Weyl semi-metals, van der Waals (vdW) materials and more. The many-body phenomena in these systems leads to broken symmetries and phase transitions which are typically manifested in the optical excitations spectrum.

In order to probe the electronic correlations in these systems I am using various optical methods and techniques such as Fourier transform infrared spectroscopy (FTIR), Time-Domain TeraHertz spectroscopy (TDS-THz), NearInfraRed/Visible/UltraViolet ellipsometry and Raman spectroscopy as a function of frequency, temperature, pressure and magnetic field. Here is a summary of my recent work:

Shaping the phase diagram of granular superconductors

From Phys. Rev. B 93, 100503(R) (2016)

Granular aluminum is composed of nano sized metallic aluminum grains coupled through thin insulating barriers (typically aluminum oxide). By controlling the coupling between the grains, one can tune the superconducting temperature of this system reaching a maximum value of about 3.2 K which is a factor of 3 higher than that of bulk Al. In our work, we measured the superconducting energy scales, the energy gap Δ and the superfluid stiffness Js (or equivalently the superfluid density ns), using THz transmission of granular Al thin films. The phase diagram is shaped by the competition between these two energy scales while the system is crossing from a BCS-like amplitude driven superconductivity to non-BCS phase driven superconducting regime. We also detected phase collective mode driven by the phase disorder and a pseudo-gap in the optical conductivity. Recently granular aluminum has gained a lot of interest in the community of qubits and photon detectors for astrophysics.

Heavy fermion plasmons in URu2Si2

sig-ac-2D
From Phys. Rev. B 94, 235101 (2016)

URu2Si2 is a heavy fermion compound which is also superconducting at low temperatures. Over the years a large effort has been dedicated in order to understand its normal properties namely an ordered phase at temperature of about 17.5 K which its mechanism is still under debate, thus notoriously called the “Hidden Order” (HO) phase. As in a typical heavy fermion system, URu2Si2 is crossing over to a coherent state at about 75 K due to hybridization of a localized band with a conducting band. However, apart from transport properties, other apparatus pointed out to lower temperatures for this crossover of the order of 30 K.

sig-loss-HO PPT2
From Phys. Rev. B 94, 235101 (2016)

In our work, we show that the crossover to the coherent state can be detected by optical spectroscopy of strain free surfaces and the hybridization gap can be observed directly in the dynamic conductivity of both a- and c-axis at about 75 K. The energy loss function shows a heavy fermion plasmon at 18 meV attributed to the hexadecapolar state. At lower temperatures we observe additional low frequency plasmons attributed to the mass renormalization and gap opening in the Fermi surface with the transition to the HO state.

Pseudo-gap and correlation-disorder induced collective mode in La doped Sr2IrO4

Fig1
From Phys. Rev. B 98, 045107 (2018)

Sr2IrO4 is a spin orbit Mott insulator which has similar structure to the La2CuO4 from the cuprates family. Although hole doped cuprates such as the La2-xSrxCuO4 exhibit high Tc superconductivity, the iridate family is not showing yet any signatures to a superconducting state. Nevertheless, both materials exhibit an anomalous pseudo-gap in the density of states in the normal state. In our work we show the suppression of the Mott gap in the optical conductivity upon electron doping of the Sr2IrO4 compound. We observe a pseudo-gap at low frequencies which persists up to high doping. In addition we detect a low energy collective mode which can be attributed to correlation-disorder effects involving either polaron dipole-dipole interactions or fluctuating charge density wave which pin the mobile carriers to the incoherent part of the spectrum.

A Breach in the “Standard Model” of the Quantum Matter

News on our Nature Physics paper about an exotic zero-frequency collective mode in the pyrochlore iridate compound Nd2Ir2O7.

THz ellipsometry

I am also developing and building new spectroscopic techniques in the THz frequency range such as our Time Domain THz ellipsometer. Ellipsometry is a powerful tool which allows direct measurement of the optical conductivity in thin films and bulk crystals without the need for a reference measurement (such as in near-normal incidence reflectivity). In combination with THz sources and detectors we are able to access and detect low energy excitations in correlated systems.

150K_conductivity
The real part of the optical conductivity (red marks) of bulk SrTiO3 crystal as was obtained from the ellipsometric parameters Psi and Delta measured at 150 K. The data fits to a model (black line) having a soft phonon mode at about 50 wavenumbers.
The THz ellipsometry setup with a homemade LabView software to control the spectrometer and acquire spectra as a function of angle of incidence, polarizer and analyzer angles and temperature.

Our recent article on the optical conductivity of strained LaNiO3 thin films where THz ellipsometry was used in order to measure the low frequency zero-frequency mode response.

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.155148

In this work we show that the changes in spectral weight of the mobile carriers are induced by the interplay between the in-plane to the out-of-plane hopping parameters. The latter are governed by rotation of the Oxygen octahedra as a function of strain going from tensile to compressive.

Other fun stuff

Rubens’ tube – flame tube that demonstrates the physics of standing waves but can also be used to have a nice flame show along with your favorite music.

As presented in the Geneva Night Science 2018

https://en.wikipedia.org/wiki/Rubens’_tube