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We present the results of a study of the vortex lattice in the heavy fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, using small-angle neutron scattering (SANS). In this material at temperatures well below ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K, the value of the upper critical field ${B}_{\mathrm{c}2}\ensuremath{\sim}2.2$ T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near ${B}_{\mathrm{c}2}$. This behavior is the effect of Pauli paramagnetism and we present a theory-based model, which can be used to describe this effect in a range of materials. The pairing in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ appears to arise from the effect of magnetic fluctuations, but the evidence for a $d$-wave order parameter is rather weak. We find that the vortex lattice structure in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for $d$-wave superconductors and observed in ${\mathrm{CeCoIn}}_{5}$; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband $s$-wave superconductivity, rather than a nodal gap structure.

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

An experimental study of the rare-earth intermetallic system LuPt2In reveals a strong enhancement of superconductivity near the charge density wave quantum critical point. This represents an unusual counter-example to cuprates, in which superconductivity and charge density waves tend to compete. Quantum critical points (QCPs), at which a second-order phase transition is continuously suppressed to zero temperature, are currently one of the central topics in solid-state physics1,2. The strong interest emerges from observations of very unusual properties at QCPs such as the onset of unconventional superconductivity (SC)3. While QCPs found at the disappearance of magnetic order are quite common and intensively studied, a QCP that results from a structural transition is scarce and poorly investigated. Here, we report on the observation of a charge density wave (CDW) type of structural ordering in LuPt2In with a second-order transition at TCDW = 490 K. Substituting Pd for Pt suppresses TCDW continuously towards T = 0, leading to a QCP at 58% Pd substitution. We find a strong enhancement of bulk SC just at the QCP, pointing to a new type of interaction between CDW and SC.

The Third Law of Thermodynamics states that the entropy of any system in equilibrium has to vanish at absolute zero temperature. At nonzero temperatures, on the other hand, matter is expected to accumulate entropy near a quantum critical point (QCP), where it undergoes a continuous transition from one ground state to another. Here, we determine, based on general thermodynamic principles, the spatial-dimensional profile of the entropy S near a QCP and its steepest descent in the corresponding multidimensional stress space. We demonstrate this approach for the canonical quantum critical compound CeCu6-xAux near its onset of antiferromagnetic order. We are able to link the directional stress dependence of S to the previously determined geometry of quantum critical fluctuations. Our demonstration of the multidimensional entropy landscape provides the foundation to understand how quantum criticality nucleates novel phases such as high-temperature superconductivity., 14 pages, 4 figures

CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below $T_{\mathrm{N}} = 8.8$ K, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of $\sim40$ K at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only $m_{\mathrm{Ce}} = 0.37(6)$ $\mu_{\mathrm{B}}$. Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies, and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure., Comment: 8 pages, 11 figures

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}$ reveal a phonon anomaly in a wide temperature range. Starting from ${T}_{\mathrm{ins}}\ensuremath{\sim}50--60\text{ }\text{ }\mathrm{K}$ where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}=27\text{ }\text{ }\mathrm{K}$, where also a ferroelectric ordering at ${T}_{\mathrm{FE}}\ensuremath{\approx}{T}_{\mathrm{N}}$ occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below ${T}_{\mathrm{N}}$, while no change of the lattice symmetry is observed across ${T}_{\mathrm{N}}$ in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.

Here, we reinvestigate the underlying charge and magnetic correlations in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ within a combined study of muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), x-ray photon correlation spectroscopy, synchrotron radiation single-crystal x-ray diffraction, and neutron-scattering measurements. For ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ around 1/3 hole-doping, the significant charge correlations that are responsible for the emergence of hour-glass magnetic spectra are found to be slowly fluctuating checkerboard charge ordering (CBCO) correlations with an onset temperature being independent of hole-doping. This reveals a local origin of the CBCO correlations in the rather ionic cobaltates, thus being distinct from the more covalent nickelates. Moreover, we report the observation of a very similar temperature dependence of the intensities of in-plane high- and low-energy excitations within the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ hour-glass spectrum. This observation shows that nano phase separation is distinct from conventional phase separation and that there is a coupling between nanometer-sized undoped and hole-doped islands. Based on this, it is possible to propose a microscopic scenario for the suppression of the outwards-dispersing branches in hour-glass magnetic spectra.

