Coherent phonons and the interplay between charge density wave and Mott phases in 1<i>T</i>-TaSe₂

1$T$-TaSe$_{2}$ is host to coexisting strongly-correlated phases including charge density waves (CDWs) and an unusual Mott transition at low temperature. Here, we investigate coherent phonon oscillations in 1$T$-TaSe$_{2}$ using a combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES) and time-resolved reflectivity (TRR). Perturbation by a femtosecond laser pulse triggers a modulation of the valence band binding energy at the $\Gamma$-point, related to the Mott gap, that is consistent with the in-plane CDW amplitude mode frequency. By contrast, TRR measurements show a modulation of the differential reflectivity comprised of multiple frequencies belonging to the distorted CDW lattice modes. Comparison of the temperature dependence of coherent and spontaneous phonons across the CDW transition shows that the amplitude mode intensity is more easily suppressed during perturbation of the CDW state by the optical excitation compared to other modes. Our results clearly identify the relationship of the in-plane CDW amplitude mode with the Mott phase in 1$T$-TaSe$_{2}$ and highlight the importance of lattice degrees of freedom.Comment: 7 pages, 4 figures, supplemental materia

Femtosecond dynamics of spin-polarized electrons in topological insulators

A faster control of spins is a major request for the new generation of computing and spintronic systems. In this framework, since many years, ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators (TIs) have received attention in the field of spintronics due to their spectacular features, in particular, the existence, within the insulating gap of bulk states, of spin-polarized surface states (Dirac-cone) that are protected from backscattering by time-reversal symmetry. We have studied the sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of Bi$_2$Te$_3$, employing circular-polarized light in time and angle resolved photoemission spectroscopy (trARPES). Exploiting the noncollinear optical parametric amplification (NOPA) besides several nonlinear optical processes resulted in tunable ultrashort visible pump pulses with 30 fs length and 1.8 eV energy and ultraviolet probe pulses with about 6 eV energy and 60 fs duration. The stable optical setup and the high repetition rate of an Yb-laser source grants a high signal-to-noise ratio in our photoemission process. The obtained 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight (TOF) energy analyzer, provides us with an exciting possibility to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism (CD) allows access to the spin state of the photoemitted electrons. We found a signature of femtosecond unpolarized bulk bands dynamics in the presence of spin-polarized electrons of the surface states. This observation aided to distinguish the bulk and surface contributions in the spin-electronic current

Structural, Electronic and Magnetic Properties of a Few Nanometer-Thick Superconducting NdBa₂Cu₃O₇ Films

Abstract: Epitaxial films of high critical temperature (Tc) cuprate superconductors preserve their transport properties even when their thickness is reduced to a few nanometers. However, when approaching the single crystalline unit cell (u.c.) of thickness, Tc decreases and eventually, superconductivity is lost. Strain originating from the mismatch with the substrate, electronic reconstruction at the interface and alteration of the chemical composition and of doping can be the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu L3 edge to study the crystal field and spin excitations of NdBa2Cu3O7x ultrathin films grown on SrTiO3, comparing 1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy and x-ray diffraction

Excitonic and lattice contributions to the charge density wave in 1T-TiSe2 revealed by a phonon bottleneck

Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above Fth = 60 microJ cm-2. This observation coincides with a shift in the relative weight of coherently coupled phonons to higher frequency modes in time-resolved reflectivity (TRR), suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials.Comment: 11 pages, 4 figure

[Hyperuricemia and Gout]

Already known to ancient Egyptians, gout is one of the first diseases which have been described as a clinical entity. To date, gout is the most common form of inflammatory arthritis. Gout is defined by the deposition of monosodium urate crystals within tissues, causing episodes of acute arthritis and the development of tophi, nephrolithiasis, and urate nephropathy. Hyperuricemia, i.e. levels of serum uric acid above 6.8 mg / dL(404mol/L), is a condition necessary, yet not sufficient for gout to develop. The increasing incidence of risk factors such as hypertension, obesity, and renal failure together with an ever-growing life expectancy has led in recent decades to a significant increase in gout prevalence, which has more than doubled when compared to the 1960s. This article addresses the issue of gout by highlighting the role played by the kidneys in uric acid homeostasis; the clinical effect of crystal deposition in tissues, including the kidney; the more recent guidelines on diagnosis and management strategies, with special regard to the use of old and new drugs in renal patients

[ADPKD: predictors of Renal Disease progression]

Factors predicting rapid progression of kidney disease in ADPKD can be divided into genetic (non-modifiable) and clinical (modifiable) risk factors. Patients harbouring PKD1 mutations, in particular if truncating, have a more severe form of ADPKD. Clinical risk factors include decrease in glomerular filtration rate and renal blood flow at a young age; high total kidney volume; hypertension and urological complications <35 years; albuminuria/proteinuria. The renal disease is also more severe in males and in subjects with family history of ESRD <55 years. In recent years, two models for predicting progression in ADPKD have been published: the Mayo model, based on height-adjusted TKV, age and eGFR, and the Brest model, based on PKD gene mutation type, gender, and early onset of hypertension and urological complications. With the emergence of new disease-modifying therapies, prediction tools are essential. However, the high variability in ADPKD makes the predicting models difficult to apply on an individual patient basis. Thus, the above-mentioned predicting models should be viewed as complimentary to clinical evaluation and follow-up. In the future, an individual risk score linking genetic, imaging and clinical data might prove the most accurate way of predicting long-term outcome