arXiv subjects

S. Tajima

Publications and source records attributed to S. Tajima.

At least 19 recordsLinked to original sources

arXiv2025

Investigation of Medium Modifications to $^{12}$C Structure Functions in the Resonance Region

We present results from a high precision experimental study of the nuclear modification of the longitudinal ($F_L$) to transverse ($F_1$) structure function ratio for bound nucleons in the resonance region. The inclusive electron scattering cross sections were measured in Jefferson Lab Experimental Hall C on carbon and deuterium nuclei for a large range of kinematics, allowing for separations of the longitudinal and transverse structure functions to be performed at a range of four-momentum transfer values $0.5 \le Q^2 \le$ 3.75 GeV$^2$. In contrast to the significant body of measurements of the nuclear modification of the $F_2$ structure function in the deep inelastic scattering region, there is very little on $F_L$ and $R = F_L / 2xF_1$ in the region of the nucleon resonances. In this paper we present measurements of the nuclear effect on $R$ for $^{12}$C ($R_C$) relative to deuterium ($R_D$). These results indicate regions in which in $R_C>R_D$, requiring that the nuclear modifications be different in all three structure functions, $F_2$, $F_1$ and $F_L$.

nucl-ex↗

arXiv2022

Observation of the bands with $d_{xy}$ orbital character near the Fermi level in NdFeAs$_{1-x}$P$_{x}$O$_{0.9}$F$_{0.1}$ using angle-resolved photoemission spectroscopy

We studied the band structure of NdFeAs$_{1-x}$P$_{x}$O$_{0.9}$F$_{0.1}$ ($x$ = 0, 0.2, 0.4 and 0.6) using angle-resolved photoemission spectroscopy (ARPES) measurements. Two of the hole bands, $α_1$ $(d_{xz})$ and $α_3$ $(d_{z^2})$, were observed at the Brillouin zone center in the $P$-polarized light configuration, while the other two hole bands, $α_2$ $(d_{yz})$ and $γ$ $(d_{xy})$, were observed in the $S$-polarized alternative. The observed $γ$ band shifts downwards as $x$ increases, which is consistent with the theoretical prediction for the change in bond angle of As/P-Fe-As/P. Furthermore, a small amount of the $d_{xy}$ orbital component was observed at the same binding energy as that of the top of the $α_1$ band, thus indicating the band reconstruction of the originally degenerate $α_1$ and $α_2$ $(d_{xz}/d_{yz})$ bands by the unoccupied $d_{xy}$ band. The change in the energy level of the $α_1$ band top with $d_{xy}$ orbital character is accompanied by a $T_{c}$ upturn at $0.2 < x < 0.4$. The $T_{c}$ continues to increase as the $α_1$ band shifts downward, crossing the Fermi level. The incipient band with the $d_{xy}$ orbital character on its top could be an important ingredient for high $T_{c}$ 1111-type iron-based superconductors.

cond-mat.str-el↗

arXiv2022

Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor

To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in non-superconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite energy part in the spin fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.

cond-mat.supr-con↗

arXiv2022

Multipole polaron in the devil's staircase of CeSb

Rare-earth intermetallic compounds exhibit rich phenomena induced by the interplay between localized $f$ orbitals and conduction electrons. However, since the energy scale of the crystal-electric-field splitting is only a few millielectronvolts, the nature of the mobile electrons accompanied by collective crystal-electric-field excitations has not been unveiled. Here, we examine the low-energy electronic structures of CeSb through the anomalous magnetostructural transitions below the N$é$el temperature, $\sim$17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4$f$ orbitals, which renormalizes the Sb 5$p$ band prominently, yielding a kink at a very low energy ($\sim$7 meV). This coupling strength is strong and exhibits anomalous step-like enhancement during the devil's staircase transition, unveiling a new type of quasiparticle, named the 'multipole polaron', comprising a mobile electron dressed with a cloud of the quadrupole crystal-electric-field polarization.

cond-mat.mtrl-sci↗

arXiv2021

Importance of dxy orbital and electron correlation in iron-based superconductors revealed by phase diagram for 1111-system

