Adaptation and pilot implementation of the Psychosocial Assessment Tool for Autism Spectrum Disorders (PAT-ASD)

Recognizing the multifaceted and chronic demands on families of children with Autism Spectrum Disorder (ASD) and challenges in providing care matched to need, we adapted the Psychosocial Assessment Tool (PAT), a brief caregiver-report screener of family psychosocial risk, for this population. Study methods included literature review, focus groups with providers, and feedback from caregivers. The PAT-ASD is consistent with the original PAT, with new items reflecting core behavioral manifestations of ASD and parent and family challenges associated with chronicity. The PAT-ASD was implemented in a four-month pilot and was completed online by 59% of families. Although further testing of its validity is necessary, the PAT-ASD is a promising means of assessing family psychosocial risk for families of children with ASD.

Evolution of magnetic surfboards and spin glass behavior in (Fe1-pGap)2TiO5

The unusual anisotropy of the spin glass (SG) transition in the pseudobrookite system Fe2TiO5has been interpreted as arising from an induced, van der Waals-like, interaction among magnetic clusters. Here we present susceptibility (χ) and specific heat data (C) for Fe2TiO5diluted with non-magnetic Ga, (Fe1-pGap)2TiO5, for disorder parameterp= 0, 0.11, and 0.42, and elastic neutron scattering data forp= 0.20. A uniform suppression ofTgis observed upon increasingp, along with a value ofχTgthat increases asTgdecreases, i.e.dχ(Tg)/dTg<0We also observeCT∝T2in the low temperature limit. The observed behavior places (Fe1-pGap)2TiO5in the category of a strongly geometrically frustrated SG.

Acceptability and Feasibility of eSCCIP: Results From a Pilot Study of the Electronic Surviving Cancer Competently Intervention Program

BACKGROUND

Providing high-quality psychosocial care to parents and other primary caregivers of children with cancer (henceforth referred to as caregivers) is important, given the numerous challenges associated with a pediatric cancer diagnosis and the increased risk for negative psychosocial sequelae among caregivers. The Electronic Surviving Cancer Competently Intervention Program (eSCCIP) is a psychosocial eHealth intervention for caregivers, developed using an iterative, user-centered process.

METHOD

eSCCIP was tested in a single-arm pilot trial at Nemours Children's Hospital, Delaware (NCT05333601). The primary outcomes were intervention acceptability and feasibility, assessed via enrollment and retention targets, and item-level acceptability ratings. Enrollment and retention targets of 45% were set based on previous work, and an item-level acceptability threshold of 80% was set. A secondary exploratory analysis was conducted examining acute distress, anxiety, symptoms of post-traumatic stress, and family functioning.

RESULTS

44 caregivers enrolled in the study and 31 completed. The intervention was rated favorably by completers, with over 80% of the sample selecting "mostly true" or "very true" for all items of the eSCCIP Evaluation Questionnaire, which was used to assess acceptability and feasibility. Enrollment and retention rates were 54% and 70%, respectively. Exploratory psychosocial outcomes showed statistically significant decreases from pre-intervention to post-intervention for overall symptoms of post-traumatic stress disorder (PTSD), cluster D symptoms of PTSD (negative mood and cognitions), and anxiety. Small-moderate effect sizes were observed for all psychosocial outcomes of interest.

CONCLUSIONS

eSCCIP is an acceptable and feasible intervention for caregivers. Results are promising regarding reductions in symptoms of post-traumatic stress and anxiety.

Telehealth and Type 1 Diabetes Care During COVID-19: Perceptions From Youth of Color, Caregivers, and Health Care Providers

Objective

At the outset of the coronavirus disease 2019 (COVID-19) pandemic, health care systems rapidly implemented telehealth services to maintain continuity of type 1 diabetes care. Youth of color are more likely to have suboptimal glycemic control and may benefit most from efforts to ensure continuity of care. However, research examining the perspectives of families of youth of color regarding telehealth for pediatric type 1 diabetes care is limited. We gathered perspectives from youth of color, their caregivers, and health care providers (HCPs) on telehealth for type 1 diabetes care during COVID-19.

Methods

Fifty participants (22 caregivers, 19 youth, and nine HCPs) completed semi-structured interviews conducted in English (n = 44) or Spanish (n = 6). Transcripts containing mentions of telehealth (n = 33) were included for qualitative analysis to extract themes pertaining to perceptions of type 1 diabetes care and telehealth use during COVID-19.

