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The layered antiferromagnet FePS$_3$ has been shown to undergo a structural transition under pressure linked to an insulator-metal transition, with two incompatible models previously proposed for the highest-pressure structure. We present a study of the high-pressure crystal structures of FePS$_3$ using both single-crystal and powder x-ray diffraction. We show that the highest pressure transition involves a collapse of the inter-planar spacing of this material, along with an increase in symmetry from a monoclinic to a trigonal structure, to the exclusion of other models. The extent of this volume collapse is shown to be sensitive to the presence of a helium pressure medium in the sample environment, indicating that consideration of such experimental factors is important for understanding high-pressure behaviours in this material., Comment: 5 pages, 3 figures

The article aims to introduce the underlying motivation and conceptual underpinning to the special issue entitled “Globalizing Local Understanding of Fragility in Eurasia.” The main purpose of this article is to problematize the popular opinion and portrayal of Central Asia (Kazakhstan, Kyrgyz Republic, Tajikistan, Turkmenistan, and Uzbekistan) and more generally the countries of Eurasia and the Caucasus as inherently fragile states which are politically unstable and thus on the brink of collapse. This article also seeks to question narratives of modernity that are singular and constantly out of reach for large swathes of the world’s populations because of the narrowness and hegemonic nature of the architecture of global governance. By carefully considering the ways and means through which international institutions categorize countries as fragile and/or failed, the article aims to provide the theoretical foreground for the special issue which focuses on locating inherent community resilience strategies. We explain how the non-participatory norm making behavior of international organizations privilege certain actors, largely the Global North, and simultaneously ignore the majority of Eurasian states. In other words, a demand predicated in the linear evaluation of institutions and norms dictated by global institutions clash with the Eurasian model of inherent complex adaptive capability and introduce fragility. The focus thus is on understanding the ‘local’ based on the historical analysis of development in the region, nodal points of urban development and community life, forms of social capital, and community resilience strategies in the wider Eurasian region.

We present the observation of an isostructural Mott insulator-metal transition in van-der-Waals honeycomb antiferromagnet V$_{0.9}$PS$_3$ through high-pressure x-ray diffraction and transport measurements. The MPX$_3$ family of magnetic van-der-Waals materials (M denotes a first row transition metal and X either S or Se) are currently the subject of broad and intense attention, but the vanadium compounds have until this point not been studied beyond their basic properties. We observe insulating variable-range-hopping type resistivity in V$_{0.9}$PS$_3$, with a gradual increase in effective dimensionality with increasing pressure, followed by a transition to a metallic resistivity temperature dependence between 112 and 124 kbar. The metallic state additionally shows a low-temperature upturn we tentatively attribute to the Kondo Effect. A gradual structural distortion is seen between 26-80 kbar, but no structural change at higher pressures corresponding to the insulator-metal transition. We conclude that the insulator-metal transition occurs in the absence of any distortions to the lattice - an isostructural Mott transition in a new class of two-dimensional material, and in strong contrast to the behavior of the other MPX$_3$ compounds., 12 pages, including appended supplementary section. Manuscript submitted to NPJ Quantum Materials

Layered van-der-Waals 2D magnetic materials are of great interest in fundamental condensed-matter physics research, as well as for potential applications in spintronics and device physics. We present neutron powder diffraction data using new ultra-high-pressure techniques to measure the magnetic structure of Mott-insulating 2D honeycomb antiferromagnet FePS3 at pressures up to 183 kbar and temperatures down to 80 K. These data are complemented by high-pressure magnetometry and reverse Monte Carlo modeling of the spin configurations. As pressure is applied, the previously-measured ambient-pressure magnetic order switches from an antiferromagnetic to a ferromagnetic interplanar interaction, and from 2D-like to 3D-like character. The overall antiferromagnetic structure within the \ud ab planes, ferromagnetic chains antiferromagnetically coupled, is preserved, but the magnetic propagation vector is altered from k=0,1,12) to k=(0,1,0), a halving of the magnetic unit cell size. At higher pressures, coincident with the second structural transition and the insulator-metal transition in this compound, we observe a suppression of this long-range-order and emergence of a form of magnetic short-range order which survives above room temperature. Reverse Monte Carlo fitting suggests this phase to be a short-ranged version of the original ambient pressure structure - with the Fe moment size remaining of similar magnitude and with a return to antiferromagnetic interplanar correlations. The persistence of magnetism well into the HP-II metallic state is an observation in contradiction with previous x-ray spectroscopy results which suggest a spin-crossover transition.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of Tc near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes., Superconductivity in doped SrTiO3 near to a ferroelectric quantum critical point emerges due to a strong interaction driving the formation of Cooper pairs, the nature of which has remained elusive for several decades. Here, the authors reveal that pairing is due to the exchange of longitudinal hybrid polar modes rather than transverse critical modes.

