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A magnetic monopole in spin ice is a novel quasiparticle excitation in condensed matter physics, and we found that the ac frequency dependent magnetic susceptibility $\chi(\omega)$ in the two-dimensional (2D) spin ice (so-called kagom\'{e} ice) of Dy$_2$Ti$_2$O$_7$ shows a single scaling form. This behavior can be understood in terms of the dynamical scaling law for 2D Coulomb gas (CG) systems [Phys. Rev. B 90, 144428 (2014)], characterized by the charge correlation length $\xi (\propto1/\sqrt{\omega_1})$, where $\omega_{1}$ is a characteristic frequency proportional to the peak position of the imaginary part of $\chi(\omega)$. It is a generic behavior among a wide variety of models such as the vortex dynamics of 2D superconductors, 2D superfluids, classical XY magnets, and dynamics of melting of Wigner crystals., Comment: 5 pages, 4 figures, Supplemental Material

The magnetization M, magnetic susceptibility M/H, and specific heat C of TmTi2Al20 single crystal have been measured in an external magnetic field along the crystalline principal directions, [001],...

Magnetic susceptibility and torque measurements have been performed for neutral BEDT-TTF crystals to investigate the diamagnetic properties. The diamagnetic susceptibility is found to be anisotropi...

The magnetic properties of the S = 1/2 distorted kagome antiferromagnet CdCu3(OH)6Cl2 have been studied by magnetic susceptibility, heat capacity, and high-field magnetization measurements. The mag...

Electronic nematic states in cuprate high-Tc superconductors are not only ubiquitous but also very diverse. Recently, a B1g-symmetry nematic transition around the pseudogap temperature T* has been ...

We have synthesized polycrystalline samples of Eu2Pt6(Al1−xGax)15 (x = 0, 0.05, 0.1, 0.2, 0.3, and 1) and examined electrical resistivity, magnetic susceptibility, high-field magnetization, and X-r...

We have synthesized single crystals of CePt6Al3 in which Ce atoms form a honeycomb lattice in a trigonal basal plane. The measurements of magnetic susceptibility χ, electrical resistivity ρ, thermo...

Magnetic properties of an Yb-based semiconductor YbCuS2 with a zigzag chain of Yb3+ ions were studied. The Curie–Weiss behavior of the magnetic susceptibility χ(T) indicates antiferromagnetic inter...

Motivated by the recent discovery of Dirac nodal line in the single-component molecular conductor [Pt(dmdt)$_{2}$], we propose a three-orbital tight-binding model based on the Wannier fitting of the first-principles calculation, and address the problems of edge states, topological properties and magnetic susceptibility. We find that logarithmic peaks of the local density of states emerge near the Fermi energy, owing to pseudo-one-dimensional edge states that appear between the Dirac nodal lines. Magnetic susceptibility calculated in our model can explain the experimental result at a high temperature. In the presence of a realistic spin-orbit coupling, we show that [Pt(dmdt)$_{2}$] is a topological nodal line semimetal with isolated electron and hole pockets.

We performed heat capacity measurements on the molecular π–d system λ-(BEDT-STF)2FeCl4 to discuss the unconventional paramagnetic-like behavior. We observed a Schottky-type anomaly below TN of 16 K...

We report on the pressure–magnetic field–temperature (p–B–T) phase diagram of BaVS3 that was constructed by measuring the magnetic susceptibility and the high-field magnetization under pressure. Th...

We report the investigation of the magnetic susceptibility (χ), specific heat (CP), electrical resistivity (ρ) and magnetoresistance (MR) studies of the compound Ce2Ru3Ge5. Ce2Ru3Ge5 crystallizes i...

Magnetism in EuNiIn4 is studied using specific heat, magnetic susceptibility, magnetization, 151Eu Mossbauer spectroscopy, and neutron diffraction experiments. The specific heat exhibited two magne...

The magnetic and transport properties of a single crystal of YbNiGe with an orthorhombic TiNiSi-type structure are presented here. The experimental results indicate that YbNiGe undergoes two succes...

We carried out neutron powder diffraction experiments on the cluster-based Haldane compound fedotovite K2Cu3O(SO4)3. Weak magnetic reflections caused by a magnetic long-range order appeared below a...

