Goran Tripun Karapetrov

Charge density waves (CDWs), electronic crystals that form within a host
solid, have long been speculated to melt into a spatially textured electronic
liquid. Though they have not been previously detected, liquid CDWs may
nonetheless be fundamental to the phase diagrams of many correlated electron
systems, including high temperature superconductors and quantum Hall states. In
one of the most promising candidate materials capable of hosting a liquid CDW,
1T-TaS2, a structural phase transition impedes its observation. Here, by
irradiating the material with a femtosecond light pulse, we circumvent the
structural phase transition to reveal how topological defect dynamics govern
the otherwise invisible CDW correlations. Upon photoexcitation, the CDW
diffraction peaks broaden azimuthally, initially revealing a hexatic state. At
higher temperatures, photoexcitation completely destroys translational and
orientational order and only a ring of diffuse scattering is observed, a key
signature of a liquid CDW. Our work provides compelling evidence for a
defect-unbinding transition to a CDW liquid and presents a protocol for
uncovering states that are hidden by other transitions in thermal equilibrium.

In a vast array of materials, including cuprates, transition metal
dichalcogenides (TMDs) and rare earth tritellurides, superconductivity is found
in the vicinity of short-range charge density wave (CDW) order. The crossover
from long-range to short-range charge order often occurs as quenched disorder
is introduced, yet it is unclear how this disorder disrupts the CDW. Here,
using x-ray photon correlation spectroscopy (XPCS), we investigate the
prototypical TMD superconductor CuxTiSe2 and show that disorder induces
substantial CDW dynamics. We observed the CDW phase fluctuation on a timescale
of minutes to hours above the nominal transition temperature while the order
parameter amplitude remains finite. These long timescale fluctuations prevent
the system from finding the global free energy minimum upon cooling and
ultimately traps it in a short-range ordered metastable state. Our findings
demonstrate how correlated disorder can give rise to a distinct mechanism of
domain formation that may be advantageous to the emergence of
superconductivity.

Cryogenic calorimeters, also known as bolometers, are among the leading
technologies for searching for rare events. The CUPID experiment is exploiting
this technology to deploy a tonne-scale detector to search for neutrinoless
double-beta decay of $^{100}$Mo. The CUPID collaboration proposed an innovative
approach to assembling bolometers in a stacked configuration, held in position
solely by gravity. This gravity-based assembly method is unprecedented in the
field of bolometers and offers several advantages, including relaxed mechanical
tolerances and simplified construction. To assess and optimize its performance,
we constructed a medium-scale prototype hosting 28 Li$_2$MoO$_4$ crystals and
30 Ge light detectors, both operated as cryogenic calorimeters at the
Laboratori Nazionali del Gran Sasso (Italy). Despite an unexpected excess of
noise in the light detectors, the results of this test proved (i) a thermal
stability better than $\pm$0.5 mK at 10 mK, (ii) a good energy resolution of
Li$_2$MoO$_4$ bolometers, (6.6 $\pm$ 2.2) keV FWHM at 2615 keV, and (iii) a
Li$_2$MoO$_4$ light yield measured by the closest light detector of 0.36
keV/MeV, sufficient to guarantee the particle identification requested by
CUPID.

We report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.

We present a design and implementation of frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC-loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby current feedline induces Meissner shielding currents in the resonator loop leading to change in the kinetic part of the total inductance of the resonator. We demonstarte continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency upconversion is achieved when kHz AC modulation signal is superimposed onto the DC bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong RF pump signal in the feedline. The simple architecture is amenable to large array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, upconverters, and parametric amplifiers operating above 4 Kelvin cryogenic temperatures when alternative high critical temperature material with high kinetic inductance is used.

We have investigated the magnetization properties and flux dynamics of superconducting Cu$_x$TiSe$_2$ single crystals within wide range of copper concentrations. We find that the superconducting anisotropy is low and independent on copper concentration ($\gamma\sim1.7$), except in the case of strongly underdoped samples ($x\leq0.06$) that show a gradual increase in anisotropy to $\gamma\sim1.9$. The vortex phase diagram in this material is characterized by broad region of vortex liquid phase that is unusual for such low-$T_c$ superconductor with low anisotropy. Below the irreversibility line the vortex solid state supports relatively low critical current densities as compared to the depairing current limit ($J_c/J_0\sim10^{-7}$). All this points out that local fluctuations in copper concentration have little effect on bulk pinning properties in this system.