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Magnetic fields are now available to 100 T (pulsed), 45 T (static) at temperatures below 0.3 K. Such technical developments allow the study and tuning of (quantum) phase transitions, unusual magnetic structures and (high-temperature) superconductors in a variety of quantum materials. An especially important class of strongly correlated electron materials is the heavy Fermi liquids (HFLs) displaying numerous reduced-moment antiferromagnets, quantum critical points, unconventional superconductivity, hidden order (HO) and other mysterious ground states. Among the ‘heavy fermions’, the duality of 5f electrons in uranium-based compounds introduces interesting behavior that can be affected by large magnetic fields. I list a few such heavy fermion materials to be considered: URu2Si2 and its tunable hidden state, UBe13 and UPt3 as very HFL paramagnets that become superconducting, the magnetic superconductors UPd2Al3 and UNi2Al3, and the ferromagnetic s UGe2, URhGe and UCoGe. There are also the suggested metamagne...

The unusual correlated state that emerges in URu$_2$Si$_2$ below T$_{HO}$ = 17.5 K is known as "hidden order" because even basic characteristics of the order parameter, such as its dimensionality (whether it has one component or two), are "hidden". We use resonant ultrasound spectroscopy to measure the symmetry-resolved elastic anomalies across T$_{HO}$. We observe no anomalies in the shear elastic moduli, providing strong thermodynamic evidence for a one-component order parameter. We develop a machine learning framework that reaches this conclusion directly from the raw data, even in a crystal that is too small for traditional resonant ultrasound. Our result rules out a broad class of theories of hidden order based on two-component order parameters, and constrains the nature of the fluctuations from which unconventional superconductivity emerges at lower temperature. Our machine learning framework is a powerful new tool for classifying the ubiquitous competing orders in correlated electron systems.

This topical review describes the multitude of unconventional behaviors in the hidden order, heavy fermion, antiferromagnetic and superconducting phases of the intermetallic compound URu

Resonant ultrasound spectroscopy and machine learning are used to rule out many theories of hidden order in URu2Si2., The unusual correlated state that emerges in URu2Si2 below THO = 17.5 K is known as “hidden order” because even basic characteristics of the order parameter, such as its dimensionality (whether it has one component or two), are “hidden.” We use resonant ultrasound spectroscopy to measure the symmetry-resolved elastic anomalies across THO. We observe no anomalies in the shear elastic moduli, providing strong thermodynamic evidence for a one-component order parameter. We develop a machine learning framework that reaches this conclusion directly from the raw data, even in a crystal that is too small for traditional resonant ultrasound. Our result rules out a broad class of theories of hidden order based on two-component order parameters, and constrains the nature of the fluctuations from which unconventional superconductivity emerges at lower temperature. Our machine learning framework is a powerful new tool for classifying the ubiquitous competing orders in correlated electron systems.

Using single crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wavevector, Qmod, is intriguingly similar to the Fermi surface nesting wavevector of URu2Si2. Qmod shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at ~ (0.42 0 0) near 180 K. Bulk measurements indicate a cross-over toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at TN = 35 K., 6 pages, 4 figures

We report the high-field-induced magnetic phases and phase diagram of a high quality $\mathrm{U}{({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08})}_{2}{\mathrm{Si}}_{2}$ single crystal prepared using a modified Czochralski method. Our paper, that combines high-field magnetization and electrical resistivity measurements, shows for fields applied along the $c$-axis direction three field-induced magnetic phase transitions at ${\ensuremath{\mu}}_{0}{H}_{c1}=21.60,\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{0}{H}_{c2}=37.90$, and ${\ensuremath{\mu}}_{0}{H}_{c3}=38.25\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, respectively. In agreement with a microscopic up-up-down arrangement of the U magnetic moments the phase above ${H}_{c1}$ has a magnetization of about one-third of the saturated value. In contrast the phase between ${H}_{c2}$ and ${H}_{c3}$ has a magnetization that is a factor of 2 lower than above the ${H}_{c3}$ where a polarized Fermi-liquid state with a saturated moment ${M}_{s}\ensuremath{\approx}2.1{\ensuremath{\mu}}_{B}/\mathrm{U}$ is realized. Most of the respective transitions are reflected in the electrical resistivity as sudden drastic changes. Most notably, the phase between ${H}_{c1}$ and ${H}_{c2}$ exhibits substantially larger values. As the temperature increases, transitions smear out and disappear above $\ensuremath{\approx}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. However, a substantial magnetoresistance is observed even at temperatures as high as 80 K. Due to a strong uniaxial magnetocrystalline anisotropy, a very small field effect is observed for fields applied perpendicular to the $c$-axis direction.

