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In the metallic magnet ${\mathrm{Nb}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{2+y}$, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess $y$ within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of magnetic order from Fe-rich, ferromagnetic ${\mathrm{Nb}}_{0.981}{\mathrm{Fe}}_{2.019}$ to approximately stoichiometric ${\mathrm{NbFe}}_{2}$, in which we can, for the first time, characterize a long-wavelength spin density wave state burying a ferromagnetic quantum critical point. The associated ordering wave vector ${\mathbf{q}}_{\mathrm{SDW}}=(0,0,{l}_{\mathrm{SDW}})$ is found to depend significantly on $y$ and $T$, staying finite but decreasing as the ferromagnetic state is approached. The phase diagram follows a two-order-parameter Landau theory, for which all of the coefficients can now be determined. Our findings suggest that the emergence of spin density wave order cannot be attributed to band structure effects alone. They indicate a common microscopic origin of both types of magnetic order and provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder.

Determining the origin and consequences of novel phase transitions is a key task in condensed matter physics research. Recently, Lu(Pt1−xPdx)2In was discovered to present a very rare case of strong...

Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0 K. Using inelastic x-ray scattering, we examine the phonon spectrum of the nonmagnetic quasiskutterudite $({\mathrm{Ca}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{)}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration $x$. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at $x=0.85$, the soft mode energy squared at the M point extrapolates to zero at $(\ensuremath{-}5.7\ifmmode\pm\else\textpm\fi{}7.7)$ K, providing the first compelling microscopic evidence of a structural QCP in $({\mathrm{Ca}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{)}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$. The enhanced phonon density of states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP.

We report selected experimental results on the spin dynamics and spin freezing at ferromagnetic quantum phase transitions to illustrate some of the most prominent escape routes by which ferromagnetic quantum criticality is avoided in real materials. In the transition metal Heusler compound Fe2TiSn we observe evidence for incipient ferromagnetic quantum criticality. High pressure studies in MnSi reveal empirical evidence for a topological non-Fermi liquid state without quantum criticality. Single crystals of the hexagonal Laves phase compound Nb1−y Fe2+y provide evidence of a ferromagnetic to spin density wave transition as a function of slight compositional changes. Last but not least, neutron depolarisation imaging in CePd1−x Rh x underscore evidence taken from the bulk properties of the formation of a Kondo cluster glass.

We have carried out a high magnetic field study on single crystalline stoichiometric NbFe 2 , a material discussed in terms quantum criticality in itinerant ferromagnets, by means of high field resistivity experiments. Our experiments have been performed at the Laboratoire National des Champs Magnetiques Intenses in Toulouse, France. The resistivity of single crystalline NbFe 2 , has been investigated in external fields up to 15.5 T aligned along the c-axis in the temperature range of 1.4–55 K. The main focus of our study lies on the method to extract TN from the magnetoresistivity measurements, because TN could not be easily observed in temperature dependent resistivity for stoichiometric NbFe 2 .

One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.

This review gives an overview of the quantum phase transition (QPT) problem in metallic ferromagnets, discussing both experimental and theoretical aspects. These QPTs can be classified with respect to the presence and strength of quenched disorder: Clean systems generically show a discontinuous, or first-order, QPT from the ferromagnetic state to a paramagnetic one as a function of some control parameter, as predicted by theory. Disordered systems are much more complicated, depending on the disorder strength and the distance from the QPT. In many disordered materials the QPT is continuous, or second order, and Griffiths-phase effects coexist with QPT singularities near the transition. In other systems the transition from the ferromagnetic state at low temperatures is to a different type of long-range order, such as an antiferromagnetic or a spin-density-wave state. In still other materials a transition to a state with glass-like spin dynamics is suspected. The review provides a comprehensive discussion of the current understanding of these various transitions, and of the relation between experimental and theoretical developments., Comment: 90 pages, 43 figures, review article

In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

We examine the low temperature states of two transition metal compounds: (i) NbFe 2 is poised on the threshold of ferromagnetism and can be pushed into a spin-aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe 2 has been reported as a rare example of low-temperature spin density wave order in a d-metal system. We have used pressure, field and composition tuning to examine the phase diagram of NbFe 2 . Near the quantum critical point, we find distinct non-Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca 3 Ru 2 O 7 undergoes first a magnetic transition (T N = 56 K) and then a structural transition (T s = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both T N and T S and, for p > 3.5 GPa, induces a third low temperature state, which is robust up to at least 7.5 GPa.

