Thermal Neutron Scattering And Critical Magnetic Fluctuations PDF Download

Strongly correlated electron systems are at the borderline of competing phases and can be tuned through different ground states by slight modifications. Therefore, they are good examples to study a quantum phase transition (QPT), to reveal how a quantum critical point (QCP) at zero temperature is responsible for the unconventional properties observed at finite temperatures. QPTs are zero-temperature phase transitions, they are more complex and less understood than common phase transitions at finite temperatures. Examples are lacking, especially in the case where disorder is involved. Recent theories predict the possibility of an exotic quantum critical point in itinerant magnets with induced disorder that is accompanied by a quantum Griffiths phase. To explore such unconventional properties in close neighborhood to a magnetic phase, we aim to reveal the relevant quantum critical fluctuations with neutron scattering. We select systems with different magnetic order and choose as tuning parameter chemical substitution to study the effect of disorder. Such experimental study aims to find key elements of a QCP with disorder. The two systems are the ferromagnetic (FM) alloy, Ni-V, tuned by the V-concentration into a paramagnetic phase, and the non-magnetic Kondo semimetal, Ce(Cu,Ni)Sn, tuned by Cu concentration into an antiferromagnetic state. We apply different neutron scattering techniques and simple models to get essential characteristics of the magnetic correlations and fluctuations close to the QCP. Ni-V is a simple FM-alloy with a random atomic distribution that undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state with sufficient substitution of Ni by V. First indication of a quantum Griffiths phase came from magnetization and [mu]SR data, but the scale of the magnetic clusters remained elusive. Optimized small angle neutron scattering (SANS) data on different polycrystalline Ni-V samples close to the QCP finally show a small magnetic scattering response within the FM state. Using full polarization analysis, we discover short-range correlations of order of several nanometers. In addition, the polarized SANS data show evidence of long-range order. The coexistence of magnetic clusters and long-range ordered domains explain the unusual properties of this disordered FM with a QCP. These results agree with the expectations of a disordered QCP with a quantum Griffith phase. The non-magnetic anisotropic Kondo insulator, CeNiSn, presents a rather unique magnetic excitation spectrum with gaps. Cu doping leads to an antiferromagnetic ordered state with a small magnetic moment. We collected the first inelastic neutron scattering data of a single crystalline sample with Cu concentration at the onset of the magnetic order and revealed the magnetic excitation spectrum at a specific wave vector. It does not show a gap anymore, but magnetic order. In addition, we identify a fluctuation spectrum, including one-dimensional critical magnetic fluctuations. These first data already shed some light on the complex behavior of this Kondo semimetal driven by anisotropic hybridization and disorder. Overall these neutron scattering data reveal relevant quantum fluctuations close to onset of magnetic order that help to characterize the nature of the transition