The duality between localized and itinerant nature of magnetism in $5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single crystalline system of UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV, and surprising absence of coherent spin-waves, suggestive of itinerant magnetism. While the gap closes above the Neel temperature, strong dynamic spin correlations persist to high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by local moment with $J=4$. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$ systems in general., Comment: see supplementary material for more details

We report a high-field-induced magnetic phase in a single crystal of $\mathrm{U}{({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08})}_{2}{\mathrm{Si}}_{2}$. Our neutron study, combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at ${\ensuremath{\mu}}_{0}{H}_{c1}=21.6$ T has an uncompensated commensurate antiferromagnetic structure with a propagation vector ${\mathbit{Q}}_{2}=\left(\frac{2}{3}00\right)$ possessing two single-$\mathbit{Q}$ domains. U moments of $1.45(9){\ensuremath{\mu}}_{B}$ directed along the $c$ axis are arranged in an up-up-down sequence propagating along the $a$ axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the ${\mathrm{U}}^{3+}$ and ${\mathrm{U}}^{4+}$ types. The low-field short-range order that emerges from pure ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ due to Rh doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition, but the $a$-axis lattice parameter increases already at low fields. Our results are in agreement with an itinerant electron model with $5f$ states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field.

We have measured the Hall effect, magnetotransport, and magnetostriction on the field-induced phases of single-crystalline ${\mathrm{UPt}}_{2}{\mathrm{Si}}_{2}$ in magnetic fields up to 60 T at temperatures down to 50 mK, firmly establishing the phase diagram for magnetic fields $B\ensuremath{\parallel}a$ and $c$ axes. Moreover, for the $B\ensuremath{\parallel}c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium $5f$ electrons, we propose that these represent field-induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the proposed Lifshitz-type transition is of a discontinuous nature, as predicted for interacting electron systems.

This Topical Review describes the multitude of unconventional behaviors in the hidden order, heavy fermion, antiferromagnetic and superconducting phases of the intermetallic compound URu$_2$Si$_2$ when tuned with pressure, magnetic field, and substitutions for all three elements. Such `perturbations' result in a variety of new phases beyond the mysterious hidden order that are only now being slowly understood through a series of state-of-the-science experimentation, along with an array of novel theoretical approaches. Despite all these efforts spanning more than 30 years, hidden order (HO) remains puzzling and non-clarified, and the search continues in 2019 into a fourth decade for its final resolution. Here we attempt to update the present situation of URu$_2$Si$_2$ importing the latest experimental results and theoretical proposals. First, let us consider the pristine compound as a function of temperature and report the recent measurements and models relating to its heavy Fermi liquid crossover, its HO and superconductivity (SC). Recent experiments and theories are surmized that address four-fold symmetry breaking (or nematicity), Isingness and unconventional excitation modes. Second, we review the pressure dependence of URu$_2$Si$_2$ and its transformation to antiferromagnetic long-range order. Next we confront the dramatic high magnetic-field phases requiring fields above 40 T. And finally, we attempt to answer how does random substitutions of other $5f$ elements for U, and $3d$, $4d$, and $5d$ elements for Ru, and even P for Si affect and transform the HO. Commensurately, recent theoretical models are summarized and then related to the intriguing experimental behavior., 75 pages, 27 figures. J. Phys.: Condensed Matter, in press