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl reveal a phonon anomaly in a wide temperature range. Starting from T_{ins}∼50-60 K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature T_{N}=27 K, where also a ferroelectric ordering at T_{FE}≈T_{N} occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below T_{N}, while no change of the lattice symmetry is observed across T_{N} in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.

We measure the quasiparticle weight in the heavy-fermion compound CeCu$_{6-x}$Au$_{x}$ ($x=0,\ 0.1$) by time-resolved THz spectroscopy for temperatures from 2 up to 300\,K. This method distinguishes contributions from the heavy Kondo band and from the crystal-electric-field satellite bands by different THz response delay times. We find that the formation of heavy bands is controlled by an exponentially enhanced, high-energy Kondo scale once the crystal-electric-field states become thermally occupied. We corroborate these observations by temperature-dependent dynamical mean-field calculations for the multi-orbital Anderson lattice model and discuss consequences for quantum critical scenarios., Comment: Published version, 6 pages (including references), 5 figures, Supplemental Material (2 pages) added

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...

Paramagnons are supposed to provide the pairing glue for unconventional superconductors. For the heavy-fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, there is indeed good evidence from inelastic neutron scattering (INS) that spin fluctuations drive the superconductivity. Here, we present the INS measurement of the inelastic response of the antiferromagnetic parent compound, `A-type' ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, to probe the relation to the excitations of the superconducting (`S-type') sample. We find that the dispersion is very similar in the antiferromagnetic state and in the normal state of the superconducting sample. Pronounced differences to the response in the superconducting state exist at low energies around the zone center. These findings are in line with observations of other unconventional superconductors.

We have investigated the local low-energy excitations in CeRu4Sn6, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce M-5 resonant inelastic x-ray scattering (RIXS). The availability of both F-2(5/2) and F-2(7/2) excitations of the Ce 4f(1) configuration in the spectra allows for the determination of the crystal-electric field (CEF) parameters that explain quantitatively the high-temperature anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all CEF states are close to being rotational symmetric. We show further that the non-negligible impact of the (sic)(6)(0) parameter on the ground state of CeRu4Sn6 leads to a reduction of the magnetic moment mu(c) due to multiplet intermixing. This improves the agreement between CEF calculations and the experimentally determined magnetic susceptibility considerably at low temperatures. Deviations that persist at low temperatures for fields within the tetragonal plane are attributed to the Kondo interaction between 4f and conduction electrons. The RIXS results are consistent with inelastic neutron scattering data and are compared to the predictions from ab initio based electronic structure calculations.

The heavy-fermion compound ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Au}}_{x}$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0\ensuremath{\le}xl0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature ${T}_{s}$ with ${T}_{s}\ensuremath{\rightarrow}0$ for $x\ensuremath{\approx}0.15$. Antiferromagnetic order sets in close to $x\ensuremath{\approx}0.1$. To shed light on the interplay between quantum-critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0\ensuremath{\le}x\ensuremath{\le}0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\ensuremath{\approx}0.1$ and 0.15. The application of hydrostatic and chemical pressure allows us to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum-critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion.

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon., 30 pages, 7 Figures, Accepted for publication in Philosophical Magazine

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. Here we trace the dynamics of heavy-fermion quasiparticles in CeCu$_{6-x}$Au$_{x}$ and monitor their evolution towards the QCP in time-resolved experiments, supported by many-body calculations. A terahertz pulse disrupts the many-body heavy-fermion state. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a coherence time 100 times longer than typically associated with correlated metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains constant -- phenomena believed to be mutually exclusive. Coexistence in the same experiment calls for revisions in our view on quantum criticality., Published version, including data on CeCu6, CeCu5.9Au0.1, and CeCu5Au1 and extended Supplementary Information. 7 pages, 4 figures, Supplementary Information: 5 pages, 3 figures

In the heavy-fermion metal CePdAl long-range antiferromagnetic order coexists with geometric frustration of one third of the Ce moments. At low temperatures the Kondo effect tends to screen the frustrated moments. We use magnetic fields $B$ to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with $B$ employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid., Comment: 5+2 pages with 4 figures

We report on inelastic neutron scattering experiments that were carried out on single-crystalline CePdAl, which is a heavy fermion system with hexagonal crystal structure and is close to an antiferromagnetic quantum critical point. The measurements yield information on the crystalline-electric-field (CEF) excitations and quasielastic spin fluctuations. Two excited CEF levels have been detected at ≈240 K and >400 K. A broad quasielastic magnetic signal could be observed, yielding a Kondo temperature TK = 5 K that agrees with the result from thermodynamic measurements.

