RIKEN Accel. Prog. Rep. 50 (2017)

Ⅲ-2. Atomic & Solid State Physics (Muon)

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Y. S. Choi, 䠆1 S.-H. Do,䠆1 Dita Puspita Sari,䠆2,䠆3 I. Watanabe,䠆2 and K.-Y. Choi䠆1, 䠆2
Quantum spin ladders consisting of leg and rungcouplings offer an outstanding opportunity to investigate
quantum-critical spin dynamics and have far-reaching
relevance to diverse fields of physics such as
Tomonaga-Luttinger liquids, magnon fractionalization,unconventional
superconductivity,andquark1)-3). Isolated two-leg ladders have a short-rangeconﬁnement.1)-3)
confinementstate.4)4). With growing interladderresonating valence bond state
couplings, a quantum phase transition is anticipated tostate.5)5).
occur to the magnetically ordered state
Ba2CuTeO6 is a prime candidate material for athree-dimensionally networked spin ladder, allowing
addressing quantum criticality in coupled two-leg ladders.
Ba2CuTeO6 features both a long-range ordering at TN =15 K
and the spin-gap excitation of Ʀ=50 K at finite6)-7)
temperatures.
. However,However, the magnetic transition istemperatures6)-7)
largely hidden, while showing no magnetic Bragg peaks
and no apparent Ȝ-like anomaly in the specific heat. Thus, it
is highly desired to identify the occurrence of the static
magnetic ordering. To resolve these issues, we performed
zero-field ȝSR experiments on the ARGUS spectrometer of
RIKEN-RAL. The collected data were analyzed using thesoftware package WiMDA.

Fig. 1. (a) Representative data of the muon polarization ofBa2CuTeO6 measured above and below TN. The solid lines
are fits described in the text. The inset zooms the early-time
behavior at various temperatures. The spectra are verticallyshifted. (b),(c),(d) The asymmetry, the muon spin
precession frequency fȝ, and the transverse relaxation rateȜT as a function of temperature. The vertical bar indicatesthe onset of magnetic ordering at TN =14.1 K.

The time decay of the muon spin polarization P(t) at
temperatures above and below TN is shown in Fig. 1(a).
Upon cooling towards TN, we observe muon-spin
precession together with a drop in the early-time asymmetry[see the inset of Fig. 1(a)], confirming the development ofstatic local magnetic fields at the muon stopping sites. The
polarization curves can be well described by the sum of an
exponentially relaxing cosine function and a simple
exponential function:
P(t)=(1-Į H[S -ȜLW ĮH[S -ȜTW FRV ʌfȝt+ࢥ),
where the two terms represent muons polarized transverseand parallel to the local magnetic fields. The temperaturedependence of the asymmetry, the muon-spin precession
frequency fȝ, and the transverse relaxation rate ȜT is plottedin Fig. 1(b)-(d). All ȝSR parameters display distinct
changes at TN. The initial asymmetry drops rapidly on
cooling to TN. The missing asymmetry is ascribed to anunresolved precession signal within the pulsed muon beamtime window.
fȝ(T), corresponding to the magnetic order parameter, isfitted to the phenomenological form fȝ(T)= f0(1-(T/TN))ȕ,f0=4.3 MHz is the frequency at T=0 K and ȕ=0.29(1) is thecritical exponent. The obtained critical exponent is notmuch different from the value ȕ=0.365, expected for the 3D
Heisenberg model. TN =14.1 K is slightly lower than thetransition temperature of 15 K determined from the uniform
susceptibility. The temperature dependence of ȜT(T) can bealso modeled with the same order-parameter fit as plotted inFig. 1(d). Taken together, a ground state of Ba2CuTeO6 ischaracterized by a conventional antiferromagnetic order,
while having persisting spin fluctuations in the orderedstate.
In this report, we have presented a combined study ofZF-ȝSR measurements on the coupled two-leg spin ladder
Ba2CuTeO6. We observe unambiguously an oscillating
signal in the ZF-ȝSR time spectra, suggesting that
Ba2CuTeO6 lies close to a quantum critical point from amagnetically ordered side.
References
1) S. Maekawa: Science 273, 1515 (1996).
2) B. Lake et al.: Nature Phys. 6, 50 (2009).
3) M. Jeong et al.: Phys. Rev. Lett. 111, 106404 (2013).
4) S. R. White et al.: Phys. Rev. Lett. 73, 886 (1994).
5) B. Normand and T. M. Rice, Phys. Rev. B 54, 7180 (1996).
6) A. S. Gibbs et al.: arXiv:1511.01477 (2015).
7) G. Narsinga Rao et al.: Phys. Rev. B 93, 104401 (2016).

䠆1

Department of Physics, Chung-Ang University
RIKEN Nishina Center
䠆3
Department of Physics, Osaka University
䠆2- 234 -