A variety of unconventional pairing mechanisms seem to be operating in heavy-fermion superconductors. We address briefly Cooper pairing mediated by magnetic excitons in UPd 2 Al 3 and by valence fluctuations in pressurized CeCu 2 Si 2 . Moreover, evidence from inelastic neutron scattering for HF superconductivity being mediated by quantum critical spin-density-wave fluctuations in CeCu 2 Si 2 is discussed.

We have investigated single crystals and polycrystals from the series ${\mathrm{Nb}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{2+y}, \ensuremath{-}0.004\ensuremath{\le}y\ensuremath{\le}0.018$, by electron spin resonance, muon spin relaxation, and M\"ossbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Fe excess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric ${\mathrm{NbFe}}_{2}$ display spin density wave order with small magnetic moment amplitudes of order $\ensuremath{\sim}0.001--0.01\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Fe}$. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in ${\mathrm{NbFe}}_{2}$.

The quasiskutterudite superconductor ${\mathrm{Sr}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ features a pronounced anomaly in electrical resistivity at ${T}^{*}\ensuremath{\sim}138\text{ }\text{ }\mathrm{K}$. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of ${T}^{*}$ as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., ${x}_{c}=0.9$). This establishes the ${({\mathrm{Ca}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x})}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.

For the prototypical heavy-fermion (HF) metal CeCu2Si2 superconductivity extends to surprisingly high pressures (PO10 GPa), quite different from other Ce-based HF superconductors, like CePd2Si2, where superconductivity is confined to a narrow P-range centered around an antiferromagnetic quantum critical point (QCP). By deliberately reducing the HF mean free path, i.e. by moderate Ge-substitution for Si, superconductivity could be weakened and two distinct superconducting regimes could be established at low and high P, respectively. The low-P regime is centered at a magnetic QCP. The nature of the magnetic phase could recently be unraveled with the aid of neutron diffractometry and appears to be an incommensurate low-moment spin-density-wave phase. q 2005 Published by Elsevier Ltd.

We present a detailed pressure study of the electrical resistivity ρ(T) and the specific heat C(T) of the non-Fermi-liquid (NFL) compound YbRh2Si2 and of ρ(T) for a single crystal in which 5 at.% of Si is replaced by isoelectronic Ge. The magnetic phase diagram is deduced up to p ~ = 2.5 GPa. A comparison of the effects of the volume change introduced by doping and/or by hydrostatic pressure will be given. We show that the NFL behaviour observed in ρ(T) as well as the magnetic phase diagram are not influenced by the disorder introduced by alloying.

The effect of pressure on the low-temperature states of the newly discovered clathrate Ba6Ge25 is investigated by means of measurements of the electrical resistivity. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230 and 180 K from metallic behaviour to a highresistivity state characterized by a mean free path of about 3 A. Interestingly, a Bardeen–Cooper–Schrieffer-like (BCS-like) superconducting transition occurs at TC ≈ 0.24 K fro mt he resulting ‘bad metal’. With increasing pressure, the structural phase transition is depressed but TC increases drastically. TC reaches am aximum value of 3.85 K at the critical pressure pC ≈ 2. 8G Pa, where the structural distortion is completely suppressed and the system exhibits metallic behaviour. Higher pressures lead to a slight decrease of TC.

We discuss physical properties of two metallic clathrate compounds with low carrier concentration: Eu8Ga16Ge30, the first clathrate known to date, for which the ‘guest’ sites are fully occupied by ‘rattling’ lanthanide ions, and Ba6Ge25. Eu8Ga16Ge30 exists in two modifications which both contain divalent Eu ions and order ferromagnetically at low temperatures (TC=10.5 K and 36 K, respectively). In Ba6Ge25 some of the ‘rattling’ Ba atoms lock, perhaps randomly, into split positions near 200 K. A BCS-like superconducting phase transition takes place at Tc=0.14 K. Tc increases almost 20-fold, as the structural distortion is suppressed under hydrostatic pressure.