We present an overview of our recent work in tuning and controlling the structural, magnetic and electronic dimensionality of 2D van-der-Waals antiferromagnetic compounds (Transition-Metal)PS3. Low-dimensional magnetic systems such as these provide rich opportunities for studying new physics and the evolution of established behaviours with changing dimensionality. These materials can be exfoliated to monolayer thickness and easily stacked and combined into functional heterostructures. Alternatively, the application of hydrostatic pressure can be used to controllably close the van-der-Waals interplanar gap and tune the crystal structure and electron exchange paths towards a 3D nature. We collect and discuss trends and contrasts in our data from electrical transport, Raman scattering and synchrotron x-ray measurements, as well as insight from theoretical calculations and other results from the literature. We discuss structural transitions with pressure common to all materials measured, and link these to Mott insulator-transitions in these compounds at high pressures. Key new results include magnetotransport and resistivity data in the high-pressure metallic states, which show potentially interesting qualities for a new direction of future work focused on low temperature transport and quantum critical physics.\ud \ud

A detailed neutron scattering study of the Shastry–Sutherland material SrCu2(BO3)2 verifies the existence of a 4-spin plaquette singlet phase in this system.

Phase transitions to a long-range-ordered state driven by a softened phonon mode are ubiquitous across condensed matter physics, but the evolution of such a mode as the system is tuned to or from the transition has never been explicitly measured until now. We report the effect of pressure on the soft mode associated with ferroelectricity in the archetypal quantum critical paraelectric SrTiO3. This is an ideal, clean, model system for exploring these effects, with pressure directly addressing the phonon modes only. We measure and report the effect of quantum critical fluctuations on the pressure and temperature dependence of the ferroelectric soft phonon mode as the system is tuned away from criticality. We show that the mean-field approximation is confirmed experimentally. Furthermore, using a self-consistent model of the quantum critical excitations including coupling to the volume strain and without adjustable parameters, we determine logarithmic corrections that would be observable only very close to the quantum critical point. Thus, the mean-field character of the pressure dependence is much more robust to the fluctuations than is the temperature dependence. We predict stronger corrections for lower dimensionalities. The same calculation confirms that the Lydanne-Sachs-Teller relation is valid over the whole pressure and temperature range considered. Therefore, the measured dielectric constant can be used to extract the frequency of the soft mode down to 1.5 K and up to 20 kbar of applied pressure. The soft mode is observed to stiffen further, raising the low-temperature energy gap and returning towards the expected shallow temperature dependence of an optical mode. This behavior is consistent with the existence of a ferroelectric quantum critical point on the pressure-temperature phase diagram of SrTiO3, which applied pressure tunes the system away from. This work represents an experimental measurement of the stiffening of a zone center soft phonon mode as a system is tuned away from criticality, a potentially universal phenomenon across a variety of phase transitions and systems in condensed matter physics. Universidad de Costa Rica/[816-B7-601]/UCR/Costa Rica NUST MISiS-Ministry of Education and Science/[ K2-2017-024]//Rusia University of Cambridge/[]//Inglaterra UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA) UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Física

