SCIENCE CHINA Physics, Mechanics & Astronomy. Charge density wave transition in Na 2 Ti 2 Sb 2 O probed by 23 Na NMR

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1 SCIENCE CHINA Physics, Mechanics & Astronomy Article December 2013 Vol.56 No.12: th Anniversary for the Institute of Physics, Chinese Academy of Sciences doi: /s Charge density wave transition in Na 2 Ti 2 Sb 2 O probed by 23 Na NMR FAN GuoZhi, ZHANG Xu, SHI YouGuo & LUO JianLin * Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing , China Received September 10, 2013; accepted November 4, 2013; published online November 14, 2013 Herein we investigated the electronic properties of layered transition-metal oxides Na 2 Ti 2 Sb 2 O by 23 Na nuclear magnetic resonance (NMR) measurement. The resistivity, susceptibility and specific heat measurements show a phase transition at approximately 114 K (T A ). No splitting or broadening in the central line of 23 Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected. The spin-lattice relaxation rate divided by T (1/T 1 T) shows a sharp drop at about 110 K which suggests a gap opening behavior. Below the phase transition temperature zone, 1/T 1 T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states (DOS) because of the gap. No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature. These results suggest a commensurate charge-density-wave (CDW) phase transition occurring. layered transition-metal oxides, charge-density-waves, NMR PACS number(s): Dd, Lr, k Citation: Fan G Z, Zhang X, Shi Y G, et al. Charge density wave transition in Na 2 Ti 2 Sb 2 O probed by 23 Na NMR. Sci China-Phys Mech Astron, 2013, 56: , doi: /s Introduction The discovery of high-temperature superconductivity in iron based compounds has stimulated extensive studies on layered transition metal oxides, such as pnictide oxides and oxychalcogenides. These transition metal compounds often have complex orbits and unexpected electronic structures, which assume different charge and magnetic ordering (e.g., superconductivity, anti- and ferromagnetism, etc). For their low dimensional characteristic, these electronic structures are leading to Fermi surface (FS) nesting frequently, resulting in CDW or spin-density-wave (SDW) formation [1,2]. With the change of some parameters e.g., carriers doping, some types of ordering is suppressed gradually. To some extend the superconductivity is emerging as the case in iron *Corresponding author ( jlluo@aphy.iphy.ac.cn) based superconductors. The competition and mutual interaction of these different orderings in these systems have been continuously researched because of their novel physics and are believed to be key factors in understanding superconductivity mechanism. Except for iron compounds, some iron-free layered titanium pnictideoxides such as BaTi 2 Sb 2 O and Na 2 Ti 2 Sb 2 O (which can be represented by the general formula ATi 2 Pn 2 O, where A = Ba, Na 2, (SrF) 2, (SmO) 2 and Pn = As or Sb) have attracted much research focus [3 5]. On the basis of the intrinsic similarity with cuprate and iron pnictide superconductors, these pnictide oxides have been proposed as possible candidates for new superconductors [6,7]. This class of compounds is built from alternatively stacking special octahedral Ti 2 Pn 2 O layers which are similar but anti to CuO 2 layers in the superconducting cuprates with the O atoms placed at the center of the Ti square and two Pn atoms respectively located above and below the center of Science China Press and Springer-Verlag Berlin Heidelberg 2013 phys.scichina.com

2 2400 Fan G Z, et al. Sci China-Phys Mech Astron December (2013) Vol. 56 No. 12 each Ti 2 O square unit [8]. In previous reports, both Na 2 Ti 2 Sb 2 O and Na 2 Ti 2 As 2 O exhibit sharp drop in magnetic susceptibility and an anomaly in the electrical resistivity at about K and 320 K [6,8 10], respectively. Barium compounds BaTi 2 Sb 2 O [3,4] and BaTi 2 As 2 O [11] also show similar behavior. The superconductivity in BaTi 2 Sb 2 O with a superconducting transition temperature T C of 1.2 K [3] and Ba 1 x Na x Ti 2 Sb 2 O (0.05 x 0.33) with a maximum T C of 5.5 K [4] has been reported. Recently, the newest superconductors BaTi 2 Bi 2 O [12], BaTi 2 (Sb 1 x