We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T_{C}\approx 0.24\,\hbox{K} out of the resulting bad metal ( \rho_{0}\approx 1.5\,\hbox{m}\Omega\;\hbox{cm} ). With increasing pressure, the structural phase transition is shifted to lower temperature but T C increases drastically. T C reaches a maximum value of 3.85 K at the critical pressure p_{C}\approx 2.8\,\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \approx 1.05\,\hbox{K}) than in th...

(Ca0.5Sr0.5)3Rh4Sn13 is a member of the substitution series (CaxSr1–x)3Rh4Sn13 which has recently been argued to feature a structural quantum critical point at xc = 0.9. In the stoichiometric compound Sr3Rh4Sn13, the structural transition at T* ≈ 138 K has been shown to be second-order. Moving towards xc, we examine the character of the structural transition in (Ca0.5Sr0.5)3Rh4Sn13 (i.e. x = 0.5, T* ≈ 55 K) using electrical resistivity, heat capacity and X-ray scattering. The absence of the thermal hysteresis in specific heat around T*, and the continuous evolution of the superlattice reflection detected by X-ray diffraction are consistent with the scenario that the structural transition associated with a modulation vector q = (0.5 0.5 0) in (Ca0.5Sr0.5)3Rh4Sn13 remains second-order on approaching the quantum critical point.

We report the pressure dependence of the superconducting transition temperature ${T}_{c}$ in ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2\ensuremath{-}x}{\mathrm{S}}_{x}$ detected via the ac susceptibility method. The pressure-temperature phase diagram constructed for ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2}$, ${\mathrm{TlNi}}_{2}{\mathrm{S}}_{2}$, and ${\mathrm{TlNi}}_{2}\mathrm{SeS}$ exhibits two unexpected features: (a) a sudden collapse of the superconducting state at moderate pressure for all three compositions and (b) a dome-shaped pressure dependence of ${T}_{c}$ for ${\mathrm{TlNi}}_{2}\mathrm{SeS}$. These results point to the nontrivial role of S substitution and its subtle interplay with applied pressure, as well as interesting superconducting properties of the ${\mathrm{TlNi}}_{2}{\mathrm{Se}}_{2\ensuremath{-}x}{\mathrm{S}}_{x}$ system.

The quasiskutterudite superconductor Sr_{3}Rh_{4}Sn_{13} features a pronounced anomaly in electrical resistivity at T^{*}∼138 K. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of T^{*} as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., x_{c}=0.9). This establishes the (Ca_{x}Sr_{1-x})_{3}Rh_{4}Sn_{13} series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.

In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

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

We report the first observation of non-Fermi-liquid (NFL) effects in a clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of YbRh(2)Si(2) present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase transition at T(N) approximately 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.

The crystallographic and low-temperature properties of the equiatomic Yb-based compounds YbPtIn, YbRhSn (hexagonal ZrNiAl structure), and YbNiGa (orthorhombic $\ensuremath{\epsilon}$-TiNiSi structure) are studied by means of x-ray diffraction, magnetic susceptibility, magnetization, electrical resistivity $(\ensuremath{\rho}),$ magnetoresistance, thermoelectric power, and specific-heat ${(C}_{P})$ measurements. The magnetic susceptibility follows a Curie-Weiss law with effective magnetic moments close to the value of ${\mathrm{Yb}}^{3+}$ and shows strong magnetocrystalline anisotropy. At low temperature the three compounds exhibit the typical properties of antiferromagnetic (AFM) Kondo lattices, namely a logarithmic increase of $\ensuremath{\rho}(T),$ enhanced electronic ${C}_{P}$ coefficients ${\ensuremath{\gamma}}_{o},$ and a reduced entropy gain below the ordering temperature. The values of ${\ensuremath{\gamma}}_{o}g350 {\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ are among the largest of known Yb-based magnetic Kondo lattices. Two AFM transitions in each compound evidence a competition between single-ion anisotropy and frustration of the magnetic interactions due to the topology of the underlying crystal structure. The ground-state properties are discussed and compared with those of other f-electron compounds showing similar characteristics concerning magnetic frustration.