Throughout the past three decades the hidden order (HO) problem in URu$_2$Si$_2$ has remained a "hot topic" in the physics of strongly correlated electron systems with well over 600 publications related to this subject. Presently in 2014 there has been significant progress in combining various experimental results embedded within electronic structure calculations using density functional theory (DFT) to give a consistent description of the itinerant behaviour of the HO transition and its low temperature state. Here we review six different experiments: ARPES, quantum oscillations, neutron scattering, RXD, optical spectroscopy and STM/STS. We then establish the consistencies among these experiments when viewed through the Fermi-surface nesting, folding and gapping framework as predicted by DFT. We also discuss a group of other experiments (torque, cyclotron resonance, NMR and XRD) that are more controversial and are presently in a "transition" state regarding their interpretation as rotational symmetry breaking and dotriacontapole formation. There are also a series of recent "exotic" experiments (Raman scattering, polar Kerr effect and ultrasonics) that require verification, yet they offer new insights into the HO symmetry breaking and order parameter. We conclude with some constraining comments on the microscopic models that rely on localised $5f$-U states and strong Ising anisotropy {for explaining} the HO transition, and with an examination of different models in the light of recent experiments., 21 pages, 12 figures; to appear in Phil. Mag

We report the crystal structure and highly anisotropic magnetic, transport, and thermal properties of an exceptionally good single crystal of $\mathrm{U}{({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08})}_{2}{\mathrm{Si}}_{2}$, prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$; however, the superconductivity, hidden order, and remnant weak antiferromagnetic orders are absent. Instead, the ground state of the doped system can be classified as a spin liquid that preserves the heavy-fermion character. $\mathrm{U}{({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08})}_{2}{\mathrm{Si}}_{2}$ exhibits a short-range magnetic order distinguished by reflections of a Lorentzian profile at ${\mathrm{q}}_{\mathit{III}}=(\frac{1}{2}\phantom{\rule{4pt}{0ex}}\frac{1}{2}\phantom{\rule{4pt}{0ex}}\frac{1}{2})$ positions that disappear above $\ensuremath{\sim}15$ K. The short-range order seems to be a precursor of a long-range magnetic order that occurs with higher Rh concentration. We indicate that these short-range fluctuations involve, at least partially, inelastic scattering processes.

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ of $ac\text{\ensuremath{-}}$ and $ab\text{-plane}$ surfaces. Our data demonstrate the itinerant character of the narrow $5f$ bands, becoming progressively coherent as the temperature is lowered below a crossover temperature ${T}^{*}\ensuremath{\sim}75$ K. ${T}^{*}$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy-fermion plasmons with $a\text{\ensuremath{-}}$ and $c\text{-axis}$ polarization. Our observation of a suppression of optical conductivity below 50 meV along both the $a$ and $c$ axes, along with a heavy-fermion plasmon at 18 meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature ${T}_{\mathrm{HO}}=17.5$ K. In the hidden order phase the low-frequency optical conductivity shows a single gap at $\ensuremath{\sim}6.5$ meV, which closes at ${T}_{\mathrm{HO}}$.

In URu$_2$Si$_2$ two types of staggered phases involving long range ordering of the uranium-5f electrons compete at low temperature when a critical parameter $x$ is tuned, where $x$ can be chemical substituent concentration, pressure or magnetic field. When cooled at below the critical value $x_c$, the non-magnetic `hidden order' (HO) phase with broken local chiral symmetry emerges, whereas above $x_c$, unconventional antiferromagnetic (AF) phase with broken local time-reversal symmetry appears. The two phases show strikingly similar electronic properties. `Janus faces' nature of the HO and AF phases has been theorized before, but the experimental signatures of the interplay between them are still lacking. Here, we use polarized Raman scattering to study the dynamical fluctuations between the two competing ground states as a function of $x$. Albeit the distinct discrete symmetries are broken above and below $x_c$, we detect a resonance continuously evolving with parameter $x$, providing evidence for a unified order parameter across the URu$_2$Si$_2$ phase diagram.

We study URu_{2-x}Fe_{x}Si_{2}, in which two types of staggered phases compete at low temperature as the iron concentration x is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration x_{c}, and unconventional antiferromagnetic (AFM) phase above x_{c}. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovectorlike A_{2g} symmetry whose energy continuously evolves with increasing x; it monotonically decreases in the HO phase until it vanishes at x=x_{c}, and then reappears with increasing energy in the AFM phase. The mode's evolution provides direct evidence for a unified order parameter for both nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of the uranium-5f electrons.