Results

Themes related to perceptions, feasibility, and quality of telehealth diabetes care were obtained. Most families had positive perceptions of telehealth. Families and HCPs described logistical and technical challenges and noted the potential for disparities in telehealth access and use. Furthermore, caregivers and HCPs felt that the lack of in-person interaction and limited access to clinical data affected the quality of care.

Conclusion

Families of youth of color with type 1 diabetes mostly had positive perceptions of telehealth but also identified issues with feasibility and quality of care. Our findings highlight a need for interventions promoting equal access to telehealth and quality care for all youth with type 1 diabetes to minimize disruptions in care.

Magnetoquantum oscillations in the specific heat of a topological Kondo insulator

Surprisingly, magnetoquantum oscillations (MQOs) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB₆. As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. We report here the first observation of MQO in the low-temperature specific heat of SmB₆. The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB₆but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminum inclusion-based origin for the MQO.

Partnering With Stakeholders to Inform the Co-Design of a Psychosocial Intervention for Prenatally Diagnosed Congenital Heart Disease

Input from diverse stakeholders is critical to the process of designing healthcare interventions. This study applied a novel mixed-methods, stakeholder-engaged approach to co-design a psychosocial intervention for mothers expecting a baby with congenital heart disease (CHD) and their partners to promote family wellbeing. The research team included parents and clinicians from 8 health systems. Participants were 41 diverse parents of children with prenatally diagnosed CHD across the 8 health systems. Qualitative data were collected through online crowdsourcing and quantitative data were collected through electronic surveys to inform intervention co-design. Phases of intervention co-design were: (I) Engage stakeholders in selection of intervention goals/outcomes; (II) Engage stakeholders in selection of intervention elements; (III) Obtain stakeholder input to increase intervention uptake/utility; (IV) Obtain stakeholder input on aspects of intervention design; and (V) Obtain stakeholder input on selection of outcome measures. Parent participants anticipated the resulting intervention, HEARTPrep, would be acceptable, useful, and feasible for parents expecting a baby with CHD. This model of intervention co-design could be used for the development of healthcare interventions across chronic diseases.

A community-based trial of a psychosocial eHealth intervention for parents of children with cancer

BACKGROUND

The Electronic Surviving Cancer Competently Intervention Program (eSCCIP), a psychosocial eHealth intervention for parents and caregivers of children with cancer (parents), was delivered in a community-based psychosocial oncology center. Primary endpoints were intervention acceptability, feasibility, and accessibility, with a secondary exploratory focus on psychosocial outcomes.

PROCEDURE

Oncology therapists in a psychosocial oncology center were trained in eSCCIP delivery. Participants were eligible for participation if they were the primary caregiver of a child with cancer between the ages 0 and 17, could read and write in English, and had reliable internet access to complete eSCCIP. Surveys were administered electronically at baseline and post intervention to evaluate study endpoints. Effect sizes (Cohen's d) were computed for exploratory psychosocial outcomes. Nineteen parents completed the intervention.

RESULTS

Parents rated eSCCIP as highly acceptable, feasible, and accessible. A large clinical effect was detected for acute distress (d = 0.79). Moderate clinical effects were reported for overall posttraumatic stress disorder (PTSD) symptoms (d = 0.37), negative mood/cognitions (d = 0.59), and symptoms of anxiety (d = 0.48).

CONCLUSIONS

Results indicate that eSCCIP is an acceptable, feasible, and accessible psychosocial intervention for parents. Exploratory analyses suggest that participation in eSCCIP may contribute to decreases in acute distress, symptoms of anxiety, and symptoms of PTSD.

Extremely Weakly Interacting ΔS_{z}=0 and ΔS_{z}=1 Excitations and Evidence for Fractional Quantization in a Magnetization Plateau: CeSb

The plateau at 1/3 of the saturation magnetization M_{s} in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured M and the specific heat C in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with ΔS_{z}=1 as measured by M, coexist with a much larger population of ΔS_{z}=0 excitations measured by C but invisible to M. The large density of ΔS_{z}=0 excitations, their energy gap, and their seeming lack of interaction with ΔS_{z}=1 excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in M(H) are seen, the size of which, 0.15%M_{s}, is consistent with fractional quantization of magnetization per site in the down-spin layers.

Strategies for Successful Recruitment and Retention of Parents in Pediatric Psychosocial eHealth Interventions: A Qualitative Study in Pediatric Oncology

OBJECTIVE

Psychosocial intervention studies with parents often have difficulty with regard to participant recruitment, retention, and engagement. Prior to conducting a pilot test of the Electronic Surviving Cancer Competently Intervention Program (eSCCIP), a psychosocial eHealth intervention for parents of children with cancer, a qualitative study was designed to preemptively generate strategies to address potential difficulties in these domains.

