Goran Tripun Karapetrov

Charge density waves, electronic crystals that form within a host solid, have long been theorized to melt into a spatially textured electronic liquid. Although such liquid charge density waves have not been previously observed, they may be central to the phase diagrams of correlated electron systems, including high-temperature superconductors and quantum Hall states. In 1T-TaS2, a promising material for hosting a liquid charge density wave, a structural phase transition hinders observation. Here we use femtosecond light pulses to bypass this transition, revealing how topological defect dynamics govern hidden charge density wave correlations. Following photoexcitation, charge density wave diffraction peaks broaden azimuthally, indicating the emergence of a hexatic state. At elevated temperatures, photoexcitation fully destroys both translational and orientational orders, leaving only a ring of diffuse scattering—the hallmark of a liquid charge density wave. These findings offer compelling evidence for a defect-unbinding transition to a charge density wave liquid. More broadly, this approach demonstrates a route to uncover electronic phases obscured by intervening transitions in thermal equilibrium.

Transition metal dichalcogenides like 2H-NbSe2 exhibit remarkable electronic properties, but their performance is highly sensitive to defects, particularly selenium vacancies. Here, we demonstrate controlled migration of Se atoms from the bulk to the surface of NbSe2 single crystal, induced by heat, electron beam irradiation, and laser excitation. Using Raman spectroscopy, atomic force microscopy, and energy-dispersive x-ray spectroscopy, we identify the formation of nanosized (200–500 nm) amorphous selenium clusters on the surface, evidenced by a distinct Raman mode at 250 cm−1. Temperature-dependent studies reveal the thermally activated nature of this process, with an activation energy of 1.12 eV—significantly higher than in related dichalocogenide 1T-TiSe2, suggesting suppressed Se diffusion in NbSe2 at elevated temperatures. The results provide an estimate of the fabrication thermal budget for future electronic and optoelectronic devices utilizing NbSe2.

Cryogenic calorimeters 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 100Mo. The CUPID collaboration proposed an innovative approach to assembling cryogenic calorimeters in a stacked configuration, held in position solely by gravity. This gravity-based assembly method is unprecedented in the field of cryogenic calorimeters 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 Li2 MoO4 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 ±0.5 mK at 10 mK, (ii) a good energy resolution of Li2 MoO4 cryogenic calorimeters, (6.6 ± 2.2) keV FWHM at 2615 keV, and (iii) a Li2 MoO4 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 1T-TiSe2 over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, TCDW, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.

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.