Electronic properties of singlecrystalline La3Ir3O11, which hosts strongly spin–orbit entangled 5d electrons on the geometrically frustrated KSbO3-type structure, have been investigated. The magnit...

We report magnetic-field-induced first-order phase transitions in the ferro-quadrupole (FQ) ordered state of PrTi2Al20, in which non-Kramers Pr3+ ions with two 4f electrons have a non-magnetic Gamma3 doublet ground state in the cubic Td crystalline electric field. For magnetic fields along [111], 27Al-NMR and magnetization experiments reveal Qz \propto 3z2-r2 type FQ order below 2 K independent of field strength. Magnetic fields along [001] or [110], however, induce discontinuous switching of order parameters within the two dimensional space spanned by Qz and Qx \propto x2-y2 at small field values less than a few tesla. A symmetry-based theoretical analysis shows that the transitions can be caused by competition between the magnetic Zeeman interaction and anisotropy in the quadrupole-quadrupole interactions, if the latter dominates over the former in low fields and vice versa in high fields. Furthermore, striking violation of proportionality between NMR Knight shift and magnetic susceptibility is observed in the symmetry-broken FQ phases, indicating significant influence of FQ order on the hybridization between conduction and f electrons, which in turn mediates the RKKY-type quadrupole interaction causing the FQ order. This feedback effect may be a specific feature of quadrupole orders not commonly observed in magnetic phase transitions and play a key role for inducing the discontinuous transitions., Comment: 20 pages, 21 figures

X-ray powder diffraction (XRPD), magnetic susceptibility and muon-spin relaxation (μSR) measurements were carried out to investigate lattice symmetries and magnetic properties of one of alkali-meta...

We study the magnetic response and valence fluctuations in the extended Anderson model on a two-dimensional Penrose lattice using real-space dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. Calculating the f-electron number, c–f spin correlations, and magnetic susceptibility at each site, we find site-dependent formation of the singlet state and valence distributions at low temperatures, which are reflected by the quasiperiodic lattice structure. The bulk magnetic susceptibility is also addressed.

The spin dynamics of the Li9V3(P2O7)3(PO4)2 insertion electrode system is explored with the results of magnetization and nuclear magnetic resonance for Li ions. The magnetic susceptibility at high ...

The successive phase transitions of the pyrochlore oxide Cd2Re2O7 are studied by polarizing microscopy and magnetic susceptibility measurements. The formation of twin domains is visualized in the polarizing images of a pristine (111) crystal surface upon cooling through the cubic-to-tetragonal transition at Ts1 ~ 200 K. Moreover, a dramatic change in the twinning pattern is observed at Ts2 ~120 K, which suggests that the tetragonal c axis flips as the strain changes its direction at the tetragonal-to-tetragonal transition. Magnetic susceptibility measurements reveal significant domain alignment upon cooling across Ts1 and Ts2 in a magnetic field of 7 T, which are due to ~10% anisotropy in the magnetic susceptibility for the low-temperature phases. Interestingly, the anisotropy is reversed at Ts2: \c{hi}c < \c{hi}a above Ts2 and vice versa below Ts2., 8 pages, 7 figures, J. Phys. Soc. Jpn, in press

We succeeded in growing single crystals of EuCu2(Ge1−xSix)2 (x = 0–1) by the Bridgman method using a Mo crucible and measured the electrical resistivity, specific heat, magnetic susceptibility, and...

We have investigated uniaxial and hydrostatic pressure effects on superconductivity in Fe1.07Te0.88S0.12 through magnetic-susceptibility measurements down to 1.8 K. The superconducting transition temperature Tc is enhanced by out-of-plane pressure (uniaxial pressure along the c-axis); the onset temperature of the superconductivity reaches 11.8 K at 0.4 GPa. In contrast, Tc is reduced by in-plane pressure (uniaxial pressure along the ab-plane) and hydrostatic pressure. Taking into account these results, it is inferred that the superconductivity of Fe1+yTe1-xSx is enhanced when the lattice constant c considerably shrinks. This implies that the relationship between Tc and the anion height for Fe1+yTe1-xSx is similar to that applicable to most iron-based superconductors. We consider the reduction of Tc by hydrostatic pressure due to suppression of spin fluctuations because the system moves away from antiferromagnetic ordering, and the enhancement of Tc by out-of-plane pressure due to the anion height effect on Tc., Comment: 5 pages, 4 figures. This paper has been accepted for publication in J. Phys. Soc. Jpn