In this paper, we present a comprehensive study of heat capacity, resistivity, neutron diffraction and magnetization measurements on Yb(Rh0.42Co0.58)2Si2. Heat capacity and resistivity measurements revealed one phase transition at Tm ≈ 820 mK into the magnetically ordered phase, but in neutron diffraction experiments the magnetic intensity at the commensurate position vanished already at a lower TL ≈ 700 mK. From magnetization measurements we could derive the existence of two magnetic phases, an antiferromagnetic phase below TL ≈ 700 mK and an intermediate phase in the temperature range , which shows surprisingly ferromagnetic properties when applying magnetic fields along the [001] direction.

We report on a single-crystal neutron diffraction study of the evolution of the antiferromagnetic order in the heavy-fermion compound CePd$_{1-x}$Ni$_x$Al which exhibits partial geometric frustration due to its distorted Kagom\'e structure. The magnetic structure is found to be unchanged with a propagation vector $Q_\mathrm{AF} \approx (0.5~0~0.35)$ for all Ni concentrations $x$ up to $x_c \approx 0.14$. Upon approaching the quantum critical concentration $x_c$, the ordered moment vanishes linearly with N\'eel temperature $T_{\rm N}$, in good agreement with CePdAl under hydrostatic pressure. For all Ni concentrations, substantial short-range magnetic correlations are observed above $T_{\rm N}$ as a result of frustration., Comment: 6 pages, 4 figures

Magnetic frustration, which is well-defined in insulating systems with localized magnetic moments, yields exotic ground states like spin ices, spin glasses, or spin liquids. In metals magnetic frustration is less well defined because of the incipient delocalization of magnetic moments by the interaction with conduction electrons, viz., the Kondo effect. Hence, the Kondo effect and magnetic frustration are antithetic phenomena. Here we present experimental data of electrical resistivity, magnetization, specific heat and neutron diffraction on CePdAl, which is one of the rare examples of a geometrically frustrated Kondo lattice, demonstrating that the combination of Kondo effect and magnetic frustration leads to an unusual ground state., Comment: 8 pages, 6 figures

We report on the results of powder and single crystal neutron diffraction to investigate the magnetic order in YbCo$_2$Si$_2$ below the N��el temperature $T_{\rm N} = 1.7~$K in detail. Two different magnetically ordered phases can clearly be distinguished. At lowest temperatures a commensurate magnetic structure with a propagation vector ${\bf k}_1 = (0.25~ 0.25~ 1)$ is found, while the intermediate phase ($T > 0.9~$K) is characterized by an incommensurate magnetic structure with ${\bf k}_2 = (0.25~ 0.086~ 1)$. The magnetic structure in YbCo$_2$Si$_2$ is in marked contrast to all other known RCo$_2$Si$_2$ compounds (R = rare earth element) likely due to some itineracy of the Yb 4f states being responsible for the magnetism., 9 pages, 7 figures, accepted for publication in Phys. Rev. B

CeAgAs 2 , an HfCuSi 2 like layered pnictide, orders antiferromagnetically at T N =6.2(1) K. The ordering process was monitored in neutron diffraction experiments in the temperature range 10 K≥ T ≥3.5 K. At T =4 K the lattice parameters are a =5.7438(1) A, b =5.7696(1) A and c =21.0067(2) A. The diffraction pattern of the antiferromagnetic phase with a propagation vector k =[0,0,0] point towards ferromagnetically ordered moments in Ce layers stacked along [001], the individual layers are coupled antiferromagnetically with a +− −+type sequence. The alignment of moments within the Ce layers cannot be determined reliably from the experimental data so that two different structure models are discussed. The proposed metamagnetic transition was confirmed by diffraction experiments applying an external magnetic field at T T N . In the interval 4 K≤ T ≤6 K, a relatively small field of μ 0 H ≈0.3 T suffices to fully suppress the antiferromagnetic ordering. The effect is completely reversible yet subject to hysteresis: After switching off the external field at any T T N the magnetic reflections gain their original intensity within several 10 min indicating the restoring of the antiferromagnetic phase.