METHODS

Two focus groups and three semi-structured interviews were conducted with parents of children with cancer. Qualitative data were analyzed using the principles of thematic and content analysis. Themes were generated to describe results and inform the development of concrete recruitment, retention, and engagement strategies to be utilized in future intervention trials.

RESULTS

Five themes emerged specific to recruitment, and three themes emerged related to retention and engagement. Parents highlighted a number of important potential recruitment, retention, and engagement strategies, including the importance of flexibility, ease of access and use, intervention timing, and pathways to follow-up care. Themes were linked to actionable recruitment, retention, and engagement strategies which can be utilized in future studies.

CONCLUSION

Results from the current study will be utilized when designing future eSCCIP intervention trials and will also be of use to the broader field of psychosocial eHealth intervention research in pediatric psychology. This study also underscores the importance of working with parent stakeholders at all phases of study design and implementation.

Evidence for undoped Weyl semimetal charge transport in Y2Ir2O7

Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature [Formula: see text] in a single particle model. Here we show that, in closed-environment-grown polycrystalline samples of Y₂Ir₂O₇, [Formula: see text] over four orders of magnitude in [Formula: see text]. While the measured prefactor, [Formula: see text], is obtained from the model using reasonable materials parameters, the [Formula: see text] behavior extends far beyond the model's range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, [Formula: see text]. Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in Y₂Ir₂O₇, are the possible origin of such 'bad' Weyl semimetal behavior.

Local Moment Instability of Os in Honeycomb Li2.15Os0.85O3

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li_2.15(3)Os_0.85(3)O₃ (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μ_B, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T^(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

Magnetic Ground States of the Rare-Earth Tripod Kagome Lattice Mg_{2}RE_{3}Sb_{3}O_{14} (RE=Gd,Dy,Er)

We present the structural and magnetic properties of a new compound family, Mg_{2}RE_{3}Sb_{3}O_{14} (RE=Gd,Dy,Er), with a hitherto unstudied frustrating lattice, the "tripod kagome" structure. Susceptibility (ac, dc) and specific heat exhibit features that are understood within a simple Luttinger-Tisza-type theory. For RE=Gd, we found long-ranged order (LRO) at 1.65 K, which is consistent with a 120° structure, demonstrating the importance of diople interactions for this 2D Heisenberg system. For RE=Dy, LRO at 0.37 K is related to the "kagome spin ice" physics for a 2D system. This result shows that the tripod kagome structure accelerates the transition to LRO predicted for the related pyrochlore systems. For RE=Er, two transitions, at 80 mK and 2.1 K are observed, suggesting the importance of quantum fluctuations for this putative XY system.

Quasispin glass in a geometrically frustrated magnet

The spin glass state in the spinel ZnCr(2(1-x))Ga(2x)O(4) is studied with magnetization and specific heat for x < 0.05. The freezing temperature is independent of disorder, despite a two-level-like density of states that varies linearly with x. This relationship implies the energy scale for freezing is independent of disorder, in contrast to mean field theories of spin glass. We suggest that the degrees of freedom are shielded spin vacancies, quasispins, which interact via an emergent long-range force mediated by the frustrated spin background.

Morphology effectively controls singlet-triplet exciton relaxation and charge transport in organic semiconductors

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

New A2/3-xRh2O4 compounds with the CaFe2O4 structure where A is a rare earth or Bi

New compounds of the type R(2/3-x)Rh(2)O(4) with the CaFe(2)O(4) structure have been prepared, where R is a rare earth. For crystals grown in a Bi/V/O flux, the rare earth was partially replaced by Bi. No evidence of ordering of the A cation vacancies is found, but the A cations are displaced from the ideal A cation site by about 0.24 A. Electrical conductivity measurements on crystals suggest that the materials are degenerate semiconductors with Seebeck measurements showing p-type behavior. This is consistent with our observation that x in R(2/3-x)Rh(2)O(4) ranges up to about 0.09. The compounds were also characterized by magnetic susceptibility and diffuse reflectance measurements.

Soft manifold dynamics behind negative thermal expansion

Minimal models are developed to examine the origin of large negative thermal expansion in underconstrained systems. The dynamics of these models reveals how underconstraint can organize a thermodynamically extensive manifold of low-energy modes which not only drives negative thermal expansion but extends across the Brillioun zone. Mixing of twist and translation in the eigenvectors of these modes, for which in ZrW2O8 there is evidence from infrared and neutron scattering measurements, emerges naturally in our model as a signature of the dynamics of underconstraint.