Filled skutterudite phosphides with heavy lanthanides Ln T 4 P 12 ( Ln = Gd, Tb, Dy, T = Fe, Ru, Os) were prepared at high temperatures and high pressures. The magnetic properties of these compounds were studied in detail through electrical resistivity ρ( T ), magnetic susceptibility, magnetization, specific heat, and thermoelectric power S ( T ) measurements. The filled skutterudite DyRu 4 P 12 was synthesized for the first time. DyRu 4 P 12 shows an antiferromagnetic transition at T N = 15 K. The dramatic increase at T N observed in ρ( T ) and S ( T ) suggest the formation of a superzone gap due to Fermi surface nesting. All the compounds undergo a ferromagnetic or antiferromagnetic transition at low temperatures except TbOs 4 P 12 . TbOs 4 P 12 is found to be paramagnetic down to 2 K. The magnetic ordered states of filled skutterudite phosphides containing heavy lanthanides are discussed.

We measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization in the first Np-based superconductor NpPd 5 Al 2 with the tetragonal crystal structure. NpPd 5 Al 2 is a paramagnet and becomes the superconductivity at T c =4.9 K. The large electronic specific heat coefficient of γ= 200 mJ/K 2 mol at T c indicates that NpPd 5 Al 2 is a heavy fermion superconductor. The upper critical field H c2 is large and highly anisotropic: H c2 (0)=37 kOe for H ∥[100] and H c2 (0)=143 kOe for H ∥[001]. Although the initial slope of H c2 at T c is very large, H c2 is strongly suppressed against the magnetic field for both field direction, indicating the existence of strong Pauli paramagnetic effect. Furthermore, the magnetization curve shows the step-like behaviors at H c2 , corresponding to the first order transition. The d -wave superconductivity with the spin-singlet state is most likely realized in NpPd 5 Al 2 .

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO 1- x F x system. A steep increase in superconducting transition temperature ( T c ) was observed for LaFePO and “optimum-doped” and “over-doped” LaFeAsO 1- x F x . Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe 2 As 2 by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1- x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

The compound, Nd 7 Rh 3 , has been recently shown by us to exhibit exceptional magnetic and transport behavior. There are at least two antiferromagnetic transitions as a function of temperature (10 and 32 K) with the 10 K-transition being first-order-like at low fields; in addition, there is a field-induced first-order (metamagnetic) transition at temperatures well below 10 K. We have investigated the influence of external pressure (

A high pressure helium gas apparatus for low temperature applications is described. The gas apparatus consists of a high pressure generator, an experimental cell and a cryostat. The high pressure cell is connected to the generator with stainless tubings of 1.6 or 3.1 mm outer diameters, so heat transfer into the cryostat is low enough. A calibrated manganin gauge, situated in a separate temperature stabilized cell, is used to measure pressure with accuracy about 0.01 GPa while helium was still in the fluid phase. Upon crystallization of helium, pressure is calculated on the basis of the measured helium-crystallization temperatures and data on EOS of helium. The apparatus can be used to create high helium pressure up to 1.5 GPa and temperature less than 2 K and to carry out electrical, magnetic and thermodynamic measurements. The technique is illustrated by the results of measurements of AC magnetic susceptibility of a single crystal of MnSi along the ferromagnetic phase-transition line up to a pressure of...

We report a detailed study of the pressure–temperature ( P – T ) phase diagrams of two Yb-based heavy-fermion compounds, namely, the antiferromagnet YbAgGe and weak ferromagnet YbRhSb. YbAgGe with a quasikagome lattice of Yb ions undergoes two magnetic phase transitions at T M1 = 0.8 K (second order) and T M2 =0.65 K (first order). With increasing pressure ( P ) up to 0.5 GPa, the transitions at T M1 and T M2 merge into one transition. For 0.5 < P < 1.6 GPa, T M2 remains constant and abruptly rises above 1.6 GPa. This rise occurs with a significant change in the specific heat anomaly from a small cusp at T M2 into a large peak at T M3 . This finding suggests that the suppressed magnetic entropy is released due to geometrical frustration at ambient pressure. On the other hand, YbRhSb undergoes a transition into an unusual magnetic state at T M1 = 2.7 K, which is associated with a small spontaneous moment of 3 ×10 -3 µ B /Yb. Measurements of the resistivity, specific heat, and ac magnetic susceptibility χ a...