The d-electron low-temperature magnet NbFe 2 can be tuned to the threshold of magnetism by varying pressure, magnetic field or composition. In particular, it is possible to select between ferromagnetism, long-wavelength spin-density-wave (SDW) (helical) order or a paramagnetic low-temperature state by adjusting the precise stoichiometry within the Nb 1 - y Fe 2 + y homogeneity range. Preparing samples close to the composition where magnetic order vanishes—the quantum critical point—enables the first detailed investigation of the thermodynamic and transport signatures of Fermi liquid breakdown in a transition metal compound. We report measurements of the heat capacity C and of the resistivity ρ in stoichiometric and slightly Nb-rich NbFe 2 samples, including a single crystal with the composition Nb 1.02 Fe 1.98 , which on the phase diagram is located very close to the quantum critical point ( T N ≃ 2.8 K ) . Both the resistivity and the heat capacity of the nearly quantum-critical single crystal display striking, robust non-Fermi liquid temperature dependences: while the heat capacity coefficient γ = C / T diverges weakly as C / T ∼ log T from 4 K down to 0.1 K, in line with theoretical predictions for 3-D ferromagnetic quantum criticality, the resistivity follows a T 3 / 2 power-law, familiar from the case of MnSi and naively predicted for the proximity of an antiferromagnetic quantum critical point.

In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

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.

Close to the border of antiferromagnetic order at high pressure the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures. The superconducting transition temperature varies rapidly with pressure near the critical pressure where the Neel temperature collapses towards absolute zero.

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 compare the cross-over from a magnetically ordered to a non-magnetic spin liquid state as a function of hydrostatic pressure in the related heavy-fermion antiferromagnets CePd2Si2 and CeRh2Si2. Both materials exhibit superconductivity at pressures close to the critical pressure pc at which their magnetic ordering temperatures extrapolate to zero. However, in CePd2Si2 the normal state resistivity varies according to ϱ ∼ Tx (1.1

The Hall effect on selected samples of the dilution series Nb1-yFe2+y is studied. Normal and anomalous contributions are observed, with positive normal Hall effect dominating at high temperatures. Consistent analysis of the anomalous contribution is only possible for Fe-rich Nb0.985Fe2.015 featuring a ferromagnetic ground state. Here, a positive normal Hall coefficient is found at all temperatures with a moderate maximum at the spin-density-wave transition. The anomalous Hall effect is consistent with an intrinsic (Berry-phase) contribution which is constant below the ordering temperature TC and continuously vanishes above TC. For stoichiometric NbFe2 and Nb-rich Nb1.01Fe1.99 - both having a spin-density-wave ground state - an additional contribution to the Hall resistivity impedes a complete analysis and indicates the need for more sophisticated models of the anomalous Hall effect in itinerant antiferromagnets.

The normal states of magnetic metals with vanishing Curie or Neel temperatures are investigated by means of measurements of the temperature and pressure dependence of the resistivity in the stoichiometric d and f compounds and . The results for the nearly ferromagnetic d metals may be described over a wide range in temperature and pressure in terms of a quantitative model of a marginal Fermi liquid based on dispersive spin-fluctuation spectra inferred from inelastic neutron scattering data. The behaviour of the antiferromagnetic f metals is also unconventional, but in a way which cannot yet be readily categorized. Near the critical pressure where , displays a resistivity of the form over nearly two decades in temperature before condensing into a short-coherence-length superconducting state below 0.4 K. The isoelectronic and isostructural compound , with a lattice cell volume slightly smaller than that of , remains normal down to 20 mK and shows a resistivity of the form over a decade below several kelvin at ambient pressure. These findings for the Ce systems have not yet been described consistently in terms of an extension of the model developed for the ferromagnetic d metals.

We report a study of the spin moment in single crystal Nb(1−y)Fe(2+y) with y=0.015 using spin-dependent Compton scattering in conjunction with ab initio electronic structure calculations. From the experiment, we find that the total spin moment is 0.245±0.004μB. Comparison of the measured spin density with theoretical results from linearized muffin-tin orbital calculations determines there to be a ferrimagnetic arrangement of Fe moments, with the 2a Fe site aligned antiparallel to the bulk moment. There is a small moment on the Nb site, also antiparallel to the net moment. The orbital moment has been determined to be 0.02±0.02μB.

