Magnetic Fluctuations and Clusters in the Itinerant Ferromagnet Ni-V Close to a Disordered Quantum Critical Point PDF Download
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Author: Ruizhe Wang Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The study of quantum phase transitions (QPT) is a promising route to comprehend the origin of unconventional properties in strongly correlated electron systems. Recent theories predict a new quantum critical point (QCP) in disordered itinerant system with exotic properties such as observable quantum Griffiths phase (QGP). The binary alloy Ni-V presents such QGP and looks like the best example to study a ferromagnetic QPT with controlled disorder. In this work, we investigate further Ni-V with advanced local methods using neutron scattering and [mu]SR techniques. Initial doubts on sample quality and sample-dependent impact on different magnetic phases and disordered scenarios can be resolved. A structural characterization indicates that the investigated Ni-V samples show a high-quality chemical structure with expected random atomic distribution. We provide direct evidence of the "disorder" with [mu]SR methods. We clarify essential details of the Ni-V phase diagram such as the nature and the boundaries of ferromagnetic and cluster glass phases. These new data reinforce the location of the QCP and the limits of the QGP in Ni-V. The main results are consistent with theories predicting an infinite randomness quantum critical point associated with a QGP.
Author: Ruizhe Wang Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The study of quantum phase transitions (QPT) is a promising route to comprehend the origin of unconventional properties in strongly correlated electron systems. Recent theories predict a new quantum critical point (QCP) in disordered itinerant system with exotic properties such as observable quantum Griffiths phase (QGP). The binary alloy Ni-V presents such QGP and looks like the best example to study a ferromagnetic QPT with controlled disorder. In this work, we investigate further Ni-V with advanced local methods using neutron scattering and [mu]SR techniques. Initial doubts on sample quality and sample-dependent impact on different magnetic phases and disordered scenarios can be resolved. A structural characterization indicates that the investigated Ni-V samples show a high-quality chemical structure with expected random atomic distribution. We provide direct evidence of the "disorder" with [mu]SR methods. We clarify essential details of the Ni-V phase diagram such as the nature and the boundaries of ferromagnetic and cluster glass phases. These new data reinforce the location of the QCP and the limits of the QGP in Ni-V. The main results are consistent with theories predicting an infinite randomness quantum critical point associated with a QGP.
Author: Shiva Bhattarai Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
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
Author: Hind A. Adawi Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The study of quantum phase transitions is a promising route for understanding the origin of unusual properties in strongly correlated electron systems. Recent theories predict a new quantum critical point (QCP) with exotic properties such as an observable quantum Griffiths phase in disordered itinerant systems. Previous studies on disordered ferromagnetic (FM) system Ni(1-x)V(x) showed only "signs" of magnetic clusters close to the critical concentration xc where FM is destroyed. This dissertation provides direct evidence of magnetic correlations at various length scales and fluctuations at different time scales close to the QCP using Ni-V and Ni-Cr samples. Polarized small-angle neutron scattering (SANS) measurements reveal short-range magnetic correlations in Ni-V samples close to QCP. This polarization study also supports long-range magnetic correlations within macroscopic domains. The structure characterization of Ni-Cr confirms a high-quality chemical structure with random atomic distribution after optimizing the annealing protocol. Muon spin rotation ([mu]SR) measurements successfully show the advantage of Ni-Cr. The response is not dominated by any strong nuclear moment like Ni-V. The [mu]SR asymmetry of Ni(1-x)Cr(x) close to xc reveals magnetic order at low temperatures with coexisting dynamic clusters. Both SANS and [mu]SR measurements support that the long-range FM order and short-range fluctuations are present in FM alloys Ni-V and Ni-Cr close to QCP.
Author: Adam Iaizzi Publisher: Springer ISBN: 9783030018023 Category : Science Languages : en Pages : 0
Book Description
This thesis is a tour-de-force combination of analytic and computational results clarifying and resolving important questions about the nature of quantum phase transitions in one- and two-dimensional magnetic systems. The author presents a comprehensive study of a low-dimensional spin-half quantum antiferromagnet (the J-Q model) in the presence of a magnetic field in both one and two dimensions, demonstrating the causes of metamagnetism in such systems and providing direct evidence of fractionalized excitations near the deconfined quantum critical point. In addition to describing significant new research results, this thesis also provides the non-expert with a clear understanding of the nature and importance of computational physics and its role in condensed matter physics as well as the nature of phase transitions, both classical and quantum. It also contains an elegant and detailed but accessible summary of the methods used in the thesis—exact diagonalization, Monte Carlo, quantum Monte Carlo and the stochastic series expansion—that will serve as a valuable pedagogical introduction to students beginning in this field.
Author: Ruizhe Wang
Author: Ruizhe Wang
Author: Shiva Bhattarai
Author: Hind A. Adawi
Author: Anna Zita Hüsges
Author: Almut Schröder
Author: Adam Iaizzi
Author: Stanford University. Microwave Laboratory
Author: 
Author: Sharon Lynn Sessions
Author: Daniel Lamago Kaffo