The effects of pressure on two prototypes of molecule-based magnets have been investigated through dc magnetic measurements in an attempt to realize these higher transition temperatures ( T C 's). The target compounds are (1) a genuine organic radical weak-ferromagnet, p -NC-C 6 F 4 -CNSSN ( T C = 35.5 K) with through-space intermolecular interaction and (2) a metal-complex ferrimagnet [Mn(en)] 3 [Cr(CN) 6 ] 2 ·4H 2 O ( T C = 69 K) with a cyanide bridge network between magnetic metal ions. In (1), T C is enhanced as d T C /d P = 22.0 K/GPa, and it exceeds 70 K at around 1.5 GPa. Exceeding a border of 70 K is worthy in the field of the genuine organic system. In (2), pressure dependences of T C and magnetic moment have been investigated over wider pressure region. The ideal material control for higher- T C was realized at around 5 GPa. Two types of pressure cells developed for these studies are also mentioned.

We grew single crystals of PrCoIn 5 , PrRhIn 5 , and PrIrIn 5 with the tetragonal crystal structure and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization and de Haas–van Alphen effect. These Pr compounds reveal no magnetic ordering. The topology of the Fermi surfaces is the same as that of a non-4 f reference compound LaRhIn 5 . The cyclotron mass and/or the electronic specific heat coefficient are also approximately the same as those of LaRhIn 5 . The magnetic susceptibility and magnetization are anisotropic, reflecting the crystal structure, which are well explained by the tetragonal crystalline electric field scheme with the singlet ground state.

A generalized periodic Anderson model with stable f 2 configuration is examined through a Gutzwiller-type variational method (GM). In particular, we focus on the effect of the f 2 crystal-field (CF) level scheme with singlet ground and triplet low-lying excited states. It is demonstrated that valence fluctuation from the f 2 states is slightly enhanced by a small singlet–triplet CF splitting whereas it becomes strongly suppressed as the splitting increases. The GM predicts that this valence change caused by the CF is discontinuous in the case of small hybridization. The influence of the CF on magnetic properties is studied in terms of the Landau parameter of magnetic susceptibility.

We have grown a single crystal of La_2-x_Sr_x_CuO_4_ in which the Sr concentration, x, continuously changes from 0.24 to 0.29 in the overdoped regime and obtained many pieces of single crystals with different x values by slicing the single crystal. From detailed measurements of the magnetic susceptibility, chi, of each piece, it has been found that the absolute value of chi at the measured lowest temperature 2 K, |chi_2K_|, on field cooling rapidly decreases with increasing x as well as the superconducting (SC) transition temperature. As the value of |chi_2K_| is regarded as corresponding to the SC volume fraction in a sample, it has been concluded that a phase separation into SC and normal-state regions occurs in a sample of La_2-x_Sr_x_CuO_4_ in the overdoped regime., 4 pages, 3 figures, ver. 2 has been accepted in J. Phys. Soc. Jpn

We have studied a change of the Fermi surface in an antiferromagnet CeIn 3 via the de Haas–van Alphen experiment under pressure up to 3 GPa. With increasing pressure P , the Neel temperature T N =10 K decreases and becomes zero at a critical pressure P c ≃2.6 GPa. In the pressure region P > P c , we have observed a large main Fermi surface named a , which indicates that the electronic state of 4 f electron in CeIn 3 changes from localized to itinerant at P c , as observed in the similar antiferromagnets CeRh 2 Si 2 and CeRhIn 5 . The cyclotron effective mass m c * of this main Fermi surface is extremely enhanced around P c : m c * ≃60 m 0 at 2.7 GPa for the magnetic field along the direction.

We discovered that oxonium (H 3 O + ) ion content is a key parameter to the understanding of unique superconductivity in sodium cobalt oxyhydrate. As Na + ions are substituted by H 3 O + ions, keep...