The structural flexibility at three substitution sites in LaFeAsO enabled investigation of the relation between superconductivity and structural parameters over a wide range of crystal compositions. Substitutions of Nd for La, Sb or P for As, and F or H for O were performed. All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling. It was found that the superconducting transition temperature $T_{c}$ is strongly affected by the pnictogen height $h_{Pn}$ from the Fe-plane that controls the electron correlation strength and the size of the $d_{xy}$ hole Fermi surface (FS). With increasing $h_{Pn}$, weak coupling superconductivity switches to the strong coupling one where the $d_{xy}$ hole FS is crucially important.

cond-mat.supr-con↗

arXiv2021

Evolution of charge dynamics in FeSe$_{1-x}$Te$_{x}$: Effects of electronic correlations and nematicity

We systematically studied in-plane optical conductivity of FeSe$_{1-x}$Te$_{x}$ thin films fabricated on CaF$_{2}$ substrates for $x$ = 0, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature $T_{\mathrm{c}}$ at $x \sim$ 0.2 and a gentle decrease in $T_{\mathrm{c}}$ with further increasing $x$. The low-energy optical conductivity spectrum is described by the sum of narrow and broad Drude components, associated with coherent and incoherent charge dynamics, respectively. With increasing Te content, the spectral weight of the narrow Drude component decreases, while the total weight of the two Drude components increases. As a consequence, the fraction of the narrow Drude weight significantly decreases, indicating that Te substitution leads to stronger electronic correlations. Below the nematic transition temperature, the narrow Drude weight decreases with decreasing temperature. This indicates the reduction of the coherent carrier density, resulting from the Fermi-surface modification induced by the development of the orbital order. The reduction of the narrow Drude weight with temperature stopped at $x \sim$ 0.2, corresponding to the disappearance of the nematic transition. Our result suggests that the increase in the coherent carrier density induced by the suppression of the nematic transition gives rise to the enhancement of $T_{\mathrm{c}}$. The decrease in $T_{\mathrm{c}}$ with further Te substitution likely arises from too strong electronic correlations, which are not favorable for superconductivity.

cond-mat.supr-con↗

arXiv2021

Control of band structure of FeSe single crystals via biaxial strain

We performed systematic transport measurements on FeSe single crystals with applying in-plane biaxial strain $\varepsilon$ ranging from -0.96% to 0.23%. Biaxial strain was introduced by firmly gluing samples to various substrate materials with different thermal expansion. With increasing $\varepsilon$, structural and superconducting transition temperatures monotonically increased and decreased, respectively. We analyzed magneto-transport results using a compensated three-carrier model. The evaluated densities of hole and electron carriers systematically changed with strain. This indicates that we succeeded in controlling the band structure of single-crystalline FeSe.

cond-mat.supr-con↗

arXiv2020

Superconducting gap and nematic resonance at the quantum critical point observed by Raman scattering in $\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$

We report comprehensive temperature and doping-dependences of the Raman scattering spectra for $\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($x =$ 0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T =$ 0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair breaking peaks observed in the $A_{\mathrm{1g}}$ and $B_{\mathrm{1g}}$ spectra, another strong $B_{\mathrm{1g}}$ peak appears below the superconducting transition temperature which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.

cond-mat.supr-con↗

arXiv2019

Band-dependent superconducting gap in SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$ studied by angle-resolved photoemission spectroscopy

The isovalent-substituted iron pnictide compound SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe$_{2}$(As$_{0.65}$P$_{0.35}$)$_{2}$ in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and $k_{z}$-independent. We propose that the SC gap of the outer hole FS changes sign around the Z-X [($0, 0, 2π$)-($π,π, 2π$)] direction.

cond-mat.supr-con↗

arXiv2019

High-$T_c$ Iron-phosphide Superconductivity Enhanced by Reemergent Antiferromagnetic Spin Fluctuations in (Sr$_4$Sc$_2$O$_6$)Fe$_2$(As$_{1-x}$P$_{x}$)$_2$ probed by NMR

