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Itsuko S. Suzuki

Publications and source records attributed to Itsuko S. Suzuki.

At least 19 recordsLinked to original sources

A proper understanding of the Davisson and Germer experiments for undergraduate modern physics course

The physical interpretation for the Davisson-Germer experiments on nickel (Ni) single crystals [(111), (100), and (110) surfaces] is presented in terms of two-dimensional (2D) Bragg scattering. The Ni surface acts as a reflective diffraction grating when the incident electron beams hits the surface. The 2D Bragg reflection occurs when the Ewald sphere intersects the Bragg rods arising from the two-dimension character of the system. Such a concept is essential to proper understanding of the Davisson-Germer experiment for undergraduate modern physics course.

physics.ed-ph

Ordered spin structures of $β$-MnO$_{2}$ (rutile-type) systems with competing exchange interactions: numerical approach using equi-energy contour plot

Using numerical calculations of equi-energy contour plot of the Fourier transform of the spin Hamiltonian, we study the magnetic phase diagram ($J_{2}$ and $J_{3}$) of the rutile type $β$-MnO$_{2}$, where $J_{1}$ ($<0$) is fixed and is the antiferromagnetic interaction along the diagonal direction, $J_{2}$ is the interaction along the c axis, and $J_{3}$ is the interaction along the a axis. The magnetic phase diagram consists of the multricritical point (the intersection $J_{2}J_{3}=J_{1}^{2}$ and $J_{2}+J_{3}=2J_{1}$), the helical order along the c axis, the $(h=1/2,k=0,l=1/2)$ phase, the helical order along the a axis, and the phase $(h=0,k=0,l=1)$. The shift of the location of the magnetic Bragg peak in the $(h,0,l)$ reciprocal lattice plane is examined with the change of $J_{2}$ and $J_{3}$ in the phase diagram. The shift is discontinuous on the first-order phase transition, and is continuous on the second-order phase transition. The detail of our magnetic phase diagram is rather different from that reported by Yoshimori.

cond-mat.str-el

Specific heat of stage-2 MnCl$_{2}$ graphite intercalation compound: co-existence of spin glass phase and incommensurate short-range spin order

Stage-2 MnCl$_{2}$ GIC magnetically behaves like a quasi 2D $XY$ antiferromagnet on the triangular lattice. It shows a typical spin glass behavior below the spin freezing temperature $T_{SG}$ (= 1.1 K). The AC magnetic susceptibility shows a peak at $T$ = $T_{SG}$ at $H$ = 0. This peak shifts to the low temperature side with increasing $H$, according to the de Almeida-Thouless critical line. We have undertaken an extensive study on the $T$ dependence of specific heat in the absence of an external field $H$ and in the presence of $H$ along the $c$ plane. The magnetic specific heat $C_{mag}$ at $H$ = 0 shows no anomaly at $T_{SG}$, but exhibits double broad peaks around 5 K and 41 K. The magnetic specific heat $C_{mag}$ at $H$ = 0 is described by $C_{mag} \propto (1/T^{2})\exp(-Δ/T)$ with $Δ= 1.41 \pm 0.03$ K, while $C_{mag}$ at $H$ = 10 kOe is proportional to $T$. The entropy due to the broad peak around 5 K is 1/3 of the total entropy. The residual entropy is 63 % of the total entropy because of highly frustrated nature of the system. The magnetic neutron scattering indicates that a short range spin order associated with the incommensurate wave vector $| {\bf Q}_{incomm}|$ ($= 0.522 Å^{-1}$ at 0.45 K) appears below 5 K and remains unchanged down to 63 mK. The in-plane spin correlation length is only 18 $Å$ at 0.45 K. The low temperature phase below $T_{SG}$ is a kind of reentrant spin glass phase where the spin glass phase coexists with a short range spin order associated with $| {\bf Q}_{incomm}|$.

cond-mat.dis-nn

Observation of a magnetic-field-induced Griffiths phase in three-dimensional Ising random magnet Ni$_{p}$Mg$_{1-p}$(OH)$_{2}$ from absorption of AC magnetic susceptibility