We review magnetic quantum-critical points (QPCs) in heavy-fermion compounds separating at zero temperature: an antiferromagnetically ordered state and a nonordered ground state. At the magnetic instability, the Fermi-liquid (FL) description valid for normal metals breaks down, giving rise to unusual, non-Fermi-liquid (NFL) low-temperature behavior. After a short introduction to phase transitions and to T = 0 phase transitions in general as well as to the physics of heavy-fermion systems, the two main theoretical scenarios describing the physics at QPCs in these systems are presented, the conventional spin-density-wave (SDW) scenario and the unconventional Kondo-breakdown scenario. Whereas for the conventional scenario experimental data for CeCu2Si2 and Ce1−xLaxRu2Si2 are discussed only briefly, we focus in more detail on the unusual behavior of CeCu6−xAux and YbRh2Si2 at their respective QPCs and show that these systems are best described within the unconventional scenario.

The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu2Si2. A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point., Comment: 30 pages, including supplementary material

The heavy-fermion system CeCu6−x Au x which exhibits antiferromagnetic order for Au concentrations x>0.1, can be easily tuned to a quantum phase transition, either by chemical composition, i.e., Au concentration, or by an external magnetic field. We present extensive inelastic neutron scattering measurements in order to study the momentum and energy dependence of the critical spin fluctuations for concentration- and magnetic field-tuned CeCu6−x Au x . The quasi-2D fluctuations, observed for x=0.1 at the magnetic instability, are already present in pure CeCu6 and persist to x=0.2, i.e., well into the magnetically ordered regime. This is a strong hint that disorder does not play a major role in this system. The magnetic response for quantum critical CeCu6−x Au x exhibits critical slowing down and obeys scaling relations that depend on the tuning parameter. While the E/T scaling of the dynamic susceptibility in concentration-tuned CeCu5.9Au0.1 is an indication for local quantum criticality, the E/T 3/2 scaling in field-tuned CeCu5.8Au0.2 points to a conventional spin-density-wave instability.

The crystal electric field (CEF) scheme of YbNi4P2 is determined, based on experimental data from inelastic neutron scattering, heat capacity, susceptibility and NMR measurements. Despite the tetragonal crystal structure, 9 parameters are needed to describe the crystal field in YbNi4P2 due to the orthorhombic site symmetry of the Yb ion. A large basal plane anisotropy is detected by the local probe NMR. Our analysis yields CEF excitation energies of 8.5, 12.5 and roughly 30 meV and a ground state wave function that is dominated by the $5/2$ state. Furthermore, we present an analysis of the CEF scheme based on density functional theory calculations, which confirms the large basal plane anisotropy.

We have investigated the antiferromagnetic (AFM) state of YbCo 2 Si 2 by means of magnetoresistance measurements for different field and current directions. We observed huge magnetoresistance effects, up to ±60% even at low fields B < 2 T, and a very rich variety of features. The results evidence a complex magnetic phase diagram, with a pronounced anisotropy even for different field directions within the basal plane. The analysis of these results gives valuable insight into the different AFM phases of YbCo 2 Si 2 .

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN2B2C was chosen as a test example because it is a conventional superconductor with a fairly high T c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.

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 report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above T c ≈ 600 mK , a truly inelastic response is detected in the superconducting state well below T c . This inelastic response is discussed in connection to the properties of the superconducting state.

CePdAl with Ce $4f$ moments forming a distorted kagome network is one of the scarce materials exhibiting Kondo physics and magnetic frustration simultaneously. As a result, antiferromagnetic (AF) order setting in at ${T}_{\mathrm{N}}=2.7$ K encompasses only two-thirds of the Ce moments. We report measurements of the specific heat, $C$, and the magnetic Gr\"uneisen parameter, ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{mag}}$, on single crystals of ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{Al}$ with $x\ensuremath{\le}0.16$ at temperatures down to 0.05 K and magnetic fields $B$ up to $8\phantom{\rule{0.28em}{0ex}}\mathrm{T}$. Field-induced quantum criticality for various concentrations is observed with the critical field decreasing to zero at ${x}_{c}\ensuremath{\approx}0.15$. Remarkably, two-dimensional AF quantum criticality of Hertz-Millis-Moriya type arises for $x=0.05$ and $x=0.1$ at the suppression of three-dimensional magnetic order. Furthermore, ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{mag}}(B)$ shows an additional contribution near $2.5\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ for all concentrations, which is ascribed to correlations of the frustrated one-third of Ce moments.

