Thermodynamic and neutron scattering study of the spin- 1/2 kagome antiferromagnet ZnCu₃(OH)₆Cl₂ : a quantum spin liquid system

New physics, such as a quantum spin liquid, can emerge in systems where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. The realization of a quantum spin liquid in two-dimensions would represent a new state of matter. It is believed that spin liquid physics plays a role in the phenomenon of high-Tc superconductivity, and the topological properties of the spin liquid state may have applications in the field of quantum information. The Zn-paratacamite family, ZnCu₃(OH)₆Cl₂ for x > 0.33, is an ideal system to look for such an exotic state in the form of antiferromagnetic Cu2+ kagome planes. The x = 1 end member, named herbertsmithite, has shown promising spin liquid properties from prior studies on powder samples. Here we show a new synthesis by which high-quality centimeter-sized single crystals of Zn-paratacamite have been produced for the first time. Neutron and synchrotron x-ray diffraction experiments indicate no structural transition down to T = 2 K. The magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured for the x = 1 sample. A small, temperature-dependent anisotropy has been observed, where Xz / Xp > 1 at high temperatures and Xz / Xp < 1 at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal anisotropies for [Theta]cw's and g-factors. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as a primary perturbation to the dominant Heisenberg nearest neighbor interaction. These results have great bearing on the interpretation of theoretical calculations based on the kagome Heisenberg antiferromagnet model to the experiments on ZnCu₃(OH)₆Cl₂. Specific heat measurements down to dilution temperatures and under strong applied magnetic fields show a super linear temperature dependence with a finite linear term. Most importantly, we present neutron scattering measurements of the spin excitations on a large deuterated single crystal sample of herbertsmithite. Our observation of a spinon continuum in a two-dimensional magnet is unprecedented. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Physics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2012

Author dc:creator

• Han, Tianheng, Ph. D. Massachusetts Institute of Technology

Advisor dc:contributor.advisor

• Young S. Lee.

• Physics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/79516

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/79516