Chen Lab - Research

Research

What are we interested in?

QUANTUM MATERIALS

The development of modern technologies in all fields highly relies on the properties of materials. The vast majority of materials’s properties can not be fully understood with only classical physics or a single atom’s behavior. Instead, it involves the understanding of collective motions of excitations with strong correlations using quantum physics, which redefines these materials to be “quantum materials”. Superconductors, quantum spin liquids (QSLs), and topological materials are three types of quantum materials that attract the most interest due to their fascinating physics and potential applications in quantum technologies.

Unconventional Superconductors

Increasing the superconducting transition temperature Tc has been thought the main direction that can promote broader applications for these materials. Fortunately, Tc was dramatically boosted up since the discovery of high-temperature superconductors. However, understanding the pairing mechanism behind these unconventional superconductors, whose properties can not be explained by the Bardeen-Cooper-Schrieffer (BCS) theory, remains the holy grail of quantum materials. Apart from superconductivity, unconventional superconductors also hold other exotic properties, such as quantum criticality, strong correlation, magnetism, pseudogap phase, charge and spin density waves, which form a complicated phase diagram for these materials. The particular question about unconventional superconductors that I would like to focus on is to identify the nodal structure of the superconducting gap by an experimental technique called low-temperature thermal conductivity. The superconducting gap structure is crucial in determining its order parameter.

Quantum Spin liquid

Quantum spin liquids (QSLs) are fascinating materials that possess highly frustrated ground states. The quest for QSLs has attracted tremendous interest due to the potential realization of non-Abelian statistics and novel exotic excitations, which could be used in realizing fault-tolerant quantum computing. Many materials have been considered as candidates for QSLs, but none of them has been experimentally confirmed to be a real QSL. Thus, looking for an experimentally achievable QSL material remains one of the major tasks in the field of quantum materials. The identifications and classifications of QSLs in various kinds of candidate materials often rely on the characterization of fractionalized excitations, such as spinons or Majorana fermions. The major question that I would like to answer about QSLs is to search for spinons by low-temperature thermal conductivity measurements and Majorana fermions by another thermal transport technique, the so-called thermal Hall effect.

Topological Materials

Topological materials are materials that have special topological arrangements in the geometry of their electronic band structure. Topological materials, such as topological insulators, Dirac and Weyl semimetals, and topological superconductors, are potential platforms for achieving the next generation of electronics. The core task of identifying a topological material is to detect its Fermi surface structure. The central problem that I want to tackle is to detect the Fermi surface of topological materials using high-magnetic-field experiments, in particular quantum oscillations and angle-dependent magnetoresistance (ADMR).

What are our experimental tools?

THERMAL TRANSPORT

We use both low-temperature thermal conductivity and thermal Hall conductivity measurements to probe charged and neutral excitations that carry entropy in various quantum materials. The thermal Hall effect is the thermal analog of the electrical Hall effect. Instead of a transverse voltage induced by a perpendicular magnetic field in the presence of an electric current, a transverse temperature difference is induced in the presence of a heat current.

Torque differential magnetometry

A highly sensitive torque differential magnetometry using the qPlus mode of a miniature-sized quartz tuning fork has been developed to measure small magnetic signals down to a dilution refrigerator temperature of 25 mK and a strong magnetic field up to 65 T. We have successfully applied this technique under vacuum, in a liquid helium environment, in a DC or pulsed magnetic field.

Electrical transport

Angle-dependent magnetoresistance (ADMR) can be used to map a Fermi surface when quantum oscillations are not detected, e.g. because of low sample quality. ADMR refers to the change of the out-of-plane resistivity, i.e. c-axis resistivity, when an external magnetic field is applied and rotated away from the crystal c-axis towards the ab plane. As a semiclassical effect, the topology of the Fermi surface will translate into the trajectory of charge carriers, which in turn affects the electrical conductivity of materials in an external magnetic field.

thermoelectric

The thermoelectric measurements include Seebeck and Nernst effects. Seebeck effect describes a longitudinal voltage generated by a longitudinal thermal gradient Vth. Nernst effect is the appearance of a transverse voltage VHall when a longitudinal thermal gradient and perpendicular magnetic field are applied.

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