We present comprehensive measurements of the structural, magnetic and electronic properties of layered van-der-Waals ferromagnet VI$_3$ down to low temperatures. Despite belonging to a well studied family of transition metal trihalides, this material has received very little attention. We outline, from high-resolution powder x-ray diffraction measurements, a corrected room-temperature crystal structure to that previously proposed and uncover a structural transition at 79 K, also seen in the heat capacity. Magnetization measurements confirm VI$_3$ to be a hard ferromagnet (9.1 kOe coercive field at 2 K) with a high degree of anisotropy, and the pressure dependence of the magnetic properties provide evidence for the two-dimensional nature of the magnetic order. Optical and electrical transport measurements show this material to be an insulator with an optical band gap of 0.67 eV - the previous theoretical predictions of d-band metallicity then lead us to believe VI$_3$ to be a correlated Mott insulator. Our latest band structure calculations support this picture and show good agreement with the experimental data. We suggest VI$_3$ to host great potential in the thriving field of low-dimensional magnetism and functional materials, together with opportunities to study and make use of low-dimensional Mott physics., 9 pages, 11 figures

Cs2CuCl4 is known to possess a quantum spin liquid phase with antiferromagnetic interaction below 2.8 K. We report the observation of a new metastable magnetic phase of the triangular frustrated quantum spin system Cs2CuCl4 induced by the application of hydrostatic pressure. We measured the magnetic properties of Cs2CuCl4 following the application and release of pressure after 3 days. We observed a previously unknown ordered magnetic phase with a transition temperature of 9 K. Furthermore, the recovered sample with new magnetic ground state possesses an equivalent crystal structure to the uncompressed one with antiferromagnetic quantum spin liquid phase., Comment: 9 pages, 5 figures 1 table

First author’s name is corrected to Charles R.S. Haines. The publisher apologizes for the inconvenience.

Proximity to magnetic order as well as low dimensionality are both beneficial to superconductivity at elevated temperatures. Materials on the border of magnetism display a wide range of novel and potentially useful phenomena: highTcs, heavy fermions, coexistence of magnetism and superconductivity and giant magnetoresistance. Low dimensionality is linked to enhanced fluctuations and, in the case of heavy fermions, has been experimentally shown to be beneficial for the fluctuations that are responsible for the rich abundance of novel emergent phases. This experimental strategy motivated us to explore 2D insulating magnets with a view to investigate phase evolution across metal-insulator and magnetic-non-magnetic boundaries. This has been a fruitful venture with totally novel results different to our expectations. We present results from 2 distinct systems. The MPS3family are highly anisotropic in both their crystal and magnetic structures. FePS3in particular is a model insulating honeycomb antiferromagnet. We find that the application of pressure to FePS3induces an insulator to metal transition. The second system, Cs2CuCl4, is a highly-frustrated quantum spin liquid at low temperature. The competition of the 3 relevant exchange couplings is delicately balanced. It has been shown to become antiferromagnetic at very low temperatures (~1 K). We have found that the application of pressure for 3 days or more followed by a return to ambient pressure stabilises a totally distinct magnetic ground state.

Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. Here we present work on a new metal-to-insulator transition system ${\mathrm{FePS}}_{3}$. This compound is a two-dimensional van der Waals antiferromagnetic Mott insulator. We report the discovery of an insulator-metal transition in ${\mathrm{FePS}}_{3}$, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure, and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.

The dielectric and magnetic polarizations of quantum paraelectrics and paramagnetic materials have in many cases been found to initially increase with increasing thermal disorder and hence exhibit peaks as a function of temperature. A quantitative description of these examples of 'order-by-disorder' phenomenona has remained elusive in nearly ferromagnetic metals and in dielectrics on the border of displacive ferroelectric transitions. Here we present an experimental study of the evolution of the dielectric susceptibility peak as a function of pressure in the nearly ferroelectric material, strontium titanate, which reveals that the peak position collapses towards absolute zero as the ferroelectric quantum critical point is approached. We show that this behaviour can be described in detail without the use of adjustable parameters in terms of the Larkin-Khmelnitskii-Shneerson-Rechester (LKSR) theory, first introduced nearly 50 years ago, of the hybridization of polar and acoustic modes in quantum paraelectrics, in contrast to alternative models that have been proposed. Our study allows us to construct for the first time a detailed temperature-pressure phase diagram of a material on the border of a ferroelectric quantum critical point comprising ferroelectric, quantum critical paraelectric and hybridized polar-acoustic regimes. Furthermore, at the lowest temperatures, below the susceptibility maximum, we observe a new regime characterized by a linear temperature dependence of the inverse susceptibility that differs sharply from the quartic temperature dependence predicted by the LKSR theory. We find that this non-LKSR low temperature regime cannot be accounted for in terms of any detailed model reported in the literature, and its interpretation poses a new empirical and conceptual challenge., Comment: Published in the Proceedings of the National Academy of Sciences, May 2020, 201922151

