Narrow Results

Some physical implications involved in a new concept, termed the "quantum protectorate" (QP), are developed and discussed. This is done by considering the idea of quantum protectorate in the context of quantum theory of magnetism. It is suggested that the difficulties in the formulation of quantum theory of magnetism at the microscopic level, that are related to the choice of relevant models, can be understood better in the light of the QP concept. We argue that the difficulties in the formulation of adequate microscopic models of electron and magnetic properties of materials are intimately related to dual, itinerant and localized behaviour of electrons. We formulate a criterion of what basic picture describes best this dual behaviour. The main suggestion is that quasi-particle excitation spectra might provide distinctive signatures and good criteria for the appropriate choice of the relevant model.

Neutron scattering experiments on a ³He layer on graphite show an unexpected behavior of the collective mode. After having been broadened by Landau damping at intermediate wave vectors, the phonon-roton mode resharpens at large wave vectors and even emerges from the particle-hole continuum at low energies. The measured spectra cannot be explained by a random phase approximation with any static interaction. We show here that the data are well described if dynamic two-pair fluctuations are accounted for. We predict similar effects for electron layers.

This paper discusses recent efforts to control magnetism with electric fields in single and multilayer oxides, which has great potential to improve a variety of technological endeavors, such as magnetic sensing and magnetoelectric (ME) logic. The importance of electrical control of magnetism is followed by a discussion of multiferroics and MEs, which are the leading contenders for this task. The focus of this paper is on complementary methods in understanding the ME coupling, an essential step to electrical control of magnetism. Neutron scattering, nonlinear optics and X-ray spectromicroscopy are addressed in providing key parameters in the study of ME coupling. While primarily direct (single-phase multiferroics) ME materials are used as examples, the techniques discussed are also valuable to the study of indirect (e.g., multilayers and pillars) magnetoelectrics. We conclude with a summary of the field and future directions.

Several recent observations of closely spaced hydrogen nuclei in condensed matter systems — water, polymers and metal hydrides — indicate that they cannot be considered as independent quantum objects when observed over very short time intervals. According to these measurements, pairs (or possibly small clusters) of protons or deuterons seem to be quantum correlated, and are able to preserve their mutual quantum phase relations over times of the order of femtoseconds even in strongly perturbing liquid or solid environments. These new experimental results are mainly based on scattering of neutrons with time windows in the atto- and femtosecond range, but there also exists corroborating evidence from other spectroscopies. When the width of the observational time window is increased above a characteristic value, the quantum coherence effects are seen to disappear. This time limit marks the onset of decoherence as the mutual phase relations between the particles are gradually lost. Possible reasons for short-time entanglement of nuclei in condensed matter systems, as well as mechanisms for its decoherence will be discussed.

A global shortage of helium gas can seriously jeopardise the scientific programmes of neutron scattering laboratories due to the use of cryogenic sample environment in the majority of the neutron scattering experiments. Recently developed cryogen-free technology allows a significant reduction or even a complete elimination of liquid helium consumption. Here we review the impact of the cryogen-free revolution on cryogenic equipment used at large neutron facilities, such as cryostats, dilution refrigerators, superconducting magnets and other cryogenic systems. Particular attention is given to the newly developed superconducting magnets for neutron diffraction and spectroscopy experiments. Use of the cryogen-free approach, as well as cutting-edge superconducting magnet technology and advanced neutron optics allows researcher to achieve extraordinary performance in their experiments, opening up new opportunities in neutron scattering research.

Neutron reflectometry (NR) was used to examine various live cells' adhesion to quartz substrates under different environmental conditions, including flow stress. To the best of our knowledge, these measurements represent the first successful visualization and quantization of the interface between live cells and a substrate with sub-nanometer resolution.

In our first experiments, we examined live mouse fibroblast cells as opposed to past experiments using supported lipids, proteins, or peptide layers with no associated cells. We continued the NR studies of cell adhesion by investigating endothelial monolayers and glioblastoma cells under dynamic flow conditions. We demonstrated that neutron reflectometry is a powerful tool to study the strength of cellular layer adhesion in living tissues, which is a key factor in understanding the physiology of cell interactions and conditions leading to abnormal or disease circumstances. Continuative measurements, such as investigating changes in tumor cell — surface contact of various glioblastomas, could impact advancements in tumor treatments. In principle, this can help us to identify changes that correlate with tumor invasiveness. Pursuit of these studies can have significant medical impact on the understanding of complex biological problems and their effective treatment, e.g. for the development of targeted anti-invasive therapies.

The characterization of quantum information quantifiers has attracted a considerable attention of the scientific community, since they are a useful tool to verify the presence of quantum correlations in a quantum system. In this context, in the present work we show a theoretical study of some quantifiers, such as entanglement witness, entanglement of formation, Bell’s inequality violation and geometric quantum discord as a function of the diffractive properties of neutron scattering. We provide one path toward identifying the presence of quantum correlations and quantum nonlocality in a molecular magnet as a Heisenberg spin-$1/2$ dimer, by diffractive properties typically obtained via neutron scattering experiments.

We describe the current technology available for producing and delivering high performance neutron beams at modern facilities for neutron scattering experiments. This overview includes optimal neutron production, optical techniques and devices, the use of the beams on the instruments, and also methods of simulation and modeling.