We report a systematic NMR study on [Sr$_4$Sc$_2$O$_6$]Fe$_2$(As$_{1-x}$P$_x$)$_2$, for which the local lattice parameters of the iron-pnictogen (Fe$Pn$) layer are similar to those of the series LaFe(As$_{1-x'}$P$_{x'}$)O, which exhibit two segregated antiferromagnetic (AFM) order phases, AFM1 at $x'$=0-0.2 and AFM2 at $x'$=0.4-0.7. Our results revealed that the parent AFM1 phase at $x$=0 disappears at $x$=0.3-0.4, corresponding to a pnictogen height ($h_{pn}$) from the Fe-plane of 1.3-1.32 Å, which is similar to that of LaFe(As$_{1-x'}$P$_{x'}$)O and various parent Fe-pnictides. By contrast, the AFM2 order reported for LaFe(As$_{0.4}$P$_{0.6}$)O does not appear at $x\sim$0.8, although the local lattice parameters of the Fe$Pn$ layer and the microscopic electronic states are quite similar. Despite the absence of the {\it static} AFM2 order, reemergent {\it dynamical} AFM spin fluctuations were observed at approximately $x\sim$0.8, which can be attributed to the instability of the AFM2 phase. We suggest this re-enhancement of AFM spin fluctuations to play a significant role in enhancing the $T_c$ to 17 K for $x$=0.8-1. Finally, we discuss the universality and diversity of the complicated magnetic ground states from a microscopic point of view, including the difference in the origins of the AFM1 and AFM2 phases, and their relations with the high superconducting transitions in Fe-pnictides.

cond-mat.supr-con↗

arXiv2019

Measurements of Non-Singlet Moments of the Nucleon Structure Functions and Comparison to Predictions from Lattice QCD for $Q^2 = 4$ $\rm GeV^2$

We present extractions of the nucleon non-singlet moments utilizing new precision data on the deuteron $F_2$ structure function at large Bjorken-$x$ determined via the Rosenbluth separation technique at Jefferson Lab Experimental Hall C. These new data are combined with a complementary set of data on the proton previously measured in Hall C at similar kinematics and world data sets on the proton and deuteron at lower $x$ measured at SLAC and CERN. The new Jefferson Lab data provide coverage of the upper third of the $x$ range, crucial for precision determination of the higher moments. In contrast to previous extractions, these moments have been corrected for nuclear effects in the deuteron using a new global fit to the deuteron and proton data. The obtained experimental moments represent an order of magnitude improvement in precision over previous extractions using high $x$ data. Moreover, recent exciting developments in Lattice QCD calculations provide a first ever comparison of these new experimental results with calculations of moments carried out at the physical pion mass, as well as a new approach which first calculates the quark distributions directly before determining moments.

nucl-ex↗

arXiv2018

Change of Fermi surface states related with two different $T_{\rm c}$-raising mechanisms in iron pnictide superconductor

Evolution of Fermi surface (FS) states of NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_{\rm c}$ and the power law exponent ($n$) of temperature-dependent resistivity ($ρ(T) = ρ_0 + AT^n$) show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced with increasing bosonic fluctuation in the same type of FS state. Around $x=$0.2, all the transport properties show anomalies, indicating that $x$$\sim$0.2 is the critical composition of drastic FS change. The angle resolved photoemission spectroscopy has more directly revealed the distinct change of FS around $x=$0.2, that one hole FS disappears at Brillouin zone center and the other FS with propeller like shape appears at zone corner with decreasing $x$. These results are indicative of the existence of two types of FS state with different nesting conditions that are related with two $T_{\rm c}$-rising mechanisms in this system.

cond-mat.supr-con↗

arXiv2018

Double pair breaking peak in Raman scattering spectra of triple layer cuprate Bi2223

We report the Raman scattering measurements on the triple layer Bi2Sr2Ca2Cu3O10 (Bi2223) crystals of four different doping levels from slightly overdoped to strongly underdoped regimes. We observed a double pair-breaking peak in the antinodal B1g configuration that we attribute to the two antinodal gaps opening on the outer and inner CuO2-plane (OP and IP) band, respectively. The doping dependence of the pair-breaking peak energy was investigated. Considering the difference in doping level between the IP and OP, all the B1g pair-breaking peak energies for OP and IP were found to align on a single line as a function of doping, which is consistent with the previous results on the double and mono-layer cuprates. Within our experimental accuracy the IP and OP peaks start to appear almost at the same temperature. These findings suggest some sort of interaction between the layers. The observed gap energy is very large, not scaling with Tc.

cond-mat.supr-con↗

arXiv2017

Three superconducting phases with different categories of pairing in hole- and electron-doped LaFeAs$_{1-x}$P$_x$O