Here, we present an alternative approach for the description of quantum critical fluctuations. These are described by Langevin random fields, which are then related to the susceptibility using the fluctuation-dissipation theorem. We use this approach to characterize the physical properties arising in the vicinity of two coupled quantum phase transitions. We consider a phenomenological model based on two scalar order parameter fields locally coupled biquadratically and having a common quantum critical point as a function of a quantum tuning parameter such as pressure or magnetic field. A self-consistent treatment shows that the uniform static susceptibilities of the two order parameter fields have the same qualitative form at low temperature even where the forms are different in the absence of the biquadratic coupling., We acknowledge support from EPSRC, Corpus Christi College, the National Science Foundation Grant No. NSF DMR-1334428, and the Increase Competitiveness Program of the Ministry of Education of the Russian Federation Grant No. NUST MISiS K2-2017-024. We are grateful to Trinity College Cambridge, the Cavendish Laboratory, and the Aspen Center for Physics, supported by National Science Foundation Grant No. NSF PHY-1066293, for hospitality. S.E.R. acknowledges support from a CONFAP Newton grant.

Superconductivity in novel bismuth-sulphur superconductors has attracted large research efforts, both experimental and theoretical, but a consensus on the nature of superconductivity in these materials has yet to be reached. Using density functional theory for superconductors, we study the electron-phonon pairing mechanism in LaO$_{0.5}$F$_{0.5}$BiS$_2$. We first confirm the presence of a commensurate charge density wave instability, in accordance with previous studies. Using a recently developed integration scheme for the electron-phonon coupling, we found that its strength is much lower than previously calculated, due to improved density of state calculations. We finally conclude that conventional phonon-mediated pairing cannot explain the high superconducting transition temperatures observed in this material.

Cs₂CuCl₄ is known to possess a quantum spin-liquid phase with antiferromagnetic interaction below 2.8 K. We report the observation of a new metastable magnetic phase of the triangular frustrated quantum spin system Cs₂CuCl₄ induced by the application of hydrostatic pressure. We measured the magnetic properties of Cs₂CuCl₄ following the application and release of pressure after 3 days. We observed a previously unknown ordered magnetic phase with a transition temperature of 9 K. Furthermore, the recovered sample with new magnetic ground state possesses an equivalent crystal structure to the uncompressed one with antiferromagnetic quantum spinliquid phase.

Quantum criticality is often found in metallic compounds that are close to being magnetic. What about insulators in which the electric moments are fluctuating? These too can be described by the same framework—over a wider temperature range than in quantum critical metals.

A density functional theory study of the BiS2 superconductors containing rare-earths: LnO1-x F x BiS2 (Ln = La, Ce, Pr, and Nd) is presented. We find that CeO0.5F0.5BiS2 has competing ferromagnetic and weak antiferromagnetic tendencies, the first one corresponding to experimental results. We show that PrO0.5F0.5BiS2 has a strong tendency for magnetic order, which can be ferromagnetic or antiferromagnetic depending on subtle differences in 4f orbital occupations. We demonstrate that NdO0.5F0.5BiS2 has a stable magnetic ground state with weak tendency to order. Finally, we show that the change of rare earth does not affect the Fermi surface, and predict that CeOBiS2 should display a pressure induced phase transition to a metallic, if not superconducting, phase under pressure.