The ternary stannide CeRuSn is a static mixed-valent cerium compound with an ordering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4:2 K) two tin sites can clearly be distinguished. Solid-state 119Sn NMR experiments were performed to probe the local hyperfine fields at the two different Sn sites. Powder 119Sn NMR spectra are nicely fitted with two Sn sites with nearly the same magnetic anisotropy, but with different absolute shift values. Both Sn sites are strongly affected by crossover-like transitions between 100 and 280 K. This local-site study confirms the superstructure modulations found in previous investigations. Towards lower temperatures the powder NMR spectra are broadened giving strong precursor evidence for the antiferromagnetically ordered ground state.

This Colloquium reviews the 25 year quest to understand the continuous (second-order), mean-field-like phase transition occurring at 17.5 K in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$. About ten years ago, the term ``hidden order'' (HO) was coined and has since been utilized to describe the unknown ordered state, whose origin cannot be disclosed by conventional solid-state probes, such as x rays, neutrons, or muons. The HO is able to support superconductivity at lower temperatures (${T}_{c}\ensuremath{\approx}1.5\text{ }\text{ }\mathrm{K}$), and when magnetism is developed with increasing pressure both the HO and the superconductivity are destroyed. Other ways of probing the HO are via Rh doping and large magnetic fields. During the last few years a variety of advanced techniques have been tested to probe the HO state and these attempts will be summarized. A digest of recent theoretical developments is also included. It is the objective of this Colloquium to shed additional light on the HO state and its associated phases in other materials.

We present a detailed comparison of the physical properties of as-cast and annealed single crystalline UPt2Si2, a compound whose properties we have shown to be governed by strain disorder on the Pt/Si ligand sites. Contrary to common knowledge, and to our surprise, from our data we do not observe a significant improvement of the physical properties of UPt2Si2 upon annealing at 900 °C for one week. We attribute this to the specific way the strain disorder is produced in UPt2Si2 by presenting evidence that it results from a first order phase transition at ambient temperatures. We discuss the implications of such phase transitions occurring at comparatively low temperatures for the ground state properties of heavy fermion systems and related correlated electron materials.

Heavy electronic states originating from the f atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope to examine the novel electronic states that emerge from the uranium f states in URu 2 Si 2 . We find that, as the temperature is lowered, partial screening of the f electrons’ spins gives rise to a spatially modulated Kondo–Fano resonance that is maximal between the surface U atoms. At T = 17.5 K, URu 2 Si 2 is known to undergo a second-order phase transition from the Kondo lattice state into a phase with a hidden order parameter. From tunneling spectroscopy, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that develops in the hidden order state. Spectroscopic imaging further reveals a spatial correlation between the hidden order gap and the Kondo resonance, suggesting that the two phenomena involve the same electronic states.

Following the border of antiferromagnetism (AF) to zero temperature is a promising route to unconventional metallic and superconducting phases. Many interesting examples of antiferromagnetic quantum phase transitions can only be reached by pressure tuning. The range of quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides now increase the beam intensity directed to mm size sample for high pressure studies. This has been demonstrated on the simple antiferromagnet NiS 2 . Neutron scattering also allows highly accurate measurements of the lattice constant via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi. We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure–temperature phase diagram of URu 2 Si 2 up to 20 kbar. URu 2 Si 2 offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the observation of magnetic order and changes in the a - and c -axis lattice constants across the phase transitions in the same experiment. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice.

We have studied the unusual low-temperature magnetic phase of Er2Ni2Pb using powder neutron diffraction measurements in zero field down to 460 mK. Our previous neutron diffraction experiments down to 1.5 K showed that magnetic Bragg reflections seen in Er2Ni2Pb can be indexed by several propagation vectors that partially coexist. All the incommensurate propagation vectors seemed to disappear in the low temperature limit. The present study, however, shows that reflections belonging to the propagation vector q’ = (0.47 0 1/2) do not disappear but remain present down to 460 mK. This highly unexpected result suggests that the magnetic structure described by this propagation vector might not be a simple sine-wave modulation. One interesting possibility here is a spin-slip structure as the ground state.