A brief review is presented that summarizes recent neutron diffuse scattering measurements on single crystal PbMg_1/3Nb_2/3O₃ (PMN) and PbZn_1/3Nb_2/3O₃ (PZN) doped with PbTiO₃ (PT). Emphasis is placed on results that suggest that the short-range, polar correlations observed in these systems are connected to the anomalous relaxor dielectric properties, and in particular to the large values of the piezoelectric coefficient d₃₃ near the morphotropic phase boundary.

Pauling's model of hydrogen disorder in water ice represents the prototype of a frustrated system. Over the years it has spawned several analogous models, including Anderson's model antiferromagnet and the statistical “vertex” models. Spin Ice is a sixteen vertex model of “ferromagnetic frustration” that is approximated by real materials, most notably the rare earth pyrochlores Ho₂Ti₂O₇, Dy₂Ti₂O₇ and Ho₂Sn₂O₇. These “spin ice materials” have the Pauling zero point entropy and in all respects represent almost ideal realisations of Pauling's model. They provide experimentalists with unprecedented access to a wide variety of novel magnetic states and phase transitions that are located in different regions of the field-temperature phase diagram. They afford theoreticians the opportunity to explore many new features of the magnetic interactions and statistical mechanics of frustrated systems. This chapter is a comprehensive review of the physics - both experimental and theoretical - of spin ice. It starts with a discussion of the historic problem of water ice and its relation to spin ice and other frustrated magnets. The properties of spin ice are then discussed in three sections that deal with the zero field spin ice state, the numerous field-induced states (including the recently identified “kagomé ice”) and the magnetic dynamics. Some materials related to spin ice are briefly described and the chapter is concluded with a short summary of spin ice physics.

We present measurements of the elementary phonon-roton and other excitations of liquid ⁴He confined in nanoporous media using inelastic neutron scattering methods. The aim is to compare phonon-roton (P-R) and superfluid density measurements and to explore the interdependence of Bose-Einstein Condensation (BEC), P-R modes and superfluidity in helium at nanoscales and in disorder. Specifically a goal is to determine the region of temperature and pressure in which well defined phonon-roton modes exist and compare this with the superfluid phase diagram. In porous media the liquid phase is extended up to 35-40 bars. A second goal is to investigate helium at higher pressures. At low temperature and at saturated vapor pressure (SVP) (p ≃ 0) liquid ⁴He supports well defined P-R modes in all porous media investigated to date (aerogel, xerogel, Vycor, MCM-41 and gelsil of several pore diameters). As temperature is increased at SVP, the P-R modes broaden but well defined modes exist above T_c in the normal phase, up to T_λ. The superfluid to normal transition temperature, T_c, in porous media always lies below the corresponding temperature, T_λ, in bulk helium. In liquid ⁴He in 25 Å and 34 Å mean pore diameter gelsil under pressure and at low temperature, we observe loss of all well defined P-R modes at p = 36.3 - 36.8 bars. Yamamoto et al. have observed a possible Quantum Phase Transition (QPT) (T≈0 K) from the superfluid to normal liquid at p = 34 bars in superfluid density measurements. The existence of P-R modes under pressure up to 36.3 bars and their subsequent loss supports the finding of a QPT. We discuss the implications of these results for the basic concepts of BEC and superfluidity in helium at nanoscales and in disorder.

Neutron scattering experiments on a ³He layer on graphite show an unexpected behavior of the collective mode. After having been broadened by Landau damping at intermediate wave vectors, the phonon-roton mode resharpens at large wave vectors and even emerges from the particle-hole continuum at low energies. The measured spectra cannot be explained by a random phase approximation with any static interaction. We show here that the data are well described if dynamic two-pair fluctuations are accounted for. We predict similar effects for electron layers.

We describe the photo-neutron source at the superconducting electron accelerator ELBE of the Helmholtz-Zentrum Dresden-Rossendorf and present first experiments to determine cross sections of inelastic scattering of neutrons in the MeV range. We discuss analysis and results of photon-scattering experiments using the bremsstrahlung facility at ELBE. A consistent determination of the dipole strength function from the combination of photon scattering and radiative neutron capture is presented.

Pauling's model of hydrogen disorder in water ice represents the prototype of a frustrated system. Over the years it has spawned several analogous models, including Anderson's model antiferromagnet and the statistical "vertex" models. Spin Ice is a sixteen vertex model of "ferromagnetic frustration" that is approximated by real materials, most notably the rare earth pyrochlores Ho₂Ti₂O₇, Dy₂Ti₂O₇ and Ho₂Sn₂O₇. These "spin ice materials" have the Pauling zero point entropy and in all respects represent almost ideal realisations of Pauling's model. They provide experimentalists with unprecedented access to a wide variety of novel magnetic states and phase transitions that are located in different regions of the field-temperature phase diagram. They afford theoreticians the opportunity to explore many new features of the magnetic interactions and statistical mechanics of frustrated systems. This chapter is a comprehensive review of the physics — both experimental and theoretical — of spin ice. It starts with a discussion of the historic problem of water ice and its relation to spin ice and other frustrated magnets. The properties of spin ice are then discussed in three sections that deal with the zero field spin ice state, the numerous field-induced states (including the recently identified "kagomé ice") and the magnetic dynamics. Some materials related to spin ice are briefly described and the chapter is concluded with a short summary of spin ice physics.

We describe the current technology available for producing and delivering high performance neutron beams at modern facilities for neutron scattering experiments. This overview includes optimal neutron production, optical techniques and devices, the use of the beams on the instruments, and also methods of simulation and modeling.