Magnetoelectric coupling phenomena in EuTiO3 are of considerable fundamental interest and are also understood to be key to reported multiferroic behavior in strained films, which exhibit distinctly different properties to the bulk. Here, the magnetoelastic coupling of EuTiO3 is investigated by resonant ultrasound spectroscopy with in situ applied magnetic field and stress as a function of temperature ranging from temperatures above the structural transition temperature Ts to below the antiferromagnetic ordering temperature Tn. One single crystal and two polycrystalline samples are investigated and compared to each other. Both paramagnetic and diamagnetic transducer carriers are used, allowing an examination of the effect of both stress and magnetic field on the behavior of the sample. The properties are reported in constant field/variable temperature and in constant temperature/variable field mode where substantial differences between both data sets are observed. In addition, elastic and magnetic poling at high fields and stresses at low temperature has been performed in order to trace the history dependence of the elastic constants. Four different temperature regions are identified, characterized by unusual elastic responses. The low-temperature phase diagram has been explored and found to exhibit rich complexity. The data evidence a considerable relaxation of elastic constants at high temperatures, but with little effect from magnetic field alone above 20 K, in addition to the known low-temperature coupling. NRF (Natl Research Foundation, S’pore) Published version

We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s. This paper recontextualizes the field in light of the discovery of superconductivity in CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how this relates to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition., 22 pages, 8 figures

Bi4O4S3 was the first superconductor discovered in the BiS2 family of novel superconductors. The subfamily of Bi-O-S systems now expanded andwas probed both theoretically and experimentally. Here we review the experimentalwork done on the Bi-O-S compounds, and compare it with theoretical results obtained using ab-initio methods. In particular we calculate the charge carrier density using an ab-initio calculated density of states, and find a really close agreement with experimental data.

Newly discovered Bi-O-S compounds remain an enigma in attempts to understand their electronic properties. A recent study of Bi$_{4}$O$_{4}$S$_{3}$ has shown it to be a mixture of two phases, Bi$_{2}$OS$_{2}$ and Bi$_{3}$O$_{2}$S$_{3}$, the latter being superconducting [W. A. Phelan et al., J. Am. Chem. Soc. 135, 5372 (2013)]. Using density functional theory, we explore the electronic structure of both the phases and the effect of the introduction of stacking faults. Our results demonstrate that the S$_{2}$ layers dope the bismuth-sulphur bands. The bands at the Fermi level are of clear two-dimensional character. One band manifold is confined to the two adjacent, square-lattice bismuth-sulphur planes, a second manifold is confined to the square lattice of sulphur dimers. We show that the introduction of defects in the stacking does not influence the electronic structure. Finally, we also show that spin-orbit coupling does not have any significant effect on the states close to the Fermi level at the energy scale considered.

The discovery of superconductivity in MgB2 sparked much interest in the diborides and led to further discoveries in this class of materials. Attention has focussed on the transitions metal diborides and the rare-earth diborides have remained sparsely investigated. Here we present electrical transport in polycrystalline samples of ErB2, TmB2 and HoB2 as a function of temperature down to 0.1 K. Particularly interesting is the large difference in the temperature dependence of resistivity, above and below the clear magnetic ordering temperature of 13.8, 9 and 7.5 K for ErB2, HoB2 and TmB2 respectively.

Historically, the geographical area of Eurasia represents the heart of the Silk Road, which was traversed by Italian merchants, Central Asian and Middle Eastern traders. It connected Central Asia with the Lower Volga and Northern China to Kara Korum (Mongolia) and for centuries facilitated trade from the east to the west and vice versa.1 The Mongol Empire in the 13th century connected peoples, goods and ideas via this Silk Road from China to the Mediterranean.2 Once the Mongols had conquered China, Russia, Transcaucasia and Iran, there was a blossoming of trade right across their empire which was in large part driven by funding provided by the Mongol Khans and their family members to Muslim, Central Asian and Chinese traders. In addition, by providing infrastructure and communication networks throughout the empire, the Mongols made trading more attractive and profitable. According to Cosmo (2005), ‘the Mongols had indeed been exceptional in their ability to provide infrastructure underpinning trade even when formal backing of European states was lacking’.3 By 1221, Central Asian traders were playing a very important role in trade across the region.4 The union of China, Turkestan, Persia and Russia in one huge empire regulated by a strict Yasa5 under princes who were concerned for the safety of the caravans and tolerant of all cults, reopened by sea and by land the world routes that had been blocked since the end of antiquity.6

In this paper we discuss our discovery [T.E. Weller, et al., Nat. Phys., submitted for publication] of superconductivity in C 6 Yb and C 6 Ca at 6.5 and 11.5 K, respectively. In addition, we present evidence based on an H – T phase diagram demonstrating that these compounds are significantly more isotropic than pure graphite. This is unexpected as the effect of introducing the intercalant is to move the graphite layers further apart. These new compounds are placed in context with previous reports on superconducting graphite intercalation compounds. This comparison raises the question of whether a simple charge transfer model, utilized to understand earlier studies, is adequate and highlights the need to revisit the theoretical model.