Measurements of low temperature transport and thermodynamic properties have been used to characterize the non-Fermi liquid state of the itinerant ferromagnet ZrZn$_2$. We observe a $T^{5/3}$ temperature dependence of the electrical resistivity at zero field, which becomes $T^2$ like in an applied field of 9 T. In zero field we also measured the thermal conductivity, and we see a novel linear in $T$ dependence of the difference between the thermal and electrical resistivities. Heat capacity measurements, also at zero field, reveal an upturn in the electronic contribution at low temperatures when the phonon term is subtracted. Taken together, we argue that these properties are consistent with a marginal Fermi liquid state which is predicted by a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions. We compare our data to quantitative predictions and establish this model as a compelling theoretical framework for understanding ZrZn$_2$., 10 pages, 10 figures

In order to examine the nature of the superconducting order parameter in UBe13, we measured the specific heat, C, as a function of temperature, T, under uniaxial stress along 〈1 0 0〉 axis, for both “L”- (T c ≈0.75 K ) and “H”-type (0.85⩽T c ⩽0.95 K ) crystals. With increasing pressure, a linear decrease of the superconducting transition temperature, Tc, was observed ( ∂ T c / ∂ p≃8.5 mK/GPa for “L”-type and ∂ T c / ∂ p≃50 mK/GPa for “H”-type) but no split of the transition could be resolved unambiguously for either of the two samples within our measurement resolution.

We present Shubnikov-de Haas measurements on LuRh2Si2, the non-magnetic reference compound to the prototypical heavy-fermion system YbRh2Si2. We find an extensive set of orbits with clear angular dependences. Surprisingly, the agreement with non-correlated band structure calculations is limited. This may be related to an uncertainty in the calculations arising from a lack of knowledge about the exact Si atom position in the unit cell. The data on LuRh2Si2 provide an extensive basis for the interpretation of measurements on YbRh2Si2 indicative of discrepancies between the high-field Fermi surface of YbRh2Si2 and the "small" Fermi surface configuration., Comment: 5 pages

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 have performed several high-pressure electrical resistance experiments on ${\text{Bi}}_{1.98}{\text{Sr}}_{2.06}{\text{Y}}_{0.68}{\text{Cu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ (Bi2212), an insulating parent compound of the high-${T}_{c}$ Bi2212 family of copper oxide superconductors. We find a resistive anomaly, a downturn at low temperature, that onsets with applied pressure in the 20--40 kbar range. Through both resistance and magnetoresistance measurements, we identify this anomaly as a signature of induced superconductivity. Resistance to higher pressures decreases ${T}_{c}$, giving a maximum of $\ensuremath{\sim}10\text{ }\text{K}$. The higher pressure measurements exhibit a strong sensitivity to the hydrostaticity of the pressure environment. We make comparisons to the pressure-induced superconductivity now ubiquitous in the iron arsenides.

Using density functional theory we investigate the evolution of the magnetic ground state of NbFe$_{2}$ due to doping by Nb-excess and Fe-excess. We find that non-rigid-band effects, due to the contribution of Fe-\textit{d} states to the density of states at the Fermi level are crucial to the evolution of the magnetic phase diagram. Furthermore, the influence of disorder is important to the development of ferromagnetism upon Nb doping. These findings give a framework in which to understand the evolution of the magnetic ground state in the temperature-doping phase diagram. We investigate the magnetic instabilities in NbFe$_{2}$. We find that explicit calculation of the Lindhard function, $\chi_{0}(\mathbf{q})$, indicates that the primary instability is to finite $\mathbf{q}$ antiferromagnetism driven by Fermi surface nesting. Total energy calculations indicate that $\mathbf{q}=0$ antiferromagnetism is the ground state. We discuss the influence of competing $\mathbf{q}=0$ and finite $\mathbf{q}$ instabilities on the presence of the non-Fermi liquid behavior in this material., Comment: 8 pages, 7 figures

We present a systematic study of transport and thermodynamic properties of the Laves phase system ${\text{Nb}}_{1\ensuremath{-}y}{\text{Fe}}_{2+y}$. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for $|y|\ensuremath{\ge}0.02$), show bulk ferromagnetism at low temperature. For stoichiometric ${\text{NbFe}}_{2}$, on the other hand, magnetization, magnetic susceptibility, and magnetoresistance results point toward spin-density wave (SDW) order, possibly helical, with a small ordering wave vector $Q\ensuremath{\sim}0.05\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$. Our results suggest that on approaching the stoichiometric composition from the iron-rich side, ferromagnetism changes into long-wavelength SDW order. In this scenario, $Q$ changes continuously from 0 to small, finite values at a Lifshitz point in the phase diagram, which is located near $y=+0.02$. Further reducing the Fe content suppresses the SDW transition temperature, which extrapolates to zero at $y\ensuremath{\approx}\ensuremath{-}0.015$. Around this Fe content magnetic fluctuations dominate the temperature dependence of the resistivity and of the heat capacity which deviate from their conventional Fermi-liquid forms, inferring the presence of a quantum critical point. Because the critical point is located between the SDW phase associated with stoichiometric ${\text{NbFe}}_{2}$ and the ferromagnetic order which reemerges for very Nb-rich ${\text{NbFe}}_{2}$, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.