Single crystals of a pressure-induced superconductor Ce 2 Ni 3 Ge 5 have been successfully grown by the flux method. The anisotropic magnetic properties due to the orthorhombic crystal structure have been studied precisely by the electrical resistivity, specific heat, magnetic susceptibility, and high-field magnetization measurements. The results of these measurements confirmed two antiferromagnetic transitions at T N1 =5.0 K and T N2 =4.3 K. The electronic specific heat coefficient obtained from the low-temperature specific heat data amounts to 90 mJ/K 2 ·mol Ce. The high-field magnetization for H ∥[100] shows four magnetic transitions at 11.8, 12.9, 17.5 and 23.9 T with a saturation moment of 0.73 µ B /Ce at 1.3 K. On the other hand, two magnetic transitions are observed for H ∥[010] and [001], with saturation moments of 0.43 and 1.1 µ B /Ce, respectively. We have also performed the crystalline electric field (CEF) analysis on the magnetic susceptibility and magnetization to understand the magnetocrysta...

Magnetic susceptibility, magnetization and heat capacity measurements are made on a single crystal sample of a spin–ladder material, Na 2 Fe 2 (C 2 O 4 ) 3 (H 2 O) 2 . The susceptibilities show a b...

We measured the electrical resistivity, specific heat, magnetic susceptibility, high-field magnetization, neutron scattering and electrical resistivity under pressure for CeRhGe. The anisotropy of ...

We report the resistivity, magnetic susceptibility, magnetization and specific heat studies on a single crystal of CePdSb 3 which crystallizes in the orthorhombic crystal structure. CePdSb 3 behave...

We have performed magnetic susceptibility, neutron diffraction and specific heat measurements on the triangular lattice antiferromagnet CuFe 1- x Al x O 2 for x ≤0.050. A magnetic phase diagram for temperature vs x was determined. The various magnetically ordered phases compete with each other on the x – T magnetic phase diagram, strongly suggesting that the quasi-Ising orderings are stabilized in CuFeO 2 with delicate balance of competing interactions. In the ground state for 0.014 < x < 0.030 [low-temperature (LT) phase], the magnetic structure consists of two magnetic modulations with slightly different wave numbers. Moreover, we observed the change in magnetic diffraction patterns in the LT phase, which corresponds to a crystallographic distortion from “hexagonal” to “orthorhombic”, suggesting that the Ising anisotropy in CuFeO 2 is closely related to the crystallographic distortion.

(La 1- x Y x )Mn 4 Al 8 (0 ≤ x ≤1) is a very unique itinerant electron system, in which the spin pseudogap can be controlled continuously and nearly uniformly in a wide range of the gap width from Δ≃250 to 500 K by the anisotropic volume shrinkage induced by the chemical pressure. The hydrostatic pressure on LaMn 4 Al 8 also increases Δ but tends to fill the gap. The strong and anisotropic volume dependence of Δ strongly supports the idea that the origin of the gap formation is associated with the one-dimensional geometry of the Mn spin arrangement.

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the effective d-p model with full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. They sensitively depend on the topology of the Fermi surfaces, which changes as the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300K, assuming that six small hole-pockets predicted by LDA band calculations are absent. When they exist, on the contrary, then ``anti-pseudo-gap behaviors'' should inevitably appear. Thus, the present study strongly supports the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent ARPES measurements. A large Fermi surface around the \Gamma-point would account for the superconductivity in water-intercalated samples., Comment: 5pages, to appear in J. Phys. Soc. Jpn. Vol.74 (2005) No.8

The low-temperature properties of Pr x La 1- x InAg 2 have been studied for a wide concentration range of Pr ions between 0≤ x ≤0.6. The susceptibility above T =15 K is well reproduced by the crystal-electric-field level scheme with a non-Kramers Γ 3 doublet in the ground state for each concentration, while that below T =15 K shows a non-Fermi-liquid behavior with a logarithmic temperature dependence. The temperature dependence of the specific heat C / T shows a monotonic enhancement below T =10 K, which can be scaled by a characteristic temperature T * defined at each concentration. These results suggest that the anomalous low-temperature properties are understood as the single-ion nature of Γ 3 quadrupolar moments. The origin of the anomalous properties is discussed in connection with the quadrupolar Kondo effect arising from the correlation between the Γ 3 moments and the conduction electrons.