We investigate the Hall coefficient R H , resistivity, and magnetization of URu 2 Si 2 and U ( Ru 0.96 Rh 0.04 ) 2 Si 2 in the high field regime up to 45 T to trace the electronic structure changes in both high and low temperature regimes. We find that at high temperatures above ∽ 7 K , temperature- and field-dependent positions of R H maxima and magnetoresistance maxima coincide with a coherent temperature T coh at each field. These systematically decrease in temperatures with increasing fields and extrapolate to zero at the field-induced putative quantum critical point (QCP). This observation suggests a collapse of a new energy scale, i.e., a coherent temperature of a heavy quasiparticle band, at the putative field-induced QCP. At lower temperatures, we show there are discontinuous changes of the Hall number n H = 1 / R H e at the phase boundaries. This behavior illustrates that the phase formation is caused by first order reconstructions of the Fermi surface, which we interpret to the polarization of Fermi surface pockets seen in de-Haas van-Alphen (dHvA) measurements.

Rare earth compounds of the type R 5 T 4 M 10 ( T = Rh , Ir and M = Si , Ge, Sn) display a variety of phase transitions ranging from charge density wave (CDW), local moment magnetism, antiferromagnetism in the heavy fermion state, superconductivity and giant positive magnetoresistance. In particular, R 5 Ir 4 Si 10 ( R = Dy – Lu ) compounds exhibit strong coupling CDW at high temperatures and superconductivity or magnetic ordering at low temperatures, while R 5 Rh 4 Ge 10 ( R = Gd – Tm ) show multiple magnetic transitions with large magnetoresistance below the magnetic transitions. Finally, the light rare earth series R 5 T 4 Sn 10 ( R = Ce , Pr , Nd , T = Rh , Ir ) display heavy fermion behavior (for Ce and Pr) and giant positive magnetoresistance (for Nd) at low temperatures.

We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at T-0 = 17.5K in URu2Si2. While confirming previous results for the HO and PM phases, our data reveal a pronounced wave-vector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter 1/Gamma which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at (Q) over right arrow (1) approximate to (1.4,0,0) show an abrupt steplike suppression of 1/Gamma below T-0, whereas excitations at (Q) over right arrow (0) = (1,0,0), associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of 1/Gamma at T-0. Therefore, at the critical HO temperature T-0, LMAF fluctuations become nearly critical as well. This is the behavior expected of a "supervector" order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.

This article reviews the 40+ year old spin-glass field and one of its earliest model interpretations as a spin density wave. Our description is from an experimental phenomenological point of view with emphasis on new spin glass materials and their relation to topical problems and strongly correlated materials in condensed matter physics. We first simply define a spin glass (SG), give its basic ingredients and explain how the spin glasses enter into the statistical mechanics of classical phase transitions. We then consider the four basic experimental properties to solidly characterize canonical spin glass behavior and introduce the early theories and models. Here the spin density wave (SDW) concept is used to explain the difference between a short-range SDW, i.e. a SG and, in contrast, a long-range SDW, i.e. a conventional magnetic phase transition. We continue with the present state of SG, its massive computer simulations and recent proposals of chiral glasses and quantum SG. We then collect and mention the various SG ‘spin-off’s'. A major section uncovers the fashionable unconventional materials that display SG-like freezing and glassy ground states, such as (high temperature) superconductors, heavy fermions, intermetallics and Heuslers, pyrochlor and spinels, oxides and chalogenides and exotics, e.g. quasicrystals. Some conclusions and future directions complete the review.

The isomorphic collapse of crystalline lattices under pressure is a rare and intriguing phenomenon–the most famous examples being samarium sulphide and cerium metal. Both lattices are cubic under ambient conditions and collapse isomorphically under pressure, remaining cubic with ∼15% volume reduction1,2,3. In SmS the transition results from a change of the 4f chemical valence. The collapse in Ce is connected with the altering contributions of the 4f electrons to the chemical bonding, the details of which are currently much debated4,5. In contrast, YCo5 is a hexagonal metallic compound with a stable valence and no 4f electrons. Here, we present a combination of high-pressure X-ray diffraction measurements and density functional electronic-structure calculations to demonstrate an entirely different type of isomorphic transition under hydrostatic pressure of 19 GPa. Our results suggest that the lattice collapse is driven by magnetic interactions and can be characterized as a first-order Lifshitz transition, where the Fermi surface changes topologically. These studies support the existence of a bistable bonding state due to the magneto-elastic interaction.