The nature of the Griffiths phase in three-dimensional Ising random magnet Ni_{p}Mg_{1-p}(OH)_{2} (p = 0.10, 0.25, 0.315, 0.50, 0.80, and 1) is studied from the measurements of absorption (the out-of phase in AC magnetic susceptibility) in the field-cooled (FC) state. The temperature (T) dependence of absorption is measured in the vicinity of the critical temperature [Néel temperature for p = 0.50, 0.80, 1, and the spin glass transition temperature for p = 0.1, 0.25, 0.315] in the presence of an external magnetic field H. A broad peak is clearly observed for absorption vs T well above the critical temperature at H > 20 kOe. This result gives a piece of evidence for the magnetic-field induced Griffiths phase.

cond-mat.dis-nn

Scaling form of zero-field-cooled and field-cooled susceptibility in superparamagnet

The scaling form of the normalized ZFC and FC susceptibility of superparamagnets (SPM's) is presented as a function of the normalized temperature $y$ ($=k_{B}T/K_{u}< V>$), normalized magnetic field $h$ ($=H/H_{K}$), and the width $σ$ of the log-normal distribution of the volumes of nanoparticles, based on the superparamagnetic blocking model with no interaction between the nanoparticles. Here $ $ is the average volume, $K_{u}$ is the anisotropy energy, and $H_{K}$ is the anisotropy field. Main features of the experimental results reported in many SPM's can be well explained in terms of the present model. The normalized FC susceptibility increases monotonically increases as the normalized temperature $y$ decreases. The normalized ZFC susceptibility exhibits a peak at the normalized blocking temperature $y_{b}$ ($=k_{B}T_{b}/K_{u}< V>$), forming the $y_{b}$ vs $h$ diagram. For large $σ$ ($σ>0.4$), $y_{b}$ starts to increase with increasing $h$, showing a peak at $h=h_{b}$, and decreases with further increasing $h$. The maximum of $y_{b}$ at $h=h_{b}$ is due to the nonlinearity of the Langevin function. For small $σ$, $y_{b}$ monotonically decreases with increasing $h$. The derivative of the normalized FC magnetization with respect to $h$ shows a peak at $h$ = 0 for small $y$. This is closely related to the pinched form of $M_{FC}$ vs $H$ curve around $H$ = 0 observed in SPM's.

cond-mat.dis-nn

Observation of superspin-glass behavior in Fe$_{3}$O$_{4}$ nanoparticles

The aging and memory effects of Fe$_{3}$O$_{4}$ nanoparticles have been studied using a series of zero-field cooled (ZFC) and field-cooled (FC) magnetization measurements at various aging protocols. The genuine ZFC magnetization after the ZFC procedure with a single stop and wait process shows an aging dip at the stop temperature on reheating. The depth of the aging dip is dependent on the wait time. The frequency dependence of the AC magnetic susceptibility is indicative of critical slowing down at a freezing temperature $T_{f}$ ($= 30.6 \pm 1.6$ K). The relaxation time $τ$ is described by a power law form with a dynamic critical exponent $x$ ($= 8.2 \pm 1.0$) and a microscopic relaxation time $τ_{0}$ [$=(1.33 \pm 0.05) \times 10^{-9}$ sec]. The ZFC-peak temperature decreases with increasing magnetic field ($H$), forming a critical line with an exponent $p = 1.78 \pm 0.26$, close to the de Almeida-Thouless exponent ($p = 3/2$). These results indicate that the superspin glass phase occurs below $T_{f}$.

cond-mat.dis-nn

Stretched exponential relaxation in three dimensional short-range spin glass Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound

Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound is a three-dimensional short-range spin glass with a spin freezing temperature $T_{SG}$ ($= 3.92 \pm 0.11$ K). The time evolution of the zero-field cooled magnetization $M_{ZFC}(t)$ has been measured under various combinations of wait time ($t_{w}$), temperature ($T$), temperature-shift ($ΔT$), and magnetic field ($H$). The relaxation rate $S_{ZFC}(t)$ [$=(1/H)$d$M_{ZFC}(t)$/d$\ln t$] shows a peak at a peak time $t_{cr}$. The shape of $S_{ZFC}(t)$ in the vicinity of $t_{cr}$ is well described by stretched exponential relaxation (SER). The SER exponent $b$ and the SER relaxation time $τ_{SER}$ are determined as a function of $t_{w}$, $T$, $H$, and $ΔT$. The value of $b$ at $T=T_{SG}$ is nearly equal to 0.3. There is a correlation between $τ_{SER}$ and $1/b$, irrespective of the values of $t_{w}$, $T$, $H$, and $ΔT$. These features can be well explained in terms of a simple relaxation model for glassy dynamics.