Bi x ) 2 O [13], and BaTi 2 (Sb 1 x Sn x ) 2 O [14] was discovered, for which the highest T C is approximately 4.6 K. The mechanism of these transitions is still unclear. It has been suggested that these anomalies arise from the SDW or CDW instability induced by FS nesting [15 17]. Singh et al. [18] identified a frustrated layered Heisenberg model, which could have a two-dimensional spin-liquid ground state. From first-principles calculations, Yan et al. [19] proposed SDW shows bi-collinear or blocked checkerboard antiferromagnetic (AF) states induced by FS nesting. However, early powder neutron diffraction studies of Na 2 Ti 2 Sb 2 O did not find any magnetic order, but detected a structural distortion in the Ti 2 Sb 2 O layer at ~120 K corresponding to the observed anomaly [20]. 121/123 Sb nuclear quadrupole resonance (NQR) measurements on BaTi 2 Sb 2 O indicate a breaking of an in-plane four-fold symmetry at the Sb site occurring below about 120 K without any internal field appearing at the Sb site, which was understood by a commensurate CDW transition [21]. To elucidate the mechanism of superconductivity, Singh [17] proposed the possibility of unconventional spin-fluctuation mediated superconductivity with sign-changing s-wave state. Subedi [22] proposed a total electron-phonon coupling is sufficiently strong to yield a conventional superconductivity. In the present study, we report a microscopic investigation by 23 Na nuclear magnetic resonance (NMR) on the layered titanium pnictideoxides Na 2 Ti 2 Sb 2 O. The measurements of 23 Na NMR spectra of spin-echo signal and spinlattice relaxation time T 1 is performed. We show that the phase transition does not induce alterations in the NMR spectra. There is no signature of additional freezing of spin degree of freedom. The temperature dependence of 1/T 1 T deviates from a Fermi-liquid behavior at about 110 K, but no peak appears in the whole temperature field which indicates no enhancement of spin fluctuations. The acute drop of 1/T 1 T indicates a gap opening behavior arising. Except phase transition temperature zone, 1/T 1 T remains constant with different value at lower or higher temperature. Combining with spectral measurement, a commensurate CDW phase transition should arise at about T A. 2 Experiments Plate-like single crystals of Na 2 Ti 2 Sb 2 O were synthesized from the NaSb flux method. The resistivity was measured using standard 4-probe method, and the specific heat was measured using relaxation method. Both measurements were performed in a Physical Property Measurement System of Quantum Design company. The susceptibility was measured in a SQUID-VSM. To perform NMR measurement, a piece of plate-like single crystal with the size 4 mm 2.5 mm 0.4 mm was used. The sample was placed in a rectangular shaped copper coil for a better filling factor with the magnetic field B ext applied along the ab plane. 23 Na NMR (nuclear spin I = 3/2, gyromagnetic ratio = MHz/T, 100% abundance) measurements were carried out using Thamway NMR spectrometer in a magnetic field of T. The NMR spectrum was obtained by summing the Fourier transform of spinecho signals following a /2 pulse sequence. The spin-lattice relaxation rate is estimated by fitting the recovery curves of the integrated spin-echo intensity measured by a conventional saturation-recovery method. 3 Results and discussion Figure 1 shows the resistivity, susceptibility and specific heat as a function of temperature between 2 K and 300 K. The in-plane resistivity exhibits an anomaly at about 114 K. Away from this temperature zone it follows metallic behavior either at lower or higher temperature. From Figure 1(b), the temperature dependence of specific heat (C p ) shows a distinct peak at the same temperature. In lowtemperature region below 11 K, the specific heat follows the form C p = T+ T 3. From the plot of C p /T versus T 2 as shown in the inset of Figure 1(b), the Sommerfeld coefficient is estimated to be 4.1 mj/mol K 2. Either the specific heat data around 114 K or that at low temperatures is not affected by external field of 70 koe (see the inset of Figure 1(b). The magnetic susceptibility measured at 50 koe with H//ab also exhibits a sharp drop at approximately 