This paper presents a brief summary of recent electrical transport, magnetization and specific heat measurements on U ( Ru 1 - x Rh x ) 2 Si 2 ( x = 0 , 2 , 2.5 , 3 , and 4%) in magnetic fields up to 45 T. The results uncover the formation of multiple phases near their putative metamagnetic quantum critical points at low temperatures. From the established magnetic field versus temperature phase diagram in our series of samples, we find a clear correlation between the field-dependence of the phases and that of the putative quantum critical point. This provides an archetypal example that the field-tuned metamagnetic quantum criticality is intimately related to the formation of thermodynamic phases.

Nuclear resonance investigations on the 23 Na and the 139 La nuclei in filled skutterudites MFe 4 Sb 12 (M=Na, La) are reported. In contrast to NaFe 4 Sb 12 for the La compound the effective magnetic moment is remarkably lower and the Curie–Weiss temperature is negative which indicates antiferromagnetic correlations. The spin-lattice relaxation rates 1/ T 1 exhibit a qualitatively different temperature behavior which provides clear evidence for dissimilar types of itinerant magnetism in the two compounds. For NaFe 4 Sb 12 1/ T 1 vs. T indicates itinerant ferromagnetism with ordering at T c = 8 5 K , whereas for the La 0.9 Fe 4 Sb 12 weak itinerant antiferromagnetism with no ordering is found. 1/ T 1 is analyzed in terms of Moriya's self-consistent renormalization theory (SCR) for itinerant magnetic materials.

We have studied the magnetic structure of the orthorhombic compound Ho2Ni2Pb by means of neutron diffraction in zero field and in magnetic fields up to 4.5 T. Both powder and single-crystalline samples were used. Previous bulk measurements suggest two distinct magnetic phase transitions: one at TN = 7.0 K and the other at 4.8 K. Our neutron diffraction measurements, which were made in the range 1.5-20 K, showed that Ho2Ni2Pb has a collinear magnetic structure with unequal number of up and down Ho moments that are aligned parallel and antiparallel to the c axis. At the lowest temperatures the Ho moments are equal in size, each 8.3 μB in agreement with magnetization data. The magnetic structure can be described as having a 5a ×b ×c magnetic unit cell. Below Ts = 3.0 K the structure is squared up. A smooth development of all the magnetic moment magnitudes indicates that the magnetic structure remains in principle the same over the whole temperature range, the “phase transition” around 4.8 K can be identified as an inflection point in the temperature dependence of one of the Ho moments. With increasing temperature there is a clear development towards a simple transverse sine-wave modulated magnetic structure that is established just below TN.

We studied the magnetic and transport properties of the orthorhombic compounds R 2 Ni 2 Pb (R = Gd, Tb and Y) by means of magnetization and magnetoresistivity measurements. These compounds form a new family of naturally layered intermetallics with a magnetic structure that can be tailored by changing the rare-earth element. Our results show a subtle interdependence between the magnetoresistance and the metamagnetic behavior in these systems. We attribute such anomalous behavior to the layered structure of the compounds, and to the intricacies introduced by the complex magnetic structure of rare earths. First-principles density-functional calculations for the electronic structure of Y 2 Ni 2 Pb are presented to support this idea.

This paper presents a brief synopsis of magnetization, electrical transport, specific heat measurements as well as other recent work on URu2Si2, together with some topical discussions of the groundstate properties in relation to metemagnetism, quantum criticality and crystal electric fields., Comment: 6 pages intended for publication in special issue of Physica B for RHMF03 conference, Toulouse

Using a special cooling procedure we have grown single crystal platelets of the newly discovered rare-earth R2Ni2Pb intermetallic compounds with R=Gd, Tb, Dy, Ho, and Er. The material was synthesized in a single-phase and on-stoichiometry form. We characterized the compounds by X-ray diffraction, electron probe micro-analysis, and Laue X-ray diffraction. Some physical properties are illustrated, e.g. magnetic susceptibility and resistivity.

X-ray diffraction with synchrotron radiation has been used to study the phase transitions in compounds M5Ir4Si10 with M a rare-earth metal. For M = Er, Ho and Er0.84Lu0.16 phase transitions have been found towards a combined commensurate/incommensurate state, that locks into a commensurate state at lower temperatures. For M = Lu and Er0.66Lu0.34 there is a single transition towards a commensurate state. The results are interpreted in terms of commensurate structural transitions that induce incommensurate CDWs.