cond-mat.dis-nn

Magnetism and superconductivity in $M_{c}$Ta$_{2}$S$_{2}$C (M = Fe, Co, Ni, and Cu)

Magnetic properties of $M_{c}$Ta$_{2}$S$_{2}$C ($M$ = Fe, Co, Ni, Cu) have been studied using SQUID DC and AC magnetic susceptibility. In these systems magnetic $M^{2+}$ ions are intercalated into van der Waals gaps between adjacent S layers of host superconductor Ta$_{2}$S$_{2}$C. Fe$_{0.33}$Ta$_{2}$S$_{2}$C is a quasi 2D $XY$-like antiferromagnet on the triangular lattice. It undergoes an antiferromagnetic phase transition at $T_{N}$ (= 117 K). The irreversible effect of magnetization occurs below $T_{N}$, reflecting the frustrated nature of the system. The AF phase coexists with two superconducting phases with the transition temperatures $T_{cu} = 8.8$ K and $T_{cl} = 4.6$ K. Co$_{0.33}$Ta$_{2}$S$_{2}$C is a quasi 2D Ising-like antiferromagnet on the triangular lattice. The antiferromagnetic phase below $T_{N} = 18.6$ K coexists with a superconducting phase below $T_{cu} = 9.1$ K. Both Ni$_{0.25}$Ta$_{2}$S$_{2}$C and Cu$_{0.60}$Ta$_{2}$S$_{2}$C are superconductors with $T_{cu}$ ($= 8.7$ K for Ni and 6.4 K for Cu) and $T_{cl}$ (= 4.6 K common to $M_{c}$Ta$_{2}$S$_{2}$C). Very small effective magnetic moments suggest that Ni$^{2+}$ and Cu$^{2+}$ spins are partially delocalized.

cond-mat.supr-con

Measurement of mutual inductance from frequency dependence of impedance of AC coupled circuit using digital dual-phase lock-in amplifier

We present a simple method to determine the mutual inductance $M$ between two coils in a coupled AC circuit by using a digital dual-phase lock-in amplifier. The frequency dependence of the real and imaginary parts is measured as the coupling constant is changed. The mutual inductance $M$ decreases as the distance $d$ between the centers of coils is increased. We show that the coupling constant is proportional to $d^{-n}$ with an exponent $n$ ($\approx$ 3). This coupling is similar to that of two magnetic moments coupled through a dipole-dipole interaction.

physics.ed-ph

Aging dip and cumulative aging in hierarchical successive transition of stage-2 CoCl$_{2}$ graphite intercalation compound

Memory and aging behaviors in stage-2 CoCl$_{2}$ GIC ($T_{cu}$ = 8.9 K and $T_{cl}$ = 6.9 K) have been studied using low frequency ($f$ = 0.1 Hz) AC magnetic susceptibility ($χ^{\prime}$ and $χ^{\prime\prime}$) as well as thermoremnant magnetization. There occurs a crossover from a cumulative aging (mainly) with a partial memory effect for the domain-growth in the intermediate state between $T_{cu}$ and $T_{cl}$ to an aging and memory in a spin glass phase below $T_{cl}$. When the system is aged at single or multiple stop and wait processes at stop temperatures $T_{s}$'s below $T_{cl}$ for wait times $t_{s}$, the AC magnetic susceptibility shows single or multiple aging dips at $T_{s}$'s on reheating. The depth of the aging dips is logarithmically proportional to the wait time $t_{s}$. Very weak aging dip between $T_{cu}$ and $T_{cl}$ indicates the existence of a partial memory effect. The sign of the difference between the reference cooling and reference heating curves changes from positive to negative on crossing $T_{cl}$ from the high $T$-side. The time dependence of $χ^{\prime}$ and $χ^{\prime\prime}$ below $T_{cl}$ is described by a scaling function of $ωt$. its a local maximum at 6.5 K just below $T_{cl}$, and drastically decreases with increasing $T$. The nature of the cumulative aging between $T_{cl}$ and $T_{cu}$ is also examined.