In cold neutron spectroscopy, there exists an increasing demand for experiments at extreme conditions with the focus on quantum magnetism, new magnetic, superconducting, frustrated or multiferroic materials, heavy-fermion, or low-dimensional systems. Investigations of these systems are often characterized by small sample sizes or small scattering cross sections in combination with extreme sample environment such as low temperatures and/or high magnetic fields. Foreseeing these trends, PANDA was designed to be a high-resolution, high-flux cold three-axis spectrometer (TAS) with a notably low background, equipped with different sample environments and constructed to use the latter optimally [1].

CeCu 6 - x Au x has become a model system to study magnetic quantum phase transitions (QPT). In this system, the QPT can be tuned by Au concentration x , pressure, or magnetic field. Previous inelastic neutron-scattering (INS) experiments have demonstrated an anomalous scaling of the dynamical susceptibility at the concentration-tuned (zero-field) QPT for x = 0.1 , i.e., χ - 1 ( q , E , T ) = c - 1 [ θ α ( q ) + ( T - i E ) α ] , where θ ( q ) is a generalized Curie–Weiss temperature. This scaling suggests local criticality, with an anomalous exponent α ≈ 0.75 . Here we report on an INS study at the field-induced magnetic QPT of CeCu 5.8 Au 0.2 where the Neel temperature is driven to zero at a critical field of B ≈ 0.4 T , supplemented by specific-heat data. The INS data can be described better by the spin-density-wave scenario, i.e., χ - 1 ( Q AF , E , T ) = a - 1 ( T 3 / 2 - i bE ) where Q AF is the antiferromagnetic ordering wave vector, than by a local quantum critical point. This constitutes direct microscopic evidence for a difference in quantum fluctuation spectra at a magnetic QPT driven by different tuning parameters.

We report on inelastic neutron scattering experiments on YbRh 2 Si 2 powder to study the low-energy spin dynamics at temperatures between T = 0.8 and 22 K. The low-energy magnetic response is quasielastic. However, it exhibits an unusual form not modelled by a simple relaxation rate yielding a Lorentzian lineshape, but can satisfactorily be described by a phenomenological model involving a distribution of relaxation rates. The lower bound of the relaxation rates varies roughly linear with temperature indicating a pronounced slowing down of the critical modes above the antiferromagnetic ordering temperature T N ≈ 70 mK .

We report on a detailed investigation of the magnetism in A-type CeCu2Si2 by neutron scattering, thermodynamic and transport measurements. The linewidths of the magnetic peaks broaden well below the Neel temperature TN ≈ 800mK pointing to a finite domain size/correlation length. In contrast, within the resolution of the experiments the magnetic order is static. Complementary measurements of the electrical resistivity observed a superconducting transition at T< 450mK well below TN. However, no anomaly has been detected in heat capacity measurements. All results indicate that A-type single crystals exhibit parasitic superconductivity limiting the size of the antiferromagnetically ordered regions. The observed behavior can be understood by the slight off-stoichiometry of A-type CeCu2Si2.

Zero-field μ SR measurements have been performed on CeCu2Si2 single crystals. Antiferromagnetic order shows in an initial rapid damping of the μ + polarization. In a single crystal exhibiting antiferromagnetic order and superconductivity, there exists clear evidence of phase separation of both phenomena with a first order transition from antiferromagnetism at higher temperature to superconductivity at low temperature.

Rare-earth intermetallic alloys and compounds, in particular those with Ce or Yb, are often close to a magnetic instability. In particular, CeCu6−xAux has become a prototype heavy-fermion (HF) system where, starting from not magnetically ordered CeCu6, Au doping introduces long-range incommensurate antiferromagnetism for x > x c ≈ 0.1 . At the critical concentration x c the system experiences a quantum phase transition (QPT). Here, the unusual magnetic fluctuations probed by inelastic neutron scattering lead to non-Fermi-liquid behavior, i.e. to anomalous low-temperature thermodynamic and transport properties. Hall-effect measurements delineate the “bandstructure” of heavy fermions across the critical concentration x c . While most rare-earth HF compounds have a tendency towards antiferromagnetic order, CeSi1.81 presents one of the comparatively few cases exhibiting ferromagnetic order below T c = 9.5 K. In a search for a ferromagnetic QPT in HF metals, we have studied the pressure dependence of the magnetization and the spontaneously ordered magnetic moment, μ S , which vanishes around p ≈ 13 kbar.

