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arXiv:1609.00438 · Physical review. B./Physical review. B

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in the dimerized quantum magnet Ba 2 CuSi 2 O 6 Cl 2

Makiko Okada, Hidekazu Tanaka, Nobuyuki Kurita, Kohei Johmoto, Hidehiro Uekusa, Atsushi Miyake, Masashi Tokunaga, Satoshi Nishimoto, Masaaki Nakamura, Marcelo Jaime, Guillaume Radtke, Andrés Saúl

Published Sep 20, 2016• 18 citations
arXivHTMLPDF

Abstract

We synthesized single crystals of composition ${\mathrm{Ba}}_{2}{\mathrm{CuSi}}_{2}{\mathrm{O}}_{6}{\mathrm{Cl}}_{2}$ and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet ${\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}$ also known as Han purple. ${\mathrm{Ba}}_{2}{\mathrm{CuSi}}_{2}{\mathrm{O}}_{6}{\mathrm{Cl}}_{2}$ has a singlet ground state with an excitation gap of $\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}=20.8$ K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the $g$ factor except for a small jump anomaly for a magnetic field perpendicular to the $c$ axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in ${\mathrm{Ba}}_{2}{\mathrm{CuSi}}_{2}{\mathrm{O}}_{6}{\mathrm{Cl}}_{2}$ is described as the quasi-2D Bose-Einstein condensation of triplets.

DOI
10.1103/physrevb.94.094421

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Han Purple, a pigment first obtained in ancient China, is one of the earliest known synthetic pigments. Also naturally occurring, as a mineral, it is known as colinowensite (Cwn). Its chemical formula is BaCuSi2_{2}2​O6_{6}6​, and its structure is a layered cyclosilicate in which magnetic Cu2+^{2+}2+ ions (S = 1/2) form dimers arranged on a square lattice, making it also the first known synthetic metal dimer compound. Most interesting magnetic properties arise from a strong intradimer spin coupling, accompanied by weaker interdimer interactions within and between Cu-dimer layers. In zero or small magnetic fields, BaCuSi2_{2}2​O6_{6}6​ remains a quantum paramagnet. However, under high magnetic fields between 23 and 49 Tesla -- about a million times stronger than Earth's magnetic field -- it undergoes magnetic ordering at subliquid Helium temperatures into an almost ideal easy-plane (XY) antiferromagnetic state regarded as a realization of a Bose-Einstein condensate of magnons. Within this experimentally accessible field range, BaCuSi2_{2}2​O6_{6}6​ serves as an extraordinary playground for testing predictions of quantum many-body physics.
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