cond-mat.dis-nn

Study on successive superconducting transitions in Ta$_{2}$S$_{2}$C from electrical resistivity and nonlinear AC magnetic susceptibility

Ta$_{2}$S$_{2}$C compound undergoes superconducting transitions at $T_{cl} = 3.60 \pm 0.02$ K and $T_{cu} = 9.0 \pm 0.2$ K. The nature of successive superconducting transitions has been studied from electrical resistivity, linear and nonlinear AC magnetic susceptibilities. The resistivity $ρ$ at $H$ = 0 shows a local maximum near $T_{cu}$, a kink-like behavior around $T_{cl}$, and reduces to zero at below $T_{0}$ = 2.1 K. The $\ln T$ dependence of $ρ$ is observed at $H$ = 50 kOe at low temperatures, which is due to two-dimensional weak-localization effect. Below $T_{cu}$ a two-dimensional superconducting phase occurs in each TaC layer. The linear and nonlinear susceptibilities $χ_{1}^{\prime\prime}$, $χ_{3}^{\prime}$, $χ_{5}^{\prime}$, and $χ_{7}^{\prime}$ as well as the difference $δχ$ ($= χ_{FC} - χ_{ZFC}$) between the FC and ZFC susceptibilities, start to appear below 6.0 K, the onset temperature of irreversibility. A drastic growth of the in-plane superconducting coherence length below 6.0 K gives rise to a three-dimensional superconducting phase below $T_{cl}$, through interplanar Josephson couplings between adjacent TaC layers. The oscillatory behavior of $χ_{3}^{\prime\prime}$, $χ_{5}^{\prime\prime}$, and $χ_{7}^{\prime\prime} $ below $T_{cl}$ is related to the nonlinear behavior arising from the thermally activated flux flow.

cond-mat.supr-con

Magnetic Ordering of CoCl_{2}-GIC: a Spin Ceramic -Hierarchical Successive Transitions and the Intermediate Glassy Phase-

Stage-2 CoCl$_{2}$-GIC is a spin ceramic and shows hierarchical successive transitions at $T_{cu}$ (= 8.9 K) and $T_{cl}$ (= 7.0 K) from the paramagnetic phase into an intra-cluster (two-dimensional ferromagnetic) order with inter-cluster disorder and then to an inter-cluster (three-dimensional antiferromagnetic like) order over the whole system. The nature of the inter-cluster disorder was suggested to be of spin glass by nonlinear magnetic response analyses around $T_{cu}$ and by studies on dynamical aspects of ordering between $T_{cu}$ and $T_{cl}$. Here, we present a further extensive examination of a series of time dependence of zero-field cooled magnetization $M_{ZFC}$ after the aging protocol below $T_{cu}$. The time dependence of the relaxation rates $S_{ZFC}(t) = (1/H)dM_{ZFC}(t)/d\ln t$ dramatically changes from the curves of simple spin glass aging effect below $T_{cl}$ to those of two peaks above $T_{cl}$. The characteristic relaxation behavior apparently indicates that there coexist two different kinds of glassy correlated regions below $T_{cu}$.

cond-mat.dis-nn

Aging dynamics of ferromagnetic and reentrant spin glass phases in stage-2 Cu$_{0.80}$C$_{0.20}$Cl$_{2}$ graphite intercalation compound

