Scientific Goals of the HZB High-Field-Magnet-Project

The aim of neutron scattering experiments in high magnetic fields is to clarify the microscopic and quantum behavior of condensed matter systems with magnetic degrees of freedom. Neutrons are ideal to investigate the magnetic structure of materials and their dynamics in elastic and inelastic scattering experiments since the wavelength of thermal neutrons is of the order of interatomic distances in matter and their magnetic moment allows probing magnetic interactions in materials. An external magnetic field is the essential parameter to tune into new phenomena in materials with important magnetic degrees of freedom. In order to achieve this goal, the external magnetic field has to be adapted to the order of magnitude of the basic magnetic interaction energies. This requires high magnetic fields of the order of several 10 T.

Condensed matter systems with magnetic degrees of freedom cover the whole range from fundamental to applied science: The basic elementary excitations in certain types of magnetic chain materials (“spinons”) allow to observe the equivalent of quarks, i.e. one of the basic constituents of matter, correlated electrons in certain layered compounds offer the chance to tailor high temperature superconductors, i.e. materials with an enormous potential for industrial applications. These topics of strong and actual interest have in common that their behavior is dominated by the laws of quantum mechanics and both neutrons and high magnetic fields are required in order to understand their microscopic behavior, be it for fundamental physics or for material science. 

A magnetic field is among the very few external parameters which can be continuously tuned to cover essential variations in the materials of interest and it is known how to combine it with low temperatures in the mK range as often required to distil the pure quantum behavior. An external magnetic field is superior to other external parameters as e.g. doping, which invariably introduces inhomogeneity and does not permit a continuous variation over a wide range. For investigations by complementary methods such as measurements of static bulk properties (specific heat, magnetization, resistivity), optical spectroscopy, NMR and ESR the availability of magnetic fields of several 10 T is by now standard, but only neutron scattering combined with high external fields permits to investigate in important regimes of typical materials the details of magnetic correlations in time and space. Existing neutron scattering facilities allow experiments in magnetic fields up to a maximum of  17 T which is sufficient to open a glance at the potential impact of varying this parameter but  on the other hand excludes qualitatively important phenomena. 

Experiments in high magnetic fields have opened the door to some of the most exciting areas of present condensed matter physics: Magnetic fields in the range up to 30 T have led to the discovery of unexpected quantum phenomena of fundamental importance: In 2D, i.e. layered semiconductors the Quantum Hall Effect[1] and the Fractional Quantum Hall Effect[2] have been discovered in transport measurements, in magnetic materials Bose-Einstein condensation has been found to occur by neutron scattering experiments[3,4]. These discoveries indicate the emergence and rapid development of a new field ‘Condensed Matter in High Magnetic Fields’, important for both fundamental and applied science. Research in this field is bringing up an increasing number of questions which require experiments combining neutron scattering and magnetic fields above 30 T. In the following we list 4 typical examples from the areas Macroscopic Quantum Phenomena in Magnetism, Correlated Electrons, Heavy Fermions  and Magnetoelasticity in High Tc -Materials,  to demonstrate the expectations and the potential impact of the proposed High Field Neutron Instrument:

High-TC Superconductivity and Antiferromagnetism:  

Do “Magnetism and Superconductivity fight for control in High-TC Superconductors”? This is a major unresolved question of condensed matter physics emphasized by recent neutron scattering experiments up to 14.5T in LaSrCuO4[5-7]. The proposed facility can answer this question: It provides the required energy scale of magnetic fields combined with neutrons as the unique probe to detect magnetic correlations. In optimum doped  LaSrCuO4, the relevant energy scale of the strongly correlated electron system can be matched by magnetic fields above 30T. Thus it will be possible to explore magnetic field regions where a new quantum state with simultaneous superconductivity and long-range magnetic order has been predicted – a key experiment to understand high-TC superconductivity.

Tuning through Gapless and Gapped Quantum Phases of a Two-dimensional Spin Liquid:

The collective dynamics of complex two-dimensional spin systems, determined by frustration and strong quantum correlations, is a fundamental problem of quantum magnetism. SrCu2(BO3)2 displays a sequence of highly frustrated correlated quantum phases in magnetic fields above 22T[8] and serves as a unique model system to meet this challenge. The phase transitions are indicated by magnetization plateaus up to 50T and are believed to correspond to transitions between itinerant (gapless) and localised (gapped) excitations. The study of the excitation spectrum by inelastic neutron scattering in fields above 22T and up to 40T will provide the obligatory microscopic information to understand a prototype material dominated by strong two-dimensional quantum behaviour.

Magnetic Moment Instabilities - Formation of Magnetic Moments in a Metallic Matrix in High Magnetic Fields

How do magnetic moments form in a metal? What are the relevant interactions causing the unusual properties of heavy-fermion and non-Fermi-liquid systems? High magnetic fields are required to answer these longstanding but yet unresolved questions: A magnetic field lifts the screening of local moments by conduction electrons and Zeeman-splits local states. This leads to a multitude of magnetic phenomena and finally to a restoration of ordinary Fermi-liquid behavior in fields of 20-40 T. Recent bulk measurements in URu2Si2  suggest a re-entrant appearance of the hidden-order phase between 36 and 39 T before reaching the field-enforced ferromagnetic state with localized 5f electrons[9]. The unique combination of neutrons and high magnetic fields provides the key for new insights into the microscopic foundations for the properties of this model system and will clarify the microscopic difference between the ground state and the reentrant phase and the nature of the ordering.

Surprising Magneto elastic Effects in Lightly doped  La2-xSr xCuO4

When a magnetic material is placed in an external field the rearrangements of the magnetic moments along the field direction cause a change in the dimensions of the unit cell. Such magnetostriction is not expected in materials with low magnetic susceptibility such as lightly doped La2-xSrxCuO4.  However, twinned orthorhombic single crystals of this material in a field of 14 T at 300 K are found to be surprisingly de-twinned. There is no theoretical model providing a mechanism of magneto elastic coupling in this or any other material with the susceptibility of paper. But, it is intuitively thought that the applied field reduces the orthorhombic distortion and poles the b-axis of the material along the field direction[10,11]. To understand this surprising and counterintuitive behavior, high quality crystallographic data at high fields are needed.

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