Nuclear magnetic resonance study of the magnetic-field-induced ordered phase in the NiCl2-4SC(NH2)2 compound

Nuclear magnetic resonance (NMR) study of the high magnetic field (H) part of the Bose-Einstein condensed (BEC) phase of the quasi-onedimensional (quasi-1D) antiferromagnetic quantum spin-chain compound NiCl2-4SC(NH2)2 (DTN) was performed. We precisely determined the phase boundary, Tc(H), down to 40 mK; the critical boson density, n_c(Tc); and the absolute value of the BEC order parameter S_perp at very low temperature (T = 0.12 K). All results are accurately reproduced by numerical quantum Monte Carlo simulations of a realistic three-dimensional (3D) model Hamiltonian. Approximate analytical predictions based on the 1D Tomonaga-Luttinger liquid description are found to be precise for Tc(H), but less so for S_perp(H), which is more sensitive to the strength of 3D couplings, in particular close to the critical field. A mean-field treatment, based on the Hartree-Fock-Popov description, is found to be valid only up to n_c = 4% (T < 0.3 K), while for higher n_c boson interactions appear to modify the density of states.Comment: Manuscript (6 pages, 3 figures) and the corresponding Supplemental material (5 pages, 6 figures), altogether 11 pages and 9 figure

Bose glass and Mott glass of quasiparticles in a doped quantum magnet

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation (BEC) and superfluidity, which have been tested experimentally in a variety of different systems. When bosons are interacting, disorder can destroy condensation leading to a so-called Bose glass. This phase has been very elusive to experiments due to the absence of any broken symmetry and of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (Br-doped dichloro-tetrakis-thiourea-Nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand-canonical ensemble; Br-doping introduces disorder in the hoppings and interaction strengths, leading to localization of the bosons into a Bose glass down to zero field, where it acquires the nature of an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the BEC (corresponding to a magnetically ordered phase) is marked by a novel, universal exponent governing the scaling on the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents the first, quantitative account of the universal features of disordered bosons in the grand-canonical ensemble.Comment: 13+6 pages, 5+6 figures; v2: Fig. 5 update

Low temperature thermodynamic properties near the field-induced quantum critical point in DTN

We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical field $H_{c1} \approx 2$\,T in DTN . A $T^{3/2}$ behavior in the specific heat and magnetization is observed at very low temperatures at $H=H_{c1}$ that is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$ behavior. Our experimental study is complemented by analytical calculations and Quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gr\"{u}neisen parameters that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$ with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the second order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.Comment: 11 paged, 10 figures, submitted to PR

Pressure-driven gapped-to-gapless phase transition in a quantum antiferromagnet with large planar anisotropy

Strongly correlated electron systems can be driven to quantum critical points via various routes. Contrary to field-induced transitions of a gapped quantum antiferromagnet into the gapless ordered state (characterized by the $z=2$ dynamic critical exponent and a quadratic excitation dispersion), pressure-induced instabilities possess $z=1$ and a linear excitation spectrum, belonging to another universality class. Employing radio-frequency susceptibility technique, we demonstrate that in the tetragonal easy-plane quantum paramagnet NiCl$_2$$\cdot$4SC(NH$_2$)$_2$ (aka DTN) the spin gap vanishes at about $4.3$ kbar, suggesting the onset of antiferromagentic order induced by pressure. Powder neutron diffraction measurements confirm that no lattice distortion occurs at this pressure and that the high spin symmetry is preserved. We are able to quantitatively describe the pressure-driven evolution of the critical fields and spin Hamiltonian parameters in DTN. The studies are complemented by high-pressure electron spin resonance measurements confirming the proposed scenario.Comment: Main text: 11 pages, 6 figures Supplement: 6 pages, 4 figure

Quantum critical scaling at a Bose-glass/superfluid transition: theory and experiment on a model quantum magnet

In this paper we investigate the quantum phase transition from magnetic Bose glass to magnetic Bose-Einstein condensation induced by a magnetic field in NiCl2.4SC(NH2)2 (dichloro-tetrakis-thiourea-Nickel, or DTN), doped with Br (Br-DTN) or site diluted. Quantum Monte Carlo simulations for the quantum phase transition of the model Hamiltonian for Br-DTN, as well as for site-diluted DTN, are consistent with conventional scaling at the quantum critical point and with a critical exponent z verifying the prediction z=d; moreover the correlation length exponent is found to be nu = 0.75(10) and the order parameter exponent to be beta = 0.95(10). We investigate the low-temperature thermodynamics at the quantum critical field of Br-DTN both numerically and experimentally, and extract the power-law behavior of the magnetization and of the specific heat. Our results for the exponents of the power laws, as well as previous results for the scaling of the critical temperature to magnetic ordering with the applied field, are incompatible with the conventional crossover-scaling Ansatz proposed by Fisher et al., [Phys. Rev. B 40, 546 (1989)], but they can all be reconciled within a phenomenological Ansatz in the presence of a dangerously irrelevant operator.Comment: 12 pages, 10 figure

O comportamento do consumidor de vinho no mercado português

A presente investigação resultou da necessidade de perceber as razões que motivam e que eventualmente poderão condicionar a frequência de consumo de vinho em Portugal. Deste modo, o principal objetivo deste trabalho é compreender as motivações de compra e consumo, dependendo da situação de uso e de caraterísticas pessoais. A literatura existente sobre as motivações de consumo de vinho salienta a existência de efeitos diretos sobre as motivações do género do comprador e da ocasião de consumo. Este estudo introduz uma formulação alternativa destes efeitos, considerando-os indiretos, i.e. completamente mediados pelo conhecimento subjetivo do comprador e pelas emoções que ele associa ao respetivo consumo. Foram desenvolvidos dois modelos, um para explicar as motivações, outro a frequência de consumo, os quais foram posteriormente validados numa amostra de 523 compradores entrevistados no momento da compra em grandes superfícies retalhistas da Grande Lisboa e do Grande Porto. Os resultados apresentados no primeiro modelo apontam para a rejeição dos efeitos diretos, sendo o conhecimento subjetivo o mediador mais importante. Sugere-se que a autoconfiança e a eficácia na decisão, que aparecem associadas ao conhecimento subjetivo, justifiquem que os homens se sintam mais motivados para o consumo. Este efeito positivo do conhecimento na motivação é ainda mais importante nas ocasiões regulares. No segundo modelo, verificou-se que a frequência de consumo é explicada predominantemente pela motivação por gratificação sensorial e, também, pela ocasião de consumo no contexto de refeição normal (efetuada no dia-a-dia). O conhecimento subjetivo revelou-se também como elemento fundamental no comportamento do consumidor, quer pela sua influência sobre as motivações, quer pela sua influência sobre a frequência de consumo. As conclusões retiradas desta investigação salientam a importância do conhecimento subjetivo no consumo de vinho. Realçam também que os efeitos de fatores como o género e a ocasião sobre as motivações para consumir vinho, reportados na literatura, poderão dever-se a mediadores de ordem psicográfica, como o conhecimento subjetivo. Esta conclusão tem implicações no marketing de vinhos para compradores regulares, nomeadamente pelo reconhecimento da necessidade de reforçar a perceção de confiança e de eficácia de decisão que costumam ser associadas aos elevados níveis de conhecimento percebido numa categoria de produto

Method for the determination of purity in metals

Método para a determinação da pureza em metaisMethod for the determination of purity in metal