We have studied the magnetic and thermal properties of Y substituted Ca 3 Co 2 O 6 . Ca 3 Co 2 O 6 exhibits the ferrimagnetic ground state due to ferromagnetic intrachain coupling and antiferromagnetic interchain coupling. As Y substitution increases, which results in variation of the interaction strength of both interchain and intrachain coupling, gradual but unique changes of ground state from ferrimagnetic to nonmagnetic state have been observed in dc susceptibility, isothermal magnetization, heat capacity, and ac susceptibility. Y substitution induces site percolation on Co–O one-dimensional (1D) chain as well as Co two-dimensional (2D) triangular lattice, and site percolation as well as frustration explains experimental results of doped Co–O 1D chain system. The spin configuration of the Y substituted Ca 3 Co 2 O 6 can be understood by using crystal field splitting model.

We grew high-quality single crystals of NpTGa 5 (T = Fe, Rh, and Ni) with a tetragonal structure by the Ga-flux method and measured their electrical resistivity, specific heat, magnetic susceptibility, and magnetization. All the investigated compounds undergo magnetic ordering. NpFeGa 5 orders antiferromagnetically at T N =118 K and shows another magnetic transition at T * =78 K. A relatively large electrical resistivity indicates that NpFeGa 5 might be a low-carrier compound. NpRhGa 5 also orders antiferromagnetically at T N1 =36 K. Below another magnetic transition at T N2 =32 K, the antiferromagnetic easy-axis is most likely changed from [001] to the (001) plane. On the other hand, NpNiGa 5 undergoes ferromagnetic ordering at T C =30 K, where the magnetic moment of Np is directed along the [001] direction. Furthermore, below another magnetic transition at T * =18 K, the ordered moment is discontinuously enlarged and a change of the magnetic structure occurs. The electronic specific heat coefficients ar...

We report on the magnetic properties of CeCoGe 3 by measuring the electrical resistivity, magnetic susceptibility, magnetization and specific heat, together with the magnetic properties of a reference compound LaCoGe 3 . The measurements were performed on a single crystal grown from a high-temperature solution using a third element, bismuth as flux. From the magnetic susceptibility and magnetization measurements, CeCoGe 3 is an antiferromagnet with a Neel temperature T N1 =21 K and undergoes two successive transitions T N2 =12 K and T N3 =8 K. The magnetization for H ∥[001] indicated a metamagnetic transition with three steps. We constructed a magnetic phase diagram from the magnetization measurements, which is very complex and is roughly similar to the previous result performed on a polycrystalline grain-aligned sample.

Magnetic susceptibility and specific heat experiments were performed on the single crystalline Tb 1- x Y x B 2 C 2 (0 ≤ x ≤0.5) compounds. The anomalous increase of magnetic susceptibility below T N in TbB 2 C 2 was similarly observed in all the compounds. The Y dilution weakens the domain preference and the in-plane anisotropy in susceptibility disappears for x =0.4 and 0.5. As x increases, the Neel temperature decreases rapidly so as to reach 0 K around x =0.6, and the specific heat anomaly around T N also decreases considerably. These results are well explained by the scheme of a Van Vleck antiferromagnetic transition. The specific heat experiments reveal that the 4 f ground state is a singlet with the first excited state of a singlet that locates around 40 K.

We grew a single crystal of the transuranium superconductor PuRhGa 5 with the tetragonal structure by the Ga-flux method, and measured its magnetic susceptibility, magnetization and AC susceptibili...

We report on the magnetic properties of double-layered perovskite (CuCl)LaNb 2 O 7 with a square lattice of S =1/2 Cu 2+ ions, prepared by topotactic ion-exchange reactions. Magnetic susceptibility exhibits a thermal-activated behavior, and the analysis of the data suggests competing ferro- and antiferromagnetic exchanges for first- and second-nearest-neighbor bonds, respectively. Neutron scattering experiments provide explicit evidence for a spin-singlet ground state with an energy gap of 2.3 meV. The well-defined low-lying triplet mode is almost independent of Q despite densely packed magnetic ions in the CuCl plane. In addition, on the basis of the isolated dimer model, we found that the intradimer distance is as long as 8.8 A, which corresponds to the fourth-nearest-neighbor bond. These results indicate a sizable geometrical frustration that should impose severe constraints on the propagation of the triplet excitation, as in the case of SrCu 2 (BO 3 ) 2 . This argument is further reinforced by the obs...