Neutron scattering studies and crystal growth on strongly correlated electronic systems

Our research focuses on using neutron scattering technique to explore the exotic properties of the strongly correlated electronic systems. Our neutron scattering experiments are carried out at the sources all over the world (see some representative sources in Fig. 1). At the campus, we maintain a sample synthesis and characterization lab. A poster describing our research can be found here.

Fig. 1, some representative neutron sources around the world.

1, Neutron scattering

What we "see" is ultimately limited by the wavelength of the media. To probe a microscopic object, we have to use something with comparable scale. Some candidates are electrons (charged particles), photons (electromagnetic wave), and neutrons. Using neutrons is analogue to using electrons and photons, as illustrated in Fig.2, and the basic principle of neutron scattering is similar as x-ray scattering. Neutron is a neutral particle with a spin 1/2. They interact with the nuclei via short-range nuclear force. Furthermore, since neutron has a spin, it can interact with the magnetic moment of the unpaired electron through dipole-dipole interaction. Because of these, they have the following advantages:

A, their wavelength in the order of angstrom is comparable with the lattice size; the energy in the order of meV is suitable to probe the elementary excitations such as phonons and spin waves which have similar energy scale.

B, since neutrons do not carry charge and interact with electrons, they can penetrate deeply into the sample. This makes probing the bulk properties under controllable sample environments possible. It also avoids the constraint on the form of the sample. For instance, liquid can be measured in an aluminum container which neutrons can easily pass through.

C, neutron scattering is the most powerful tool exploring the magnetic structure and excitations because of the spins they carry with.

D, the scattering intensity does not depend on the number of electrons but on the scattering length instead, which makes them useful identifying light elements and distinguishing isotope.

Fig. 2, schematic of electron, photon and neutron interacting with matter.

With their unique properties, neutron scattering is a must-have technique in many areas such as engineering, biomedical science, energy research, basic science, etc.

2, Crystal growth

Neutron scattering requires large samples in the order of gram. The interaction cross section of neutrons with the sample is small; neutron flux from the source is several orders of magnitude less than that from the photon source. Both result in low scattering intensity. In order to get reliable signals within limited beamtime, one has to increase the sample size. For this reason, we have to have the capability of making large-size single crystals. Typically, the floating-zone and Bridgman technique are used for the crystal growth. Both methods and crystals obtained are shown in Figs. 3 and 4.

                       

Fig. 3, floating-zone technique and La2-xBaxCuO4 crystals obtained.


                              

Fig. 4, Bridgman technique and Fe1+yTe1-xSex crystals obtained.

3, Systems of interests

Unconventional superconductors

Since the discovery of the Cu-based superconductors in 1986, it is generally agreed that the theory developed by Bardeen, Cooper and Schrieffer in 1956 is difficult to explain the unconventional superconductivity. However, there has not been an alternative that is commonly accepted. In these superconductors, including Cu- and Fe-based, and heavy-fermion superconductors, magnetism often has a close interplay with superconductivity. It is thus hoped that by using neutron scattering to study the magnetic interactions one will be able figure out the mechanism underlying the superconductivity.

Quantum spin liquids

Quantum spin liquids (QSLs) are an exotic topological state of matter in which strong quantum fluctuations prevent conventional magnetic order from establishing down to zero temperature. They are closely relevant to the understanding of high-temperature superconductivity, and are promising for quantum computation. However, experimental realizations of these materials and relevant investigations are still rare. We will look for new QSLs materials and use inelastic neutron scattering to identify their magnetic interactions.

Multiferroics

Multiferroics are materials where several ferro-orders, such as ferroelectric, and ferromagnetic / antiferromagnetic / ferrimagnetic orders coexist and couple with each other. Such materials can find applications in high-capacity storage, and multifunctional devices. However, not much has been known on their exotic properties, especially how these orders are linked. Using neutron scattering to study the crystal and magnetic structure, phonons and magnetic excitations will shed lights on these issues.

Relaxor ferroelectrics

Relaxor ferroelectrics are a special class of material that exhibit an enormous electromechanical response and are easily polarized with an external field. These properties make them attractive for applications as sensors and actuators. However, how they possess such properties remains elusive. By carrying out neutron scattering experiments, we are hoping to solve this problem.

Other systems

Examples of some other systems that we are interested in are topological insulators and superconductors, transition metal oxides, and low-dimensional magnets.