Cupric oxide as an induced-multiferroic with high-TC

Materials that combine coupled electric and magnetic dipole order are termed 'magnetoelectric multiferroics'. In the past few years, a new class of such materials, 'induced-multiferroics', has been discovered, wherein non-collinear spiral magnetic order breaks inversion symmetry, thus inducing ferroelectricity. Spiral magnetic order often arises from the existence of competing magnetic interactions that reduce the ordering temperature of a more conventional collinear phase. Hence, spiral-phase-induced ferroelectricity tends to exist only at temperatures lower than approximately 40 K. Here, we propose that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions can be good candidates for induced-multiferroics with high Curie temperature (T(C)). In fact, we demonstrate ferroelectricity with T(C)=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-T(c) (critical temperature) superconductors. Our result provides an important contribution to the search for high-temperature magnetoelectric multiferroics.

Direct transition from a disordered to a multiferroic phase on a triangular lattice

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)(2). Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.

Magnetically driven ferroelectric order in Ni3V2O8

We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

Electric polarization rotation in a hexaferrite with long-wavelength magnetic structures

We report on the control of electric polarization (P) by using magnetic fields (B) in a hexaferrite having magnetic order above room temperature (RT). The material investigated is hexagonal Ba0.5Sr1.5Zn2Fe12O22, which is a nonferroelectric helimagnetic insulator in the zero-field ground state. By applying B, the system undergoes successive metamagnetic transitions, and shows concomitant ferroelectric order in some of the B-induced phases with long-wavelength magnetic structures. The magnetoelectrically induced P can be rotated 360 degrees by external B. This opens up the potential for not only RT magnetoelectric devices but also devices based on the magnetically controlled electro-optical response.

Landau quantization effects in the charge-density-wave system (Per)2M(mnt)2 (where M = Au and Pt)

A finite transfer integral t(a) orthogonal to the conducting chains of a highly one-dimensional metal gives rise to empty and filled bands that simulate an indirect-gap semiconductor upon formation of a charge-density wave (CDW). In contrast to semiconductors such as Ge and Si with band gaps approximately 1 eV, the CDW system possesses an indirect gap with a greatly reduced energy scale, enabling moderate laboratory magnetic fields to have a major effect. The consequent variation of the thermodynamic gap with magnetic field due to Zeeman splitting and Landau quantization enables the electronic band structure parameters (transfer integrals, Fermi velocity) to be determined accurately. These parameters reveal the orbital quantization limit to be reached at approximately 20 T in (Per)2M(mnt)(2) salts, making them highly unlikely candidates for a recently proposed cascade of field-induced CDW states.

Strongly enhanced hole-phonon coupling in the metallic state of the dilute two-dimensional hole gas

We have studied the temperature dependent phonon emission rate P(T) of a strongly interacting (r(s) > or =22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that P(T) changes from P(T) approximately T5 to P(T) approximately T7 above 100 mK, indicating a crossover from screened piezoelectric (PZ) coupling to screened deformation potential (DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is almost 20 times stronger than expected from theory. The density dependence of P(T) shows that it is easier to cool low-density 2D holes in GaAs than higher density 2D hole systems.

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.

Competing magnetic phases on a kagomé staircase

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.

Amorphouslike density of gap states in single-crystal pentacene

We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.

Bias-dependent generation and quenching of defects in pentacene

We describe a defect generation phenomenon that is new to organic semiconductors. A defect in pentacene single crystals can be created by bias-stress and persists at room temperature for an hour in the dark but only seconds with 420 nm illumination. The defect gives rise to a hole trap at Ev+0.38 eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67 eV activation energy with a small (10(8) s(-1)) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.

Monocrystal elastic constants of the negative-thermal-expansion compound zirconium tungstate (ZrW2O8)

We measured zirconium tungstate's elastic constants C(ij). This compound shows relatively soft, nearly isotropic elastic constants with normal Poisson ratios and no approach to Born instability. ZrW2O8 shows normal ambient-temperature elastic constants C(ij), but remarkable dC(ij)/dT that show dominant low-frequency acoustic-vibration modes. From the bulk modulus, we estimated the total ambient-temperature thermodynamic Grüneisen parameter as gamma = -1.2. The dB/dT slope gives a Grüneisen parameter gamma = -7. The 300-0 K bulk-modulus increase (40%) seems unprecedented and breaks Birch's law of corresponding states.

