3He adsorbed on molecular hydrogen surfaces.

Using a diffusion Monte Carlo technique, we calculated the phase diagram of 3He adsorbed on a first solid layer of a molecular hydrogen isotope (H2, HD, and D2) on top of graphite. The results are qualitatively similar in all cases: a two-dimensional gas spanning from the infinite dilution limit to a second-layer helium density of 0.048 ± 0.004 Å-2. That gas is in equilibrium with a 7/12 commensurate structure, more stable than any incommensurate triangular solid of similar density. These findings are in reasonably good agreement with available experimental data.

Ultradilute Quantum Liquid of Dipolar Atoms in a Bilayer.

We show that ultradilute quantum liquids can be formed with ultracold bosonic dipolar atoms in a bilayer geometry. Contrary to previous realizations of ultradilute liquids, there is no need for stabilizing the system with an additional repulsive short-range potential. The advantage of the proposed system is that dipolar interactions on their own are sufficient for creation of a self-bound state and no additional short-range potential is needed for the stabilization. We perform quantum Monte Carlo simulations and find a rich ground-state phase diagram that contains quantum phase transitions between liquid, solid, atomic gas, and molecular gas phases. The stabilization mechanism of the liquid phase is consistent with the microscopic scenario in which the effective dimer-dimer attraction is balanced by an effective three-dimer repulsion. The equilibrium density of the liquid, which is extremely small, can be controlled by the interlayer distance. From the equation of state, we extract the spinodal density, below which the homogeneous system breaks into droplets. Our results offer a new example of a two-dimensional interacting dipolar liquid in a clean and highly controllable setup.

Quantum halo states in two-dimensional dipolar clusters.

A halo is an intrinsically quantum object defined as a bound state of a spatial size which extends deeply into the classically forbidden region. Previously, halos have been observed in bound states of two and less frequently of three atoms. Here, we propose a realization of halo states containing as many as six atoms. We report the binding energies, pair correlation functions, spatial distributions, and sizes of few-body clusters composed by bosonic dipolar atoms in a bilayer geometry. We find two very distinct halo structures, for large interlayer separation the halo structure is roughly symmetric and we discover an unusual highly anisotropic shape of halo states close to the unbinding threshold. Our results open avenues of using ultracold gases for the experimental realization of halos composed by atoms with dipolar interactions and containing as many as six atoms.

Erratum: Superfluid and Supersolid Phases of

This corrects the article DOI: 10.1103/PhysRevLett.124.205301.

Superfluid and Supersolid Phases of

We revisited the phase diagram of the second layer of ^{4}He on top of graphite using quantum Monte Carlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a 7/12 registered phase. The 4/7 commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.

Fragmento cristalino subretiniano en tomografía de coherencia óptica tras cirugía de catarata complicada / Sub-retinal lens fragment in optical coherence tomography after complicated cataract surgery

No disponible

Evaluación y abordaje del estreñimiento en el paciente crítico / Evaluation and handling of constipation in critical patients

Objetivo: Valorar la eficacia de los cuidados enfermeros frente al estreñimiento e identificar, analizar y evaluar sus causas y consecuencias. Metodología: Estudio observacional, descriptivo y prospectivo, en UCI polivalentes de un hospital de tercer nivel (2013-2015). Criterios de inclusión: >18 años, estancia >7 días, con ventilación mecánica, portadores de sonda nasogástrica y nutrición enteral o mixta. Se excluyeron pacientes con enfermedad digestiva, encefalopáticos y con yeyunostomía/ileostomía. Las variables estudiadas (edad, sexo, peso, talla, enfermedad, tratamiento médico, tipo de nutrición y volumen, características deposicionales, cantidad y frecuencia, medidas correctoras y complicaciones) se recogieron mediante parrilla ad hoc. Dispone de autorización CEIC. Resultados: Se analizaron 139 pacientes con edad media de 62 años y estancia media de 11 días; un 63% padecieron estreñimiento. Opiáceos y antiácidos fueron los fármacos más administrados (99%), aunque los relajantes musculares, suplementos de hierro y/o calcio y antihipertensivos fueron los que dieron más estreñimiento (77, 75 y 70%). La dieta sin fibra fue la más utilizada (60% estreñidos), seguida de dieta con fibra (51% estreñidos) y la combinación de ambas (85% estreñidos) Un 56% usó laxantes como medida correctora, siendo el hidróxido de magnesio el más utilizado; un 54% las iniciaron el primer día. La retención gástrica fue la complicación más relevante (49%). Conclusión: El estreñimiento es un problema real multifactorial. Recomendamos: · Intensificar la vigilancia en pacientes con fármacos que favorecen el estreñimiento. · Utilizar dietas con fibra desde el inicio. ·Aplicar de forma precoz y combinada procinéticos y laxantes. Creemos necesario crear un protocolo para la profilaxis y manejo del estreñimiento (AU)

