Dimensional reduction by pressure in the magnetic framework material ${\mathrm{CuF}}_{2}{({\mathrm{D}}_{2}\mathrm{O})}_{2}$(pyz): From spin-wave to spinon excitations

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Dimensional reduction by pressure in the magnetic framework material ${CuF}_{2} {(D_{2} O)}_{2}$ (pyz): From spin-wave to spinon excitations

M. Skoulatos, M. Månsson, C. Fiolka, K. W. Krämer, J. Schefer, J. S. White, and Ch. Rüegg

Phys. Rev. B 96, 020414(R) – Published 28 July 2017

Abstract

Metal organic magnets have enormous potential to host a variety of electronic and magnetic phases that originate from a strong interplay between the spin, orbital, and lattice degrees of freedom. We control this interplay in the quantum magnet ${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ by using high pressure to drive the system through structural and magnetic phase transitions. Using neutron scattering, we show that the low pressure state, which hosts a two-dimensional square lattice with spin-wave excitations and a dominant exchange coupling of 0.89 meV, transforms at high pressure into a one-dimensional spin chain hallmarked by a spinon continuum and a reduced exchange interaction of 0.43 meV. This direct microscopic observation of a magnetic dimensional crossover as a function of pressure opens up new possibilities for studying the evolution of fractionalised excitations in low-dimensional quantum magnets and eventually pressure-controlled metal–insulator transitions.

Received 23 December 2016
Revised 14 July 2017

DOI:https://doi.org/10.1103/PhysRevB.96.020414

Physics Subject Headings (PhySH)

Magnetic interactions Magnetic order Magnetic phase transitions Magnetism Orbital order

Magnetic systems

Spin lattice models

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. Skoulatos^1,2, M. Månsson^2,3,4, C. Fiolka⁵, K. W. Krämer⁵, J. Schefer², J. S. White^2,3, and Ch. Rüegg^2,6

¹Heinz Maier-Leibnitz Zentrum (MLZ) and Physics Department E21, Technische Universität München, D-85748 Garching, Germany
²Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH–5232 Villigen, Switzerland
³Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne, Station 3, CH-1015 Lausanne, Switzerland
⁴Department of Materials and Nanophysics, KTH Royal Institute of Technology, SE-164 40 Kista, Sweden
⁵Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
⁶Department of Quantum Matter Physics, University of Geneva, CH-1211 Geneva 4, Switzerland

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Vol. 96, Iss. 2 — 1 July 2017

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Figure 1
${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ crystal structure. (a) The $α$ phase (ambient $P$ ) hosts 2D magnetic interactions on a slightly distorted square lattice. (b) The magnetic interactions switch to 1D in the $β$ phase ( $P =$ 6.1 kbar). The brown rhombi denote the half-occupied magnetic orbital of ${Cu}^{2 +}$ .
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Figure 2
Spin-wave excitations and magnetic order parameter of ${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ at $P = 0$ kbar [(a) and (b)] and at $P = 3.6$ kbar [(c) and(d)]. (a) and (c) show typical 2D spin-wave dispersions obtained at 1.5 K, with the data fitted using linear spin-wave theory. The solid (dashed) line are fits to the $Q_{l}$ ( $Q_{h}$ ) dispersion. (b) and (d) show the $T$ dependence of the Bragg peak intensity at $Q = (1 / 2 0 0)$ , with a power-law fit for the low- $T$ region (see text).
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Figure 3
INS data from ${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ at all measured pressures and 1.5 K. (a)–(c) $Q$ scans at 0.5, 1, and 1.5 meV energy transfers, respectively. (d) shows energy-transfer scans at $Q = (0.5 0 - 0.7)$ . The sharp spin-wave modes completely disappear at $P = 6.1$ kbar [open points in (b)]. Gaussian fits are used throughout (solid lines). The feature denoted as “BG” in (b) for $P > 0$ is due to background from the pressure cell. It clearly disappears at $P = 0$ , where the cell was not in use. Note that the data of 0 and 3.6 kbar belong to the same phase (square lattice) and this serves as an independent criterion for this background.
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Figure 4
Magnetic INS intensity maps of the spectrum from ${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ in the high- $P$ phase. (a) shows the theoretical spinon dynamic structure factor [27]. (b) shows the experimentally measured spectrum constructed from individual energy-transfer scans equally spaced in $Q$ . The dashed lines correspond to the lower and upper spinon boundaries according to the “des Cloizeaux-Pearson” analytical result [29].
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Figure 5
Excitation spectra of ${CuF}_{2}$ ( $D_{2} O$ ) $_{2} (pyz)$ in the 1D state. (a)–(c) show energy scans along the spin-chain direction $Q_{h}$ . The solid lines are fits according to a theoretical spinon model [27]. (d) shows the spectrum to display no $Q_{l}$ dependence. The scans in all panels are obtained after subtracting an energy scan at $Q_{h} = 0$ where no magnetic signal is expected (see Fig. 4). Despite this background subtraction, the signal denoted “BG” in (a) for $Q_{L} = - 0.25$ is assigned to a parasitic, temperature-independent scattering process from the pressure cell as explained in the caption of Fig. 3. (a) includes an extra data set at a different $Q_{L}$ value (= $- 0.8$ ) with better conditions achieved, resulting to this background being absent.
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Physical Review B

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Dimensional reduction by pressure in the magnetic framework material ${CuF}_{2} {(D_{2} O)}_{2}$ (pyz): From spin-wave to spinon excitations

M. Skoulatos, M. Månsson, C. Fiolka, K. W. Krämer, J. Schefer, J. S. White, and Ch. Rüegg

Phys. Rev. B 96, 020414(R) – Published 28 July 2017

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Physical Review B

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Dimensional reduction by pressure in the magnetic framework material CuF2(D2O)2(pyz): From spin-wave to spinon excitations

M. Skoulatos, M. Månsson, C. Fiolka, K. W. Krämer, J. Schefer, J. S. White, and Ch. Rüegg

Phys. Rev. B 96, 020414(R) – Published 28 July 2017

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Dimensional reduction by pressure in the magnetic framework material ${CuF}_{2} {(D_{2} O)}_{2}$ (pyz): From spin-wave to spinon excitations