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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

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.

(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.