114 K corresponding with the anomalies in the resistivity and specific heat. These behaviors are similar to what has been observed in ironbased oxypnictide parent compound LaFeAsO in which the abnormal behavior is associated with the structure transition and SDW order [23]. As has been mentioned, these anomalies at 114 K for Na 2 Ti 2 Sb 2 O are likely because of a CDW/ SDW transition. Same batch samples were measured by infrared optical spectroscopy which revealed significant spectral change across the phase transition and formation of a density-wave type energy gap that removed most part of the free carrier spectral weight and caused a substantial reduction of the carrier scattering rate [24]. 23 Na spin-echo spectra at 100 K and 140 K are shown in Figure 2. Note that both spectra are fairly sharp with a full width at half maximum 21.4 khz at 100 K and 21 khz at 140 K, respectively obtained by Gauss function. The knight shift almost remains unchanged across T A with the value of

3 Fan G Z, et al. Sci China-Phys Mech Astron December (2013) Vol. 56 No about 0.102%. The absence of apparent temperature dependence of the Knight shift above and below T A indicates that the compound is basically non-magnetic. If any magnetic order exits, the 23 Na nuclei will experience a magnetic field which is the sum of the applied external magnetic field and the internal field originated from the ordered moments. For an AF order, the NMR spectrum below T A should exhibit splitting or broadening because of the internal field. For a ferromagnetic order, the position of resonance peak should move to a lower external field. The present results indicate that no detectable internal field is produced by magnetic order either below or above T A. There is other possibility that internal field may not be detected if the ordered moment is small or have not sufficient influence on Na nuclei. Regarding Na atoms located between the Ti 2 Sb 2 O layers by an ionic interaction, the shape of the NMR spectral may indicate that the Na site is insensitive to the electronic state which is affected by CDW or SDW modulation. For the present, the phase transition at T A should be contributed to CDW transition. The symmetric, sharp spectral shape below T A indicates that charge densities at the Ti sites should have a commensurate correlation, since there would be several Sb sites induced below T A if a CDW ordering possesses an incommensurate correlation. The spin-lattice relaxation rate can give more information about this transition. To obtain the spin-lattice relaxation time, the nuclear magnetization recovery curve was fitted by the following theoretical function as expected for the central line of the spectrum of the nuclear spin I = 3/2 of 23 Na nucleus, 1 M(t)/M 0 = 0.9exp( 6t/T 1 ) + 0.1exp( t/t 1 ), (1) Figure 1 Physical property measurements of Na 2 Ti 2 Sb 2 O single crystal performed at K. (a) In-plane resistivity with an abrupt slope at about 114 K which exhibits a metal-metal phase transition. (b) Temperature dependence of specific heat with a sharp peak at about 114 K. Inset, the plot of C p /T versus T 2 between 2 and 11 K from which the Sommerfeld coefficient is estimated to be 4.1 mj/mol K 2. (c) In-plane magnetic susceptibility at the field of 70 koe with a sharp drop around 114 K. Figure 2 (Color online) 23 Na NMR spectra obtained by summing the Fourier transform of spin-echo signals measured in the field of T at 100 K and 140 K, respectively. Resonance frequency of the peak is almost same for the two spectra. Both spectra are fairly sharp with no splitting appearing. where M(t) is nuclear magnetization after a time t from the NMR saturation pulse and M 0 is thermal equilibrium magnetization. Figure 3 shows the temperature dependence of 1/T 1 T in the temperature range of K. A deviation from a simple Fermi-liquid behavior occurs at about 110 K which agrees with the anomaly observed by the measurements of transport property. The abrupt decrease of 1/T 1 T below 120 K indicates the suppression of the low energy spin excitations with the decreasing temperature which is reminiscent of a gap opening behavior corresponding with the infrared optical measurement [24]. Either above or under this