Bandstruktur oder lokalisierte Elektronen: Hochfeldmagnetismus in metallischen Uranverbindungen

We have studied the uranium intermetallic compounds U(Pd1-xPtx)3 and UPt2Si2 to determine the degree of localization or itinerancy of the uranium 5f electrons in their respective environments. For single crystalline UPt2Si2 an extensive high magnetic field study was performed including measurements of the magnetization in pulsed magnetic fields up to 52 T and measurements of resistivity, magnetoresistivity and Hall effect in DC magnetic fields up to 35 T. From these measurements we have constructed the magnetic phase diagrams along the a and c axes. For the magnetic field applied along the a axis we find a new region indicated by strong hysteresis below ~20 K in magnetic fields of ~35 to 48 T resulting in a very uncommon magnetic phase diagram along this direction. For the magnetic field applied along the c axis at least three field induced phases are observed above 24 T at temperatures below ~18 K leading to a very exotic magnetic phase diagram. From a careful analysis of our Hall effect data including anomalous contributions and a comparison to recent band structure calculations we identified at least one of the field induced phase transitions as a Lifshitz or electronic topological transition. For the quadrupolar ordering compound U(Pd1-xPtx)3 we have characterized single crystalline samples with low alloying levels of x = 0.5% and 1% by means of specific heat, resistivity and susceptibility measurements. Against expectation for Pt levels as low as 0.5% we observe a strong reduction of the ordering temperatures by approximately one half upon isoelectronic alloying at the Pd sites and only short range order at x = 1%. From our data we have constructed the magnetic phase diagrams along the a and c axes for x = 0.5% and an alloying phase diagram. From the magnetic field dependency of the ordering temperatures we conclude that the physical mechanisms are identical for UPd3 and U(Pd1-xPtx) at low alloying levels. To identify critical parameters which cause the observed strong reduction of ordering temperatures we have set up mean field modeling allowing for shifts of crystal electric field levels, Zeeman splitting and multiple quadrupolar order parameters. In such models we reproduce large parts of the experimentally determined magnetic phase diagram. Furthermore, in these models we find an extraordinary large sensitivity of the ordering temperatures against changes of the quadrupolar coupling constants which may account for our experimental observations.Wir haben die metallischen Uranverbindungen U(Pd1-xPtx)3 und UPt2Si2 studiert um den Grad der Lokalisierung oder Itineranz der 5f Elektronen des Urans in seiner jeweiligen Umgebung zu bestimmen. An einkristallinem UPt2Si2 wurde eine Hochmagnetfeldstudie mit Messungen der Magnetisierung in gepulsten Feldern bis 52 T und Messungen des Widerstandes, Magnetowiderstandes und Hall Effektes in statischen Feldern bis 35 T durchgeführt. Aus den Daten haben wir die magnetischen Phasendiagramme entlang der a und c Achsen bestimmt. Für ein magnetisches Feld entlang der a Achse beobachten wir unterhalb von ~20 K in Feldern von ~35 bis 48 T eine Region die sich durch starke Hysterese auszeichnet und zu einem sehr ungewöhnlichen magnetischen Phasendiagram entlang dieser Richtung führt. Für ein Magnetfeld entlang der c Achse beobachten wir mindestens drei feldinduzierte Phasen oberhalb von 24 T bei Temperaturen unterhalb von ~18 K und damit ein sehr exotisches magnetischen Phasendiagram. Durch eine Analyse unserer Hall Effekt Daten und einen Vergleich zu neuesten Bandstrukturrechnungen konnten wir mindestens einen der feldinduzierten Phasenübergänge als Lifshitz-Übergang identifizieren. Vom quadrupolar ordnenden Material U(Pd1-xPtx)3 wurden einkristalline Proben mit kleinen Legierungsbeiträgen von x=0,5% und 1% durch Messungen der spezifischen Wärme, des Widerstandes und der Suszeptibilität charakterisiert. Wider Erwarten beobachten wir infolge der isoelektronischen Legierung bereits bei niedrigen Platinanteilen von 0,5% eine Reduktion der Ordnungstemperautren um etwa die Hälfte und nur kurzreichweitige Ordnung für x=1%. Aus den Daten haben wir die magnetischen Phasendiagramme entlang der a und c Achsen für x=0,5% und ein Legierungsphasendiagram erstellt. Aus den magnetischen Phasendigrammen ist ersichtlich dass die physikalischen Mechanismen in UPd3 und U(Pd1-xPtx)3 bei kleinen x identisch sind. Um kritische Paramter zu identifizieren die die starke Reduktion der Ordnungstemperaturen verursachen haben wir mean field Rechnungen unter Berücksichtigung von Verschiebungen der Kristallfeldniveaus, der Zeemann Aufspaltung und verschiedener quadrupolarer Ordnungsparameter durchgeführt. In solchen Modellen werden große Teile des experimentellen magnetischen Phasendigrams reproduziert und wir finden eine außergewöhnlich starke Empfindlichkeit der Ordnungstemperaturen gegenüber Änderungen der quadrupolaren Kopplungskonstanten was unsere experimentellen Beobachtungen erklärt

A setup for fast cooling of liquids in sealed containers

We present a simple layout of a fast cooling system for liquids in sealed containers utilizing the large temperature gradients of cold nitrogen gas. Our system is optimized for about 20 cylindrical containers of 500 cm3, but the setup allows for simple up- and down-scaling as well as the use of arbitrary containers. We have characterized the cooling performance of our system experimentally for temperatures of the liquid in the range from room temperature down to the freezing point at ≈−2 ºC. With our system we achieve container cooling times as low as 3 min, a significant reduction if compared to cooling times with common methods in the range of 8 to 40 min. Modeling of the cooling process proves that convection within the liquid is crucial for quick heat transfer. For the cases studied here, the most important factor limiting the cooling rate is the thermal conductivity of the container material

Field induced phases in UPt2Si2

The tetragonal compound UPt2Si2 has been characterised as a moderately mass enhanced system with an antiferromagnetic ground state below T_N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt2Si2. Along the crystallographic a axis, at low temperatures, we find a metamagnetic-like transition in fields of the order 40 T, possibly indicating a first order transition. Along the crystallographic c axis, in magnetic fields of B>= ~24 T, we find distinct anomalies in both properties. From our analysis of the data we can distinguish new high field phases above the AFM ground state. We discuss the emergence of these new phases in the context of Fermi surface effects and the possible occurrence of a Lifshitz or electronic topological transition, this in contrast to previous modellings of UPt2Si2 based on crystal electric field effects.Comment: 10 pages, submitted to PR

Magnetic phase diagram and electronic structure of UPt2Si2 at high magnetic fields: a possible field-induced Lifshitz transition

Theoretical Physic

Heavy fermions in high magnetic field

We give an overview on experimental studies performed in the last 25 years on heavy-fermion systems in high magnetic field. The properties of field-induced magnetic transitions in heavy-fermion materials close to a quantum antiferromagnetic-to-paramagnetic instability are presented. Effects of a high magnetic field to the Fermi surface, in particular the splitting of spin-up and spin-down bands, are also considered. Finally, we review on recent advances on the study of non-centrosymmetric compounds and ferromagnetic superconductors in a high magnetic field.Comment: 37 pages, 26 figures, Special Issue of the "Comptes Rendus de l'Acad\'emie des Sciences" on the Physics in High Magnetic Fiel