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-delta, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)2O2-delta block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

In charge ordered materials such as the transition metal dichalcogenides, in which the lattice modes are strongly coupled to the charges, the critical fluctuations surrounding a quantum critical point will consist of slow charge modulations as well as soft phonons. The presence of both these types of fluctuations offers an excellent opportunity for novel phases and unconventional forms of supercondictivity to arise. One such material, TiSe 2 , has been found to superconduct upon intercalation of copper [Morosan et al., Nature Phys. 2, 544 (2006)], with a concomitant suppression of CDW order. The transition is driven by an imbalance of charge carriers, leading to conventional superconductivity. Very recently however, Kusmartseva et al. [Phys. Rev. Lett. 103, 236401 (2009)] also discovered superconductivity in the pure system under pressure. This finding cannot be explained solely by the an imbalance of charge carriers and hints at the possibility of exciton-mediated superconductivity made manifest in a clean experimental system. Here we analyze the electronic structure of TiSe 2 using a tight binding model to fit its band structure. The resulting values found for the various overlap integrals in this material strongly suggest that TiSe 2 is best viewed as a system of weakly linked one-dimensional chains. We argue that in these chains excitons and Jahn-Teller phonons will strongly interact, and that it is therefore likely that both the charge density wave transition and the superconductivity which arises upon suppression of the CDW phase are driven by excitonic as well as phononic modes.

High purity samples of the paramagnetic 4f-electron metal CeNi2Ge2 exhibit a non-Fermi-liquid form of the resistivity Δρ~Tx with x

A variety of synchrotron radiation techniques like soft X-ray magnetic circular dichroism, near-edge as well as extended X-ray absorption fine structure, and synchrotron radiation X-ray diffraction have been used to characterise the crystal and electronic structure as well as the magnetic properties of superparamagnetic nanocrystalline samples with the nominal composition γ-Fe2O3. The results are compared with data from bulk reference samples like Fe3O4, Fe3—δO4, α-Fe2O3, and FeO.

A growing number of metals have been found to show systematic deviations from the predictions of Landau Fermi-liquid theory. The most straightforward examples are magnetic metals in which the Curie ( T C ) or Neel ( T N ) temperature has been suppressed to 0 K by the application of hydrostatic pressure. Such systems are discussed here within a spin fluctuation framework which provides quantitatively accurate descriptions of the d-electron systems MnSi and ZrZn 2 near their ‘quantum critical points’, but which appears to be less successful (in its simplest form) in antiferromagnetic f-electron systems, particularly CePd 2 Si 2 , which at its critical pressure has a resistivity of the form Δρ ∝ T 1.2 = 0.1 over a temperature range extending from around 40 K to below 1 K. CeNi 2 Ge 2 , which is believed to be close to a quantum critical point at ambient pressure, shows similar behaviour. CeIn 3 , CePd 2 Si 2 and CeNi 2 Ge 2 exhibit superconductivity, in the first two cases limited to a narrow region near the critical pressure, making these the first unambiguous examples of magnetically mediated superconductivity.

The unconventional nonmagnetic metal CeNi 2 Ge 2 is characterised at ambient pressure by temperature dependences of the specific heat and of the resistivity which deviate strongly from standard Fermi-liquid predictions and are reminiscent of the behaviour observed in its sibling system CePd 2 Si 2 above the critical pressure at which magnetic order is suppressed. We have explored the CePd 2 Si 2 /CeNi 2 Ge 2 phase diagram in a series of resistivity measurements under high hydrostatic pressure, p . At p > 15 kbar, a new superconducting transition appears below 220 mK in CeNi 2 Ge 2 and shifts to higher temperatures with increasing pressure, reaching ≈400 mK at p ≈ 26 kbar.

