Open Access

The correlation-driven metal-insulator transition (MIT) of BaVS3 was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: The quasi-1D metallic transport of A1g electrons is superimposed on the isotropic hopping conduction of localized Eg electrons. The “bad-metal” character and the weak anisotropy are the consequences of the large effective mass meff≈7me and scattering rate Γ≥160  meV of the quasiparticles in the A1g band. There is a pseudogap above TMI=69  K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with Δch≈42  meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.

The metal-insulator transition (MIT) of BaVS3 is suppressed under pressure, and above the critical pressure of pcr≈2GPa the metallic phase is stabilized. We present the results of detailed magnetoresistivity measurements carried out at pressures near the critical value in magnetic fields up to B=12T. We found that slightly below the critical pressure the structural tetramerization—which drives the MIT—is combined with the onset of magnetic correlations. If the zero-field transition temperature is suppressed to a sufficiently low value (TMI≤15K), the system can be driven into the metallic state by application of magnetic field. The main effect is not the reduction of TMI with increasing B, but rather the broadening of the transition due to the applied magnetic field. We tentatively ascribe this phenomenon to the influence on the magnetic structure coupled to the bond order of the tetramers.

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electronic-structural changes in correlated electron systems as the optical pressure cell, equipped with large free-aperture diamond window, allows the measurement of optical reflectivity down to ω≈20–30cm−1 for hydrostatic pressures up to p≈26kbar. The efficiency of the technique is demonstrated by the investigation of the two-dimensional charge-density-wave 1T−TaS2 whose electronic structure shows high sensitivity to external pressure. The room-temperature semimetallic phase of 1T−TaS2 is effectively extended by application of pressure and stabilized as the ground state above p=14kbar. The corresponding fully incoherent low-energy optical conductivity is almost temperature independent below T=300K. For intermediate pressures, the onset of the low-temperature insulating phase is reflected by the sudden drop of the reflectivity and by the emergence of sharp phonon resonances.

We have measured Raman spectra of barium vanadium sulfide (BaVS3) single crystals in a broad temperature range (15 K–300 K). Three Raman active modes are found at about 193, 350, and 366cm−1. The assignment of the observed modes of BaVS3 is carried out on the basis of the symmetry coordinate analysis. The mode at 350cm−1 shows a dramatic intensity increase and a line asymmetry by lowering the temperature below 30 K. We believe that this behavior is connected with the orbital and spin ordering in this compound.

We carried out magnetotransport experiments under hydrostatic pressure in order to study the nature of the metal-insulator transition in BaVS3. Scaling relations for ρ(T,H,p) are established and the pressure dependence of the spin gap is determined. Our results, in conjunction with a reanalysis of earlier specific-heat and susceptibility data, demonstrate that the transition is weakly second order. The nature of the phase diagram in the T−p−H space is discussed.

Static electrical and magnetic properties of single crystal BaVS3 were measured over the structural (TS=240K), metal insulator (TMI=69K), and suspected orbital ordering (TX=30K) transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at TX signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal-field quasidoublet. The disordered insulator at TX<T<TMI is described as a classical liquid of nonmagnetic pairs.

IMPORTANCE Vaccine responses are decreased in solid organ transplant (SOT) recipients, and given the complexity of implementation, vaccination programs may be suboptimal. The actual burden of vaccine-preventable infections (VPIs) among SOT recipients remains unclear. OBJECTIVES To assess the incidence rate of VPIs among SOT recipients and to evaluate whether SOT recipients are at increased risk for specific VPIs compared with the general population. DESIGN, SETTING, AND PARTICIPANTS This nationwide cohort study used data from the Swiss Transplant Cohort Study on VPIs in individuals who underwent SOT from May 2008 to June 2019 (follow-up until December 2019) and data from the Swiss Federal Office of Public Health on notifiable VPIs in the general population in the same period. Data were analyzed from January 2021 to June 2022. EXPOSURES Solid organ transplant. MAIN OUTCOMES AND MEASURES The main outcomes were the incidence rate of the following VPIs in SOT recipients: hepatitis A and B, diphtheria, Haemophilus influenzae infection, influenza, measles, mumps, pertussis, pneumococcal disease, poliomyelitis, meningococcal disease, rubella, tetanus, tick-borne encephalitis, and varicella zoster virus infection. Age-adjusted standardized incidence ratios were used to assess whether VPIs occurred more frequently in SOT recipients compared with the general population. For SOT recipients, factors associated with occurrence of VPIs were explored and the associated morbidity and mortality assessed. RESULTS Of 4967 SOT recipients enrolled (median age, 54 years [IQR, 42-62 years]; 3191 [64.2%] male), 593 (11.9%) experienced at least 1 VPI. The overall VPI incidence rate was higher in the population that underwent SOT (30.57 per 1000 person-years [PY]; 95% CI, 28.24-33.10 per 1000 PY) compared with the general population (0.71 per 1000 PY). The standardized age-adjusted incidence ratio for notifiable VPIs in SOT recipients was higher compared with the general population (27.84; 95% CI, 25.00-31.00). In SOT recipients, influenza and varicella zoster virus infection accounted for most VPI episodes (16.55 per 1000 PY [95% CI, 14.85-18.46 per 1000 PY] and 12.83 per 1000 PY [95% CI, 11.40-14.44 per 1000 PY], respectively). A total of 198 of 575 VPI episodes in the population that underwent SOT (34.4%) led to hospital admission, and the occurrence of a VPI was associated with an increased risk for death and/or graft loss (hazard ratio, 2.44; 95% CI, 1.50-3.99; P = .002). In multivariable analysis, age 65 years or older at the time of transplant (incidence rate ratio [IRR], 1.29; 95% CI, 1.02-1.62) and receipt of a lung (IRR, 1.77; 95% CI, 1.38-2.26) or a heart (IRR, 1.40; 95% CI, 1.05-1.88) transplant were associated with an increased risk of VPI occurrence.