Ferroelectricity and giant magnetocapacitance in perovskite rare-earth manganites

The relationships among magnetism, lattice modulation, and dielectric properties have been investigated for RMnO3 (R=Eu, Gd, Tb, and Dy). These compounds show a transition to an incommensurate lattice structure below their Néel temperature, and subsequently undergo an incommensurate-commensurate (IC-C) phase transition. For TbMnO3 and DyMnO3 it was found that the IC-C transition is accompanied by a ferroelectric transition, associated with a lattice modulation in the C phase. DyMnO3 shows a gigantic magnetocapacitance with a change of dielectric constant up to Deltaepsilon/epsilon approximately 500%.

Magnetodielectric effects from spin fluctuations in isostructural ferromagnetic and antiferromagnetic systems

We report on the effects of spin fluctuations, magnetic ordering, and external magnetic field on the dielectric constant of the ferromagnet SeCuO3, and the antiferromagnet TeCuO3. A model based on the coupling between uniform polarization and the q-dependent spin-spin correlation function is presented to explain the different behaviors for these isostructural compounds. The large magnetocapacitance near the transition temperature in the ferromagnet SeCuO3 suggests routes to enhancing the magnetodielectric response for practical applications.

Frustrated soft modes and negative thermal expansion in ZrW2O8

Negative thermal expansion (NTE) in cubic ZrW2O8 has generated much interest due to its large, isotropic, and temperature independent behavior. Here, x-ray absorption fine structure data are presented for various atom pairs, providing evidence that the low-energy modes causing NTE correspond to the correlated vibrations of a WO4 tetrahedron and its three nearest ZrO6 octahedra. This involves translations of the WO4 as a rigid unit along each of the four <111> axes. The interconnectivity of these modes prevents an anisotropic soft mode from developing, a new geometrical phenomenon that we call the "frustrated soft mode."

Multiple field-induced phase transitions in the geometrically frustrated dipolar magnet: Gd(2) Ti(2)O(7)

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd(2)Ti(2)O(7), where crystal-field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real space and spin space, so the transitions in Gd(2)Ti(2)O(7) arise from field-induced "cooperative anisotropy," reflecting the broken spatial symmetries of the pyrochlore lattice.

Optical response of high-dielectric-constant perovskite-related oxide

Optical conductivity measurements on the perovskite-related oxide CaCu3Ti4O12 provide a hint of the physics underlying the observed giant dielectric effect in this material. A low-frequency vibration displays anomalous behavior, implying that there is a redistribution of charge within the unit cell at low temperature. At infrared frequencies (terahertz), the value for the dielectric constant is approximately 80 at room temperature, which is far smaller than the value of approximately 10(5) obtained at lower radio frequencies (kilohertz). This discrepancy implies the presence of a strong absorption at very low frequencies due to dipole relaxation. At room temperature, the characteristic relaxation times are fast (less than or approximately 500 nanoseconds) but increase dramatically at low temperature, suggesting that the large change in dielectric constant may be due to a relaxor-like dynamical slowing down of dipolar fluctuations in nanosize domains.

Non-Fermi-liquid behaviour in La4Ru6O19

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization.

Superconductivity in the non-oxide perovskite MgCNi3

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.

Geometric magnetic frustration in Ba(2)Sn(2)Ga(3)ZnCr(7)O(22): a two-dimensional spinel based Kagomé lattice

The properties of a two-dimensional geometrically frustrated magnetic material based on the Kagomé net, Ba(2)Sn(2)ZnGa(3)Cr(7)O(22), are reported. The Kagomé net is fully filled with magnetic ions. A Curie-Weiss theta theta(W) = -312 K is found with a spin glass transition at approximately 1.5 K, indicating strong geometrical magnetic frustration. This compound is the most two dimensional of a structural series with the geometrically frustrated materials ZnCr(2)O(4) and SrCr(8)Ga(4)O(19). The comparison of their properties tests the influence of different degrees of coupling between Kagomé layers on magnetic frustration within a single chemical and structural family.

Entropy balance and evidence for local spin singlets in a Kagome-like magnet

We have measured the specific heat C(T) of the S = 3 / 2 Kagome-lattice-containing compound SrCr (9p)Ga (12-9p)O (19). We find little field dependence of the low-temperature C(T), consistent with a low-energy spectrum dominated by many-body singlet excitations. At high temperatures, we recover only approximately 50% of the total R ln4 entropy.

Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.

Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl23+Mn24+O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.