Objective: To evaluate the effectiveness of nursing care against constipation and to identify, analyze and evaluate causes and consequences. Methodology: Observational, descriptive and prospective study in polyvalent ICU tertiary hospital (2013-2015). Inclusion criteria: >18 years, stay >7 days, connected to respiratory support, with nasogastric tube and enteral or mixed nutrition. Patients with gastrointestinal pathology, encephalopathic and jejunostomy/ileostomy were excluded. The studied variables (age, sex, weight, height, pathology, medical treatment, nutrition and volume type, depositional characteristics, quantity and frequency, corrective measures and complications) were collected by ad hoc grill. It is authorized by the CEIC. Results: 139 patients with a mean age of 62 years and average stay of 11 days were analyzed; 63% suffered from constipation. Opiates and antacid were the drugs administered most frequently (99%), even though patients who took muscle relaxants, iron supplements and/or calcium and anti-hypertensive were the ones who suffered most from constipation (77%; 75%; 70%) The fiber free diet was the most widely used (60% constipated), followed by dietary fiber (51% constipated), and the combination of both (85% constipated). 56% used laxatives as a corrective measure, Magnesium Hydroxide being the most widely used; 54% began the first day. Gastric retention was the most relevant complication (49%). Conclusion: Constipation is a real multifactorial problem. We recommend: ·Intensified surveillance in patients with drugs that promote constipation. ·Use high-fiber diets from the outset. ·Apply laxatives and prokinetics early and in combination. We need to create a protocol for prophylaxis and management of constipation (AU)

Evaluation and handling of constipation in critical patients. / Evaluación y abordaje del estreñimiento en el paciente crítico.

OBJECTIVE: To evaluate the effectiveness of nursing care against constipation and to identify, analyze and evaluate causes and consequences. METHODOLOGY: Observational, descriptive and prospective study in polyvalent ICU tertiary hospital (2013-2015). INCLUSION CRITERIA: >18 years, stay >7 days, connected to respiratory support, with nasogastric tube and enteral or mixed nutrition. Patients with gastrointestinal pathology, encephalopathic and jejunostomy/ileostomy were excluded. The studied variables (age, sex, weight, height, pathology, medical treatment, nutrition and volume type, depositional characteristics, quantity and frequency, corrective measures and complications) were collected by ad hoc grill. It is authorized by the CEIC. RESULTS: 139 patients with a mean age of 62 years and average stay of 11 days were analyzed; 63% suffered from constipation. Opiates and antacid were the drugs administered most frequently (99%), even though patients who took muscle relaxants, iron supplements and/or calcium and anti-hypertensive were the ones who suffered most from constipation (77%; 75%; 70%) The fiber free diet was the most widely used (60% constipated), followed by dietary fiber (51% constipated), and the combination of both (85% constipated). 56% used laxatives as a corrective measure, Magnesium Hydroxide being the most widely used; 54% began the first day. Gastric retention was the most relevant complication (49%). CONCLUSION: Constipation is a real multifactorial problem. We recommend: • Intensified surveillance in patients with drugs that promote constipation. • Use high-fiber diets from the outset. • Apply laxatives and prokinetics early and in combination. We need to create a protocol for prophylaxis and management of constipation.

Observation of dynamic atom-atom correlation in liquid helium in real space.

Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atom-atom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pair-density function (DPDF) analysis we show that 4He atoms in the Bose-Einstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snap-shot pair-density function is found at 2.3 Å, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atom-atom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.

Ground-state properties of weakly bound helium-alkali trimers.

Weakly bound triatomic molecules consisting of two helium atoms and one alkali metal atom are studied by means of the diffusion Monte Carlo method. We determined the stability of 4He2A, 4He3HeA, and 3He2A, where A is one of the alkali atoms Li, Na, K, Rb, or Cs. Some of the trimers with 3He are predicted to be self-bound for the first time, but this is observed to be dependent on the He-A interaction potential model. In addition to the ground-state energy of the trimers, we determined their density, radial, and angular distributions. Many of them are spatially very extended, which qualifies them as quantum halo states.

Dipolar Bose Supersolid Stripes.

We study the superfluid properties of a system of fully polarized dipolar bosons moving in the XY plane. We focus on the general case where the polarization field forms an arbitrary angle α with respect to the Z axis, while the system is still stable. We use the diffusion Monte Carlo and the path integral ground state methods to evaluate the one-body density matrix and the superfluid fractions in the region of the phase diagram where the system forms stripes. Despite its oscillatory behavior, the presence of a finite large-distance asymptotic value in the s-wave component of the one-body density matrix indicates the existence of a Bose condensate. The superfluid fraction along the stripes direction is always close to 1, while in the Y direction decreases to a small value that is nevertheless different from zero. These two facts confirm that the stripe phase of the dipolar Bose system is a clear candidate for an intrinsic supersolid without the presence of defects as described by the Andreev-Lifshitz mechanism.

Droplets of Trapped Quantum Dipolar Bosons.

Strongly interacting systems of dipolar bosons in three dimensions confined by harmonic traps are analyzed using the exact path integral ground-state Monte Carlo method. By adding a repulsive two-body potential, we find a narrow window of interaction parameters leading to stable ground-state configurations of droplets in a crystalline arrangement. We find that this effect is entirely due to the interaction present in the Hamiltonian without resorting to additional stabilizing mechanisms or specific three-body forces. We analyze the number of droplets formed in terms of the Hamiltonian parameters, relate them to the corresponding s-wave scattering length, and discuss a simple scaling model for the density profiles. Our results are in qualitative agreement with recent experiments showing a quantum Rosensweig instability in trapped Dy atoms.