Aging dynamics of a reentrant ferromagnet stage-2 Cu$_{0.8}$Co$_{0.2}$Cl$_{2}$ graphite intercalation compound has been studied using DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures $T_{c}$ ($\approx 8.7$ K) and $T_{RSG}$ ($\approx 3.3$ K). The relaxation rate $S_{ZFC}(t)$ exhibits a characteristic peak at $t_{cr}$ below $T_{c}$. The peak time $t_{cr}$ as a function of temperature $T$ shows a local maximum around 5.5 K, reflecting a frustrated nature of the ferromagnetic phase. It drastically increases with decreasing temperature below $T_{RSG}$. The spin configuration imprinted at the stop and wait process at a stop temperature $T_{s}$ ($<T_{c}$) during the field-cooled aging protocol, becomes frozen on further cooling. On reheating, the memory of the aging at $T_{s}$ is retrieved as an anomaly of the thermoremnant magnetization at $T_{s}$. These results indicate the occurrence of the aging phenomena in the ferromagnetic phase ($T_{RSG}<T<T_{c}$) as well as in the reentrant spin glass phase ($T<T_{RSG}$).

cond-mat.dis-nn

Memory and aging effect in hierarchical spin orderings of stage-2 CoCl_{2} graphite intercalation compound

Stage-2 CoCl$_{2}$ graphite intercalation compound undergoes two magnetic phase transitions at $T_{cl}$ (= 7.0 K) and $T_{cu}$ (= 8.9 K). The aging dynamics of this compound is studied near $T_{cl}$ and $T_{cu}$. The intermediate state between $T_{cl}$ and $T_{cu}$ is characterized by a spin glass phase extending over ferromagnetic islands. A genuine thermoremnant magnetization (TRM) measurement indicates that the memory of the specific spin configurations imprinted at temperatures between $T_{cl}$ and $T_{cu}$ during the field-cooled (FC) aging protocol can be recalled when the system is re-heated at a constant heating rate. The zero-field cooled (ZFC) and TRM magnetization is examined in a series of heating and reheating process. The magnetization shows both characteristic memory and rejuvenation effects. The time $(t)$ dependence of the relaxation rate $S_{ZFC}(t)=(1/H)$d$M_{ZFC}(t)$/d$\ln t$ after the ZFC aging protocol with a wait time $t_{w}$, exhibits two peaks at characteristic times $t_{cr1}$ and $t_{cr2}$ between $T_{cl}$ and $T_{cu}$. An aging process is revealed as the strong $t_{w}$ dependence of $t_{cr2}$. The observed aging and memory effect is discussed in terms of the droplet model.

cond-mat.soft

Effect of random disorder and spin frustration on the reentrant spin glass phase and ferromagnetic phase in stage-2 Cu_{0.93}Co_{0.07}Cl_{2} graphite intercalation compound near the multicritical point

Stage-2 Cu$_{0.93}$Co$_{0.07}$Cl$_{2}$ graphite intercalation compound magnetically behaves like a reentrant ferromagnet near the multicritical point ($c_{MCP} \approx 0.96$). It undergoes two magnetic phase transitions at $T_{RSG}$ ($= 6.64 \pm 0.05$ K) and $T_{c}$ ($= 8.62 \pm 0.05$ K). The static and dynamic nature of the ferromagnetic and reentrant spin glass phase has been studied using DC and AC magnetic susceptibility. Characteristic memory phenomena of the DC susceptibility are observed at $T_{RSG}$ and $T_{c}$. The nonlinear AC susceptibility $χ_{3}^{\prime}$ has a positive local maximum at $T_{RSG}$, and a negative local minimum at $T_{c}$. The relaxation time $τ$ between $T_{RSG}$ and $T_{c}$ shows a critical slowing down: $τ$ with $x = 13.1 \pm 0.4$ and $τ_{0}^{*} = (2.5 \pm 0.5) \times 10^{-13}$ sec. The influence of the random disorder on the critical behavior above $T_{c}$ is clearly observed: $α= -0.66$, $β= 0.63$, and $γ= 1.40$. The exponent of $α$ is far from that of 3D Heisenberg model.

cond-mat.dis-nn

Aging dynamics across a dynamic Almeida-Thouless line in three dimensional short-range Issuing spin glass Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound

Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite intercalation compound is a three-dimensional short-range Ising spin glass with a spin freezing temperature $T_{c}$ ($= 3.92 \pm 0.11$ K). The stability of the spin glass phase in the presence of a magnetic field $H$ is examined from (i) the spin freezing temperature $T_{f}(ω,H)$ at which the differential field-cooled (FC) susceptibility $\partial M_{FC}(T,H)/\partial H$ coincides with the dispersion $χ^{\prime}(ω,T,H=0)$ with the angular frequency $ω$, and (ii) the time dependence of zero-field cooled (ZFC) susceptibility $χ_{ZFC}$ after a ZFC aging protocol with a wait time $t_{W}$ ($= 1.0 \times 10^{4}$). The relaxation rate $S(t)$ (= d$χ_{ZFC}$/d$\ln t$) exhibits a local maximum at a characteristic time $t_{cr}$, reflecting non-equilibrium aging dynamics. The peak time $t_{cr}(T,H)$ decreases with increasing $H$ at the fixed temperature $T$ (2.9 K $\leq T<T_{c}$). The spin freezing temperature $T_{f}(ω,H)$ provides evidence for the instability of the spin glass phase in thermal equilibrium in a finite magnetic field. Both $t_{cr}(T,H)$ and $T_{f}(ω,H)$ exhibit certain scaling behavior predicted from the droplet picture, suggesting a dynamic crossover from SG dynamics to paramagnetic behavior in the presence of $H$.

cond-mat.dis-nn

Aging dynamics in reentrant ferromagnet: Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound

Aging dynamics of a reentrant ferromagnet Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound has been studied using AC and DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures $T_{c}$ ($= 9.7$ K) and $T_{RSG}$ ($= 3.5$ K). The relaxation rate $S(t)$ exhibits a characteristic peak at $t_{cr}$ close to a wait time $t_{w}$ below $T_{c}$, indicating that the aging phenomena occur in both the reentrant spin glass (RSG) phase below $T_{RSG}$ and the ferromagnetic (FM) phase between $T_{RSG}$ and $T_{c}$. The relaxation rate $S(t)$ ($=\text{d}χ_{ZFC}(t)/\text{d}\ln t$) in the FM phase exhibits two peaks around $t_{w}$ and a time much shorter than $t_{w}$ under the positive $T$-shift aging, indicating a partial rejuvenation of domains. The aging state in the FM phase is fragile against a weak magnetic-field perturbation. The time ($t$) dependence of $χ_{ZFC}(t)$ around $t \approx t_{cr}$ is well approximated by a stretched exponential relaxation: $χ_{ZFC}(t) \approx \exp[-(t/τ)^{1-n}]$. The exponent $n$ depends on $t_{w}$, $T$, and $H$. The relaxation time $τ$ ($\approx t_{cr}$) exhibits a local maximum around 5 K, reflecting a chaotic nature of the FM phase. It drastically increases with decreasing temperature below $T_{RSG}$.

cond-mat.dis-nn

Successive superconducting transitions and Anderson localization effect in Ta$_{2}$S$_{2}$C

A complex carbide Ta$_{2}$S$_{2}$C consists of van der Waals (vdw)-bonded layers with a stacking sequence $...$ C-Ta-S-vdw-S-Ta-C- $...$ along the c axis. The magnetic properties of this compound have been studied from DC and AC magnetic susceptibility. Ta$_{2}$S$_{2}$C undergoes successive superconducting transitions of a hierachical nature at $T_{cl} = 3.61 \pm 0.01$ K [$H_{c1}^{(l)}(0) = 28 \pm 2$ Oe and $H_{c2}^{(l)}(0) = 7.7 \pm 0.2$ kOe] and $T_{cu} = 9.0 \pm 0.2$ K [$H_{c2}^{(u)}(0) = 6.0 \pm 0.3$ kOe]. The intermediate phase between $T_{cu}$ and $T_{cl}$, where $δχ(= χ_{FC} - χ_{ZFC}) \approx 0$, is an intra-grain superconductive state occurring in the Ta-C layers in Ta$_{2}$S$_{2}$C. The low temperature phase below $T_{cl}$, where $δχ$ clearly appears, is an inter-grain superconductive state. The magnetic susceptibility at $H$ well above $H_{c2}^{(l)}(0)$ is described by a sum of a diamagnetic susceptibility and a Curie-like behavior. The latter is due to the localized magnetic moments of conduction electrons associated with the Anderson localization effect, occurring in the 1T-TaS$_{2}$ type structure in Ta$_{2}$S$_{2}$C.

cond-mat.supr-con