temperature range, 1/T 1 T obeys the Korringa relation for canonical Fermi liquid, that is, 1/(T 1 TK 2 ) = constant. The constant behavior of 1/T 1 T below T A suggests the system is in a renormalized Fermi liquid state after the phase transition. The result is consistent with the resistivity measurement that suggests a metal-metal transition. These behaviors are related to the structural nature of the FS which induces a partial energy gap to be opened at the Fermi energy because of FS nesting. If hyperfine interactions with the relevant nuclear spins are limited to the Fermi contact interaction as the case in normal metal, the value of 1/T 1 T will be proportional

4 2402 Fan G Z, et al. Sci China-Phys Mech Astron December (2013) Vol. 56 No. 12 where A(q) is the wave-vector q-dependent hyperfine form factor, (q, f ) is the imaginary part of the dynamical electron spin susceptibility (spin fluctuations), and f is the NMR frequency. The magnetic dipole T 1 processes are divided broadly into those driven by spin and orbital hyperfine field. In the case of SDW phase transition, 1/T 1 T increases toward T SDW because of the enhancement of spin fluctuations, and exhibits a peak consistent with a critical slowing down of spin fluctuations. Specifically, one may assume 1/T 1 T = (1/T 1 T) AF + (1/T 1 T) 0, where (1/T 1 T) AF is contributed to the susceptibility at the AF wave vector q, and (1/T 1 T) 0 is contributed to s-band electrons and the orbital hyperfine interaction [25]. No increase or peak appears in our measurement. Our result is quite different from Ba(Fe 1 x Co x ) 2 As 2 [26] and LaFeAs(O 1 x F x ) [27] where a dramatic enhancement of the 75 As 1/T 1 T was observed when system undergoes a SDW transition. Kitagawa et al. [28] showed that hyperfine form factor A = 0 for commensurate spin fluctuation because of cancelation of the transferred hyperfine fields, thus the enhancement of 1/T 1 T cannot occur. Considering the compound structure, this is seldom possible in our experiment. In the case of CDW, there is often a weak or no anomaly at T A [29,30]. By combining spectral measurement, the anomaly of 1/T 1 T at T A is likely attributed to CDW transition. Note that the relaxation rate is determined by the component of the dynamic susceptibility perpendicular to the effective field. Our measurement was done in the direction of B ext ab. This should exclude the possibility of the spin fluctuation in c direction. To make clear spin fluctuation in other directions, anisotropic measurement is needed. 4 Conclusions Figure 3 1/T 1 T plotted as a function of temperature shows a sharp drop at about 110 K. It follows Fermi liquid behavior either lower or higher than T A. Loss of DOS about 53% at low temperature is estimated by the relationship that 1/T 1 T is proportional to the square of DOS at FS. to the square of DOS at FS. Thus DOS below phase transition temperature is decreased approximately 53% estimated by the value of 1/T 1 T. The result is not as large as that measured by infrared optical spectrum where about 95% of carriers are removed [24], but much more than that of 121/123 Sb NQR measurement where the decrease of DOS is only 9% [21]. Next let us consider the possibility of CDW/SDW phase transition from 1/T 1 T. Generally, 1/T 1 T can be expressed as: 1 2 ( q, f ) A( ), TT q q (2) f 1 To summarize, we have reported the 23 Na NMR investigation of the layered titanium pnictideoxides Na 2 Ti 2 Sb 2 O with B ext ab. The phase transition does not change NMR spectra structure. No magnetic order momentum is detected. The temperature dependence of 1/T 1 T shows a sharp drop suggesting a metal-metal transition with a gap opening behavior at the phase transition temperature. The loss of DOS at low temperature is estimated about 53%. Our results show evidence of the presence of a commensurate CDW phase transition. The authors are grateful to WANG NanLin for helpful discussions and LI Zheng for experimental help. This work was supported by the National Natural Science Foundation of China (Grant No ) and the National Basic Research Program of China (Grant No. 2011CB921701). 1 Wilson J A, Di Salvo F J, Mahajan S, et al. 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