Liquid and Solid Phases of

Recent heat-capacity experiments show quite unambiguously the existence of a liquid ^{3}He phase adsorbed on graphite. This liquid is stable at an extremely low density, possibly one of the lowest found in nature. Previous theoretical calculations of the same system, and in strictly two dimensions, agree with the result that this liquid phase is not stable and the system is in the gas phase. We calculated the phase diagram of normal ^{3}He adsorbed on graphite at T=0 using quantum Monte Carlo methods. Considering a fully corrugated substrate, we observe that at densities lower than 0.006 Å^{-2} the system is a very dilute gas that, at that density, is in equilibrium with a liquid of density 0.014 Å^{-2}. Our prediction matches very well the recent experimental findings on the same system. On the contrary, when a flat substrate is considered, no gas-liquid coexistence is found, in agreement with previous calculations. We also report results on the different solid structures, and on the corresponding phase transitions that appear at higher densities.

Quantum Monte Carlo estimation of complex-time correlations for the study of the ground-state dynamic structure function.

We present a method based on the path integral Monte Carlo formalism for the calculation of ground-state time correlation functions in quantum systems. The key point of the method is the consideration of time as a complex variable whose phase δ acts as an adjustable parameter. By using high-order approximations for the quantum propagator, it is possible to obtain Monte Carlo data all the way from purely imaginary time to δ values near the limit of real time. As a consequence, it is possible to infer accurately the spectral functions using simple inversion algorithms. We test this approach in the calculation of the dynamic structure function S(q, ω) of two one-dimensional model systems, harmonic and quartic oscillators, for which S(q, ω) can be exactly calculated. We notice a clear improvement in the calculation of the dynamic response with respect to the common approach based on the inverse Laplace transform of the imaginary-time correlation function.

Atomic monolayer deposition on the surface of nanotube mechanical resonators.

We study monolayers of noble gas atoms (Xe, Kr, Ar, and Ne) deposited on individual ultraclean suspended nanotubes. For this, we record the resonance frequency of the mechanical motion of the nanotube, since it provides a direct measure of the coverage. The latter is the number of adsorbed atoms divided by the number of the carbon atoms of the suspended nanotube. Monolayers form when the temperature is lowered in a constant pressure of noble gas atoms. The coverage of Xe monolayers remains constant at 1/6 over a large temperature range. This finding reveals that Xe monolayers are solid phases with a triangular atomic arrangement, and are commensurate with the underlying carbon nanotube. By comparing our measurements to theoretical calculations, we identify the phases of Ar and Ne monolayers as fluids, and we tentatively describe Kr monolayers as solid phases. These results underscore that mechanical resonators made from single nanotubes are excellent probes for surface science.

Universality in molecular halo clusters.

The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T↓, D↓, ^{3}He, ^{4}He, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.

Spin-polarized hydrogen adsorbed on the surface of superfluid 4He.

The experimental realization of a thin layer of spin-polarized hydrogen H↓ adsorbed on top of the surface of superfluid (4)He provides one of the best examples of a stable, nearly two-dimensional (2D) quantum Bose gas. We report a theoretical study of this system using quantum Monte Carlo methods in the limit of zero temperature. Using the full Hamiltonian of the system, composed of a superfluid (4)He slab and the adsorbed H↓ layer, we calculate the main properties of its ground state using accurate models for the pair interatomic potentials. Comparing the results for the layer with the ones obtained for a strictly 2D setup, we analyze the departure from the 2D character when the density increases. Only when the coverage is rather small the use of a purely 2D model is justified. The condensate fraction of the layer is significantly larger than in 2D at the same surface density, being as large as 60% at the largest coverage studied.

Zero-temperature phase diagram of D2 physisorbed on graphane.

We determined the zero-temperature phase diagram of D2 physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was C-graphane, an allotropic form of the compound that has been experimentally obtained through hydrogenation of graphene. We found that the ground state is the δ phase, a commensurate structure observed experimentally when D2 is adsorbed on graphite, and not the registered √3 x √3 structure characteristic of H2 on the same substrate.

Onset temperature of Bose-Einstein condensation in incommensurate solid (4)He.

The temperature dependence of the one-body density matrix in (4)He crystals presenting vacancies is computed with path integral Monte Carlo simulations. The main purpose of this study is to estimate the onset temperature T(0) of Bose-Einstein condensation in these systems. We see that T(0) depends on the vacancy concentration X(v) of the simulated system, but not following the law T(0) ~ X(v)(2/3) obtained assuming noninteracting vacancies. For the lowest X(v) we study, that is X(v)= 1/256, we get T(0) = 0.15 ± 0.05 K, close to the temperatures at which a finite fraction of nonclassical rotational inertia is experimentally observed. Below T(0), vacancies do not act as classical point defects becoming completely delocalized entities.