Scientists at the London Centre for Nanotechnology report a major breakthrough in the study of low-dimensional quantum systems.
Quantum effects in low-dimensional magnets have been the subject of intense research for many decades dating right back to the birth of quantum mechanics itself. This is due to the fact that magnetic systems are particularly amenable to theoretical description and experimentation, allowing the testing of fundamental concepts, and from the fact that the properties of low-dimensional magnets are exploited in a raft of technologies.
Of particular interest are the intriguing properties of one-dimensional (1D) systems such as magnetic ladders – literally a magnetic analogue of a step ladder – in which the magnetic moments carried by individual atoms are coupled together through rungs and legs. In 1D, long-range magnetic order is destroyed by quantum fluctuations, and theory predicts that instead a particular kind of exotic magnetic quantum liquid forms, known as a Luttinger liquid (LL). When a magnetic field is applied, this fascinating state of quantum matter becomes a key component of the extraordinary rich phase diagram of the ladder and can be studied using extremely sensitive magnetometers in the laboratory and high-resolution neutron spectroscopy. In the presence of weak magnetic links between ladders, the system can even display Bose-Einstein condensation, which underpins the remarkable properties of superfluids and superconductors.
Figure: Magnetic field vs. temperature phase diagram of the spin-ladder compound (C5H12N)2CuBr4, showing quantum disordered (QD), quantum critical (QC), and spin Luttinger liquid (LL) phases. Quantum phase transitions occur at Bc and Bs. Inset: lattice structure of (C5H12N)2CuBr4 in projection along the b-axis, with Cu atoms blue and Br white.
In two recent publications in Physical Review Letters, and one in Physical Review: Rapid Communications, an international research team led by Dr. Christian Ruegg from the LCN has reported the first comprehensive study of a magnetic quantum ladder over its entire phase diagram (see, for example, the Figure below). This work provides an unprecedented insight into quantum effects and their microscopic control in low dimensional magnets, and was enabled by recent progress in instrumentation, synthetic chemists supplying samples of the highest quality, and most advanced theoretical methods. In the words of Dr. Ruegg, “For many years the arrangement of spins on a ladder-like structure has been the prototypical system for theoretical studies in quantum magnetism, but suitable model materials were lacking, in particular for experiments in a magnetic fields. Well, I love it when a plan comes together”.
Copies of the original articles can be accessed via the links below:
Direct observation of magnon fractionalization in the quantum spin ladder.
B. Thielemann, Ch. Rüegg, H.M. Ronnow, A.M. Läuchli, J.-S. Caux, B. Normand, D. Biner, K.W. Krämer, H.-U. Güdel, J. Stahn, K. Habicht, K. Kiefer, M. Boehm, D.F. McMorrow, and J. Mesot. Phys. Rev. Lett. 102, 107204 (2009).
Field-controlled magnetic order in the quantum spin-ladder system (Hpip)2CuBr4.
B. Thielemann, Ch. Rüegg, K. Kiefer, H.M. Ronnow, B. Normand, P. Bouillot, C. Kollath, E. Orignac, R. Citro, T. Giamarchi, A.M. Läuchli, D. Biner, K.W. Krämer, F. Wolff-Fabris, V.S. Zapf, M. Jaime, J. Stahn, N.B. Christensen, B. Grenier, D.F. McMorrow, and J. Mesot. Phys. Rev. B 79, 020408(R) (2009).
Thermodynamics of the spin Luttinger-liquid in a model ladder material.
Ch. Rüegg, K. Kiefer, B. Thielemann, D.F. McMorrow, V.S. Zapf, B. Normand, M.B. Zvonarev, P. Bouillot, C. Kollath, T. Giamarchi, S. Capponi, D. Poilblanc, D. Biner, and K.W. Krämer. Phys. Rev. Lett. 101, 247202 (2008).