We present an overview of unconventional phenomena arising close to ferromagnetic and ferroelectric quantum phase transitions. The applicability and potential breakdown of traditional field theories of quantum criticality and the emergence of a multiplicity of critical fields in particular will be discussed.

We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO3. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment; we hence deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials. Published version

We present resistivity measurements on high-purity single crystals of Sr 2 RuO 4 and Sr 3 Ru 2 O 7 under hydrostatic pressure. The Sr 2 RuO 4 samples have an ambient pressure superconducting transition temperature T s ≈1.5 K. From the rate of decrease of the superconducting transition temperature in Sr 2 RuO 4 with pressure, we deduce a critical pressure of the superconducting phase of the order of 80 kbar. The decrease of the T 2 -coefficient of the resistivity with increasing pressure in both materials suggests a magnetic cross-over at negative rather than at positive pressures.

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties., 13 pages, 8 figures, 2 tables

We report on the observation of a superconducting phase in the itinerant electron ferromagnet UGe 2 . The superconductivity observed below 1 K and in a limited pressure range immediately below the critical pressure where ferromagnetism is abruptly suppressed, would seem to arise from the same electrons that produce band magnetism. This superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions and via a spin-triplet rather than a spin-singlet pairing normally associated with nearly anti-ferromagnetic metals.

We report the first Raman scattering measurements of BaC$_6$, SrC$_6$, and YbC$_6$, which along with new data on CaC$_6$, permits a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC$_6$, SrC$_6$, YbC$_6$, and CaC$_6$. This is due to increasing charge in the $\pi^*$ electronic band. Electron-phonon interaction effects between the in-plane carbon modes at $\approx1500$\invcm and the $\pi^*$ electrons cause a strong non-adiabatic renormalization. As charge is transferred into the $\pi^*$ band these non-adiabatic effects are found to increase concurrent with a reduction in the phonon lifetime., Comment: 5 pages, 3 figures, published version

We present the results of a neutron scattering study of the high energy phonons in the superconducting graphite intercalation compound CaC$_6$. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC$_6$, and in particular any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density functional theory (DFT). A description is given of the analysis methods developed to account for the highly-textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC$_6$. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies., Comment: 5 pages, 4 figures, updated version with changes to figure 4 (Accepted in PRB)

M. Otero-Leal,1 F. Rivadulla,2 S. S. Saxena,3 K. Ahilan,3 and J. Rivas1 1Applied Physics Department, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain 2Physical Chemistry Department, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain 3Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom Received 27 August 2009; published 27 October 2009

The location of a tricritical point in the $(p,T)$ phase diagram of MnSi and the nature of the phase transition at both sides of this point are under discussion. While the tricritical point was traditionally placed at ${p}_{\text{tc}}=12\text{ }\text{kbar}$ and ${T}_{\text{tc}}=12\text{ }\text{K}$, with the first-order transition at $pg{p}_{\text{tc}}$, recent studies [Stishov et al., Phys. Rev. B 76, 052405 (2007)] suggested that it should be located at ${p}_{\text{tc}}=3.5\text{ }\text{kbar}$ and ${T}_{\text{tc}}=25\text{ }\text{K}$ with the transition being second order for $pg{p}_{\text{tc}}$. Based on a direct analysis of the magnetic phase transition under pressure, we demonstrate that the coordinates of the tricritical point are indeed ${p}_{\text{tc}}=3.5(3)\text{ }\text{kbar}$ and ${T}_{\text{tc}}=24\text{ }\text{K}$ but with the magnetic phase transition changing from second to first order as pressure increases. We also discuss the effect of spin fluctuations in the transport and magnetic properties of MnSi.

We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $\omega >$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.

Reference LNNME-ARTICLE-2007-006View record in Web of Science URL: http://link.aip.org/link?prb/76/045426 Record created on 2007-08-15, modified on 2016-08-08

Reference LNNME-ARTICLE-2007-008View record in Web of Science URL: http://link.aip.org/link?prb/74/024505 Record created on 2007-08-15, modified on 2016-08-08