The series of intermetallic compounds RE 5 Ir 4 Si 10 (RE=rare-earth) shows unusual charge density wave (CDW) order for temperatures lower than 160 K . This work presents a muon spin rotation (μSR) study, in zero external field, on single crystalline samples of the magnetic Er 5 Ir 4 Si 10 ( T CDW =155 and 55 K ) and polycrystalline samples of Er 5 Ir 4 Ge 10 (no CDW). The muon relaxation rate λ is strongly temperature dependent in both compounds. Furthermore, upon cooling below the CDW transition temperatures, a kink is seen in λ in Er 5 Ir 4 Si 10 . We interpret this result as a signature of the change in the density of states due to the establishment of the CDW.

When matter is cooled from high temperatures, collective instabilities develop among its constituent particles that lead to new kinds of order. An anomaly in the specific heat is a classic signature of this phenomenon. Usually the associated order is easily identified, but sometimes its nature remains elusive. The heavy fermion metal URu(2)Si(2) is one such example, where the order responsible for the sharp specific heat anomaly at T(0) = 17 K has remained unidentified despite more than seventeen years of effort. In URu(2)Si(2), the coexistence of large electron electron repulsion and antiferromagnetic fluctuations leads to an almost incompressible heavy electron fluid, where anisotropically paired quasiparticle states are energetically favoured. Here we develop a proposal for the nature of the hidden order in URu(2)Si(2). We show that incommensurate orbital antiferromagnetism, associated with circulating currents between the uranium ions, can account for the local fields and entropy loss observed at the 17 K transition. We make detailed predictions for the outcome of neutron scattering measurements based on this proposal, so that it can be tested experimentally.

Er5Ir4Si10 exhibits three phase transitions upon cooling below room temperature. At TCDW = 151 K a combined commensurate and incommensurate superstructure develops, that has been attributed to the formation of charge density waves (CDWs). At TLI = 60 K (LI = lock-in) the superstructure becomes commensurate, and at TN = 2.8 K a state with long-range antiferromagnetic order develops. In this contribution we report the results of high-intensity, high-resolution x-ray diffraction for the temperature region encompassing all four phases. We have found that above TCDW the critical scattering of the commensurate superlattice reflections persists up to much higher temperatures than the critical scattering of the incommensurate satellites. It is argued that this finding substantiates the hypothesis in which the mechanism of the CDW transition involves a structural transition towards a twofold superstructure. The superlattice reflections are found to be broader in the lock-in phase than above TLI. This suggests that the lock-in transition results in relatively small domains, that are responsible for the broadening of the reflections. Finally, the antiferromagnetic order is observed by resonant x-ray scattering. The commensurate superlattice reflections persist down to 1.87 K, and no effect of the magnetic transition on their positions or intensities is found. Thus the magnetic order and the CDW coexist below TN in this compound.

The splitting of the Kondo peak on the differential resistance–voltage characteristic in a magnetic field is investigated in CuMn, CuCr, and AuFe point contacts of different diameter. Empirical formulas are obtained which can take into account the variation of the energy position of the maxima of the split Kondo peak, both in external and internal (the spin-glass state) magnetic fields, as a function of the temperature of the experiment and the Kondo temperature and also as the diameter of the contacts is decreased.

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed., 3 figures, 1 table

Uncovering the symmetry of a hidden order Cooling matter generally makes it more ordered and may induce dramatic transitions: Think of water becoming ice. With increased order comes loss of symmetry; water in its liquid form will look the same however you rotate it, whereas ice will not. Kung et al. studied the symmetry properties of a mysteriously ordered phase of the material URu 2 Si 2 that appears at 17.5 K. They shone laser light on the crystal and studied the shifts in the frequency of the light. The electron orbitals of the uranium had a handedness to them that alternated between the atomic layers. Science , this issue p. 1339

We show on the basis of electronic structure calculations that the uranium $5f$ magnetic moment in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ exhibits a unique Ising behavior, which surprisingly arises from itinerant electronic states. The origin of the unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge nested electronic structure of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ involving its strong spin-orbit interaction. The Ising anisotropy has pertinent implications for theories applicable to explaining the hidden order phase in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$.