We investigate the evolution of the electrical resistivity of BaFe2As2 single crystals with pressure. The samples used were from the same batch grown from self flux and showed properties that were highly reproducible. Samples were pressurised using three different pressure media: pentane-isopentane (in a piston cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a Bridgman cell). Each pressure medium has its own intrinsic level of hydrostaticity, which dramatically affects the phase diagram. An increasing uniaxial pressure component in this system quickly reduces spin density wave order and favours the appearance of superconductivity, similar to what is seen in SrFe2As2., Comment: 11 pages

The $d$-electron low temperature magnet ${\mathrm{NbFe}}_{2}$ is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the ${\mathrm{Nb}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{2+y}$ homogeneity range. In a nearly critical single crystal ($y=\ensuremath{-}0.01$), we observe a ${T}^{3/2}$ power-law dependence of the resistivity $\ensuremath{\rho}$ on temperature $T$ and a logarithmic temperature dependence of the Sommerfeld coefficient $\ensuremath{\gamma}=C/T$ of the specific heat capacity $C$ over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.

In the archetypal strongly correlated electron superconductor CeCu2Si2 and its Ge-substituted alloys CeCu2(Si1-xGex)2 two quantum phase transitions--one magnetic and one of so far unknown origin-can be crossed as a function of pressure. We examine the associated anomalous normal state by detailed measurements of the low temperature resistivity (rho) power-law exponent alpha. At the lower critical point (at pcl, 1

In the archetypal strongly correlated electron superconductor CeCu2Si2 and its Ge-substituted alloys CeCu2(Si1-xGex)2 two quantum phase transitions--one magnetic and one of so far unknown origin-can be crossed as a function of pressure. We examine the associated anomalous normal state by detailed measurements of the low temperature resistivity (rho) power-law exponent alpha. At the lower critical point (at pcl, 1or=alphaor=1.5) alpha depends strongly on Ge concentration x and thereby on disorder level, consistent with a Hlubina-Rice-Rosch scenario of critical scattering off antiferromagnetic fluctuations. By contrast, alpha is independent of x at the upper quantum phase transition (at pc2, alpha approximately equal to 1), suggesting critical scattering from local or q=0 modes, in agreement with a density- or valence-fluctuation approach.

Electrons in metals can self-organise. The complex interplay between lattice dynamics, electrostatic interaction and band structure brings forth numerous types of electronic order. Because of its spectacular phenomenology, superconductivity has enjoyed a central place among these, since its discovery nearly 100 years ago. This short introduction into one of the largest fields of condensed matter research focuses on the most fundamental experimental signatures of superconductivity – perfect conductivity and perfect diamagnetism – and their explanation. A conventional broken symmetry argument is presented, which introduces a superconducting order parameter in analogy to the case of superfluid 4He, and discusses its microscopic origin in the framework of the BCS model of superconductivity. New materials have brought to light novel forms of superconductivity. Many cases are now known which fall outside the orthodox BCS model, ranging from the high temperature superconductors, to various organic and d- and f- metal compounds. The article presents key concepts from this intense area of research and touches on the equally puzzling behaviour of many of these materials above their superconducting transition temperature.

The cross-over from a magneticallyordered to a non-magnetic spin liquid state has been investigated in a series of resistance measurements under hydrostatic pressures of up to 30 kbar and at temperatures down to below 200 mK in the heavy fermion antiferromagnet CePd2Si2. The electrical resistivity changes dramatically with increasing pressure. Near the critical pressure, at which the magnetic ordering temperature is extrapolated to zero, it exhibits a quasi-linear variation over two orders of magnitude in temperature. This non-Fermi liquid formof π(T) extends down to the onset of a new superconducting transition below 430 mK.