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 report on extensive neutron diffraction on single crystalline CeCu 2 ( Si 1 - x Ge x ) 2 to determine the antiferromagnetic order in the regime with low Ge content, initially called A phase due to the unknown origin of the magnetic order. Starting from the x = 0.45 alloy we followed the magnetic order to lower Ge concentrations and to pure CeCu 2 Si 2 . For all Ge concentrations x ⩽ 1 an incommensurate magnetic order with a propagation vector close to τ = ( 0.25 0.25 0.5 ) is observed. The magnetic order is determined by the nesting properties of the Fermi surface.

The heavy fermion (HF) compound YbNi4P2 is a promising new system for the study of ferromagnetic quantum criticality. Its particular crystal structure in which the Yb ions form chains along the c-axis might cause quasi-1D magnetic interactions. In thermodynamic and transport measurements, highly anisotropic behaviour has been observed. To clarify the role of the crystalline electric field (CEF) on this anisotropy, we have measured its level scheme by inelastic neutron scattering. We observed a broad magnetic excitation feature between 7 and 13 meV that is composed of two or three CEF transitions. We discuss the probability of an additional CEF level at either very high or very low energy.

Neutron-diffraction experiments on TbNi2B2C single crystals are reported for temperatures below 20 K. The evidence of magnetic satellite reflections of 2nd order and the observation of satellite reflections close to nuclear reflections instead of ferromagnetic signals request modifications of the known model for the magnetic structure.

We here present inelastic neutron scattering results on the strongly correlated cubic superconductor UBe13 (T-c = 0.85 K) obtained on a large single crystal by high-resolution cold neutron three-axis spectroscopy. We observed spin dynamics at a unique momentum space position building up below T similar to 50 K and changing significantly on entering the superconducting state. The observed short-range longitudinal character of the correlations can be understood as a result of competing magnetic interactions. The energy dependence in the normal state reflects the energy scales determined from specific heat, whereas the low-temperature data suggest the opening of a superconducting gap. Our findings are consistent with a superconducting order parameter exhibiting s +/- or d-wave symmetry and placing pure UBe13 in the strong coupling regime. (Less)

The effect of magnetic field on the static and dynamic spin correlations in the non-centrosymmetric heavy-fermion superconductor CePt$_3$Si was investigated by neutron scattering. The application of a magnetic field B increases the antiferromagnetic (AFM) peak intensity. This increase depends strongly on the field direction: for B${\parallel}$[0 0 1] the intensity increases by a factor of 4.6 at a field of 6.6 T, which corresponds to more than a doubling of the AFM moment, while the moment increases by only 10 % for B${\parallel}$[1 0 0] at 5 T. This is in strong contrast to the inelastic response near the antiferromagnetic ordering vector, where no marked field variations are observed for B${\parallel}$[0 0 1] up to 3.8 T. The results reveal that the AFM state in CePt$_3$Si, which coexists with superconductivity, is distinctly different from other unconventional superconductors., 5 pages, 4 figures, accepted for publication in Phys. Rev. B

We report on inelastic neutron scattering experiments on a CeCu 6 single crystal using polarized neutrons to study the crystalline electric field (CEF) excitations. CEF levels have been detected at ℏ ω ≈ 6.9 and 13.2 meV in close accordance to previous results. Our measurements yield new information about the transition matrix elements in the CEF Hamiltonian and will allow a reliable determination of the CEF parameters.

Magnetic susceptibility measurements and neutron powder diffraction were performed on the rare-earth transition-metal sulfide Ce 2 Fe 2 S 5 to study the effect of the substitution of nonmagnetic La 3 + ( 4 f 0 ) by Ce 3 + 4 f 1 ) . Ce 2 Fe 2 S 5 , which is isostructural to La 2 Fe 2 S 5 , contains Fe 2 + ions within chains of iron–sulfur octahedra and tetrahedra that are interconnected by Ce 2 + ions. Two antiferromagnetic phases, both with the propagation vector τ = ( 1 2 1 2 0 ) , are observed: at 5 K ⩽ T ⩽ 80 K ordering of the Fe 2 + moments is found; at T ⩽ 5 K the Ce 3 + ions also participate in the magnetic ordering.