The phase diagram of LaFeAs$_{1-x}$P$_x$O system has been extensively studied through hole- and electron-doping as well as As/P-substitution. It has been revealed that there are three different superconducting phases with different Fermi surface (FS) topologies and thus with possibly different pairing glues. One of them is well understood as spin fluctuation-mediated superconductivity within a FS nesting scenario. Another one with the FSs in a bad nesting condition must be explained in a different context such as orbital or spin fluctuation in strongly correlated electronic system. In both phases, $T$-linear resistivity was commonly observed when the superconducting transition temperature $T_{\rm c}$ becomes the highest value, indicating that the strength of bosonic fluctuation determines $T_{\rm c}$. In the last superconducting phase, the nesting condition of FSs and the related bosonic fluctuation are moderate. Variety of phase diagram characterizes the multiple orbital nature of the iron-based superconductors which are just near the boundary between weak and strong correlation regimes.

cond-mat.supr-con↗

arXiv2016

Carrier localization due to local magnetic order induced by magnetic impurities in Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM = Mn and Cr) as seen via optical spectra

The charge dynamics of Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM = Mn and Cr) has been investigated by optical spectroscopy. It was found that in addition to the strong suppression of the coherent charge transport, the magnetic impurity induces a novel spectral feature related to the carrier localization in the far-infrared region above the antiferromagnetic transition temperature. We attribute it to the cooperative effect between conduction electrons and local magnetic order induced by magnetic impurities. The present results demonstrate that Mn and Cr are not conventional magnetic pair-breakers in iron pnictides.

cond-mat.supr-con↗

arXiv2016

Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-based superconductors LaFe(As1-xPx)(O1-yFy) revealed by 31P-NMR Studies

We report on 31P-NMR studies of LaFe(As_{1-x}P_x)(O_{1-y}F_{y}) over wide compositions for 0<x<1 and 0<y<0.14, which provide clear evidence that antiferromagnetic spin fluctuations (AFMSFs) are one of the indispensable elements for enhancing Tc. Systematic 31P-NMR measurements revealed two types of AFMSFs in the temperature evolution, that is, one is the AFMSFs that develop rapidly down to Tc with low-energy characteristics, and the other, with relatively higher energy than the former, develops gradually upon cooling from high temperature. The low-energy AFMSFs in low y (electron doping) over a wide x (pnictogen height suppression) range are associated with the two orbitals of d_{xz/yz}, whereas the higher-energy ones for a wide y region around low x originate from the three orbitals of d_{xy} and d_{xz/yz}. We remark that the nonmonotonic variation of Tc as a function of x and y in LaFe(As_{1-x}P_x)(O_{1-y}F_y) is attributed to these multiple AFMSFs originating from degenerated multiple 3d orbitals inherent to Fe-pnictide superconductors.

cond-mat.supr-con↗

arXiv2016

Effect of magnetism on lattice dynamics in SrFe$_2$As$_2$ as seen via high resolution inelastic x-ray scattering

Phonon spectra of detwinned {SrFe$_2$As$_2$} crystals, as measured by inelastic x-ray scattering, show clear anisotropy accompanying the magneto-structural transition at 200 K. We model the mode splitting using magnetic DFT calculations, including a phenomenological reduction in force-constant anisotropy that can be attributed to magnetic fluctuations. This serves as a starting point for a general model of phonons in this material applicable to both magnetic and non-magnetic phase. Using this model, the measured splitting in the magnetic phase below $\it T_{N}$, and the measured phonon linewidth, we set a lower bound on the mean magnetic fluctuation frequency above $\it T_{N}$ at 210 K.

cond-mat.supr-con↗

arXiv2015

Optical Study of BaFe$_2$As$_2$ under pressure: Coexistence of spin-density-wave gap and superconductivity

Temperature-dependent reflectivity measurements in the frequency range 85 - 7000 cm$^{-1}$ were performed on BaFe$_2$As$_2$ single crystals under pressure up to $\sim$5 GPa. The corresponding pressure- and temperature-dependent optical conductivity was analyzed with the Drude-Lorentz model to extract the coherent and incoherent contributions. The gradual suppression of the spin-density wave (SDW) state with increasing pressure and the appearance of the superconducting phase coexisting with the SDW phase at 3.6~GPa were observed. At 3.6~GPa, the reflectivity reaches unity below $\sim$95~cm$^{-1}$ indicating the opening of the superconducting gap and shows a full gap tendency at 6 K.

cond-mat.supr-con↗