The phase of condensed matter known as spin glasses has become a vital and productive area of research. Historically, experiment has suggested unusual effects which have brought the theoretical study of the spin Glass Problem Onto The Same Footing As The Experimental Study. Experiments in the late 1960s on magnetic alloys presented interesting effe

Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T

Single-phase, textured samples of a new orthorhombic intermetallic compound Ho2Ni2Pb have been fabricated (space group Cmmm). Here the bulk magnetic properties are presented as determined via magnetization, susceptibility, heat capacity and resistivity measurements. The results exhibit two distinct magnetic transitions and large metamagnetic effects. Such behaviour is related to the unusual rare-earth symmetry of the highly anisotropic crystal structure.

Moderate levels of crystallographic disorder can dramatically affect the ground-state properties of heavy fermion compounds. In particular, the role of disorder close to a quantum critical point has been investigated in detail. However, crystallographic disorder is equally effective in altering the properties of magnetically ordered heavy fermion compounds like URh2Ge2, where disorder-induced spin-glass behavior has been observed. In this system, moreover, the magnetic ground state can be tuned from a spin-glass to a long-range ordered antiferromagnetic one by means of an annealing treatment. The transformation of the magnetic state is accompanied by a transition in the transport properties from “quasi-insulating” (dρ/dT

We report the magnetic structure of the local moment magnetism in high-quality single crystals of the intermetallic compound Er 5 Ir 4 Si 10 , which shows two charge-density-wave (CDW) transitions (T CDW (1) =155 K and T CDW (2) =55 K). Bulk measurements demonstrate that the system has an effective Curie-Weiss magnetic moment of 9.7μ B and undergoes long-range antiferromagnetic (AFM) ordering at 2.8 K. The neutron-diffraction data establish the AFM structure of the Er 3+ with unequal moments at different Er sites aligned along the c axis where also the CDW transitions occur.

Low-temperature electrical transport measurements were performed on large arrays of 150-nm-wide superconducting vanadium (V) wires covered with either a thin layer of Au (V/Au) or Fe (V/Fe). The measurements were conducted using various electrical contact geometries. Our results show that a particular arrangement of the electrical contacts in combination with the superconducting proximity effect can result in a pronounced ``negative resistance'' anomaly below the resistive transition. We demonstrate that this ``negative resistance'' can be clearly reproduced by constructing resistor circuits based upon the particular contact arrangement.

An uniaxial pressure cell for low temperature use is described in detail. Then we present data of the electrical resistance of single crystals of Lu5Ir4Si10, which is known to show a charge-density-wave transition around 83 K and to become superconducting near 3.8 K, both phenomena being anticorrelated under pressure. Since the CDW in Lu5Ir4Si10 is a quasi one-dimensional phenomenon because of a chain-like structure, it responds to uniaxial pressure in a specific way.

We report the observation of a new type of charge-density wave (CDW) in the large magnetic-moment rare-earth intermetallic compound, ${\mathrm{Er}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$, which then orders magnetically at low temperatures. Single crystal x-ray diffraction shows the development of a 1D incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized ${\mathrm{Er}}^{3+}$ moments are antiferromagnetically ordered below 2.8 K. We observe very sharp anomalies in the specific heat at 145 and 2.8 K, signifying the bulk nature of these transitions. Our data suggest the coexistence of strongly coupled CDW with local-moment antiferromagnetism in ${\mathrm{Er}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$.

We present a study of annealing effects on the physical properties of tetragonal single-crystalline ${\mathrm{URh}}_{2}{\mathrm{Ge}}_{2}.$ This system, which in its as-grown form was recently established as the first metallic three-dimensional random-bond heavy-fermion spin glass, is transformed by an annealing treatment into a long-range antiferromagnetically ordered heavy-fermion compound. The transport properties, which in the as-grown material were dominated by the structural disorder, exhibit in the annealed material signs of typical metallic behavior along the crystallographic a axis. From our study ${\mathrm{URh}}_{2}{\mathrm{Ge}}_{2}$ emerges as exemplary material highlighting the role and relevance of structural disorder for the properties of strongly correlated electron systems. We discuss the link between the magnetic and electronic behavior and how they are affected by the structural disorder.