arXiv subjects

C. W. Chu

Publications and source records attributed to C. W. Chu.

At least 37 records · Page 2Linked to original sources

arXiv2013

High pressure study of the normal and superconducting states of the layered pnictide oxide Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O with x = 0, 0.10, and 0.15

Here we present a systematic study of the effects of pressure on the superconducting and spin/charge density wave (SDW/CDW) transitions of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O (x = 0, 0.10, and 0.15) by means of resistivity measurements. For x = 0 and 0.10, external pressure results in a decease of the SDW/CDW transition temperature T$_c$; however, no measurable change is observed for the x = 0.15. The pressure effect on the superconducting transition temperature is different for all three samples. For BaTi$_2$Sb$_2$O (x=0), T$_c$ increases significantly from 1.2 K at zero pressure to $\sim$ 2.9 K at 16.1 kbars. The 10 % Na-doped sample shows an initial T$_c$ increase up to 4.2 K with pressure which saturates at higher pressure values. For higher Na concentrations (x=0.15), T$_c$ continuously decreases with increasing pressure.

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arXiv2013

Superconductivity in the Mn5Si3-type Zr5Sb3 system

We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-structure type. Transport, magnetic, and calorimetric measurements and band structure calculations show it to be a phonon-mediated BCS superconductor, with a relatively large density of states at the Fermi level associated with the d-electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3. More superconductors with even higher transition temperatures are expected to be found in this family of compounds.

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arXiv2013

The simultaneous occurrence of superparamagnetism and superconductivity in nominal single crystalline (Ca1-xPrx)Fe2As2 with 0 <= x <= 0.13

The observation of non-bulk superconductivity with an unexpectedly high onset transition temperature Tc up to ~ 49 K in non-superconducting single crystalline CaFe2As2 upon rare-earth doping has raised interesting questions concerning its origin. Several possibilities including interfacial mechanism have been proposed to account for the unusual observation. In an attempt to differentiate such propositions, we have carried out a systematic compositional and magnetic study on single crystals of nominal (Ca1-xPrx)Fe2As2 with 0 <= x <= 0.13 throughout the solubility range of Pr, which covers both the non-superconducting and superconducting regions. We found the unusual simultaneous occurrence of superparamagnetism and superconductivity with an x-independent Tc and a close correlation of the superconducting volume fraction with the magnetic cluster density and As-defect density. The finding demonstrates a close relationship among superconductivity, superparamagnetism, and defects, consistent with the previously proposed interface-mechanism, and offers a possible future path to higher Tc.

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arXiv2013

Anomalous hysteresis as an evidence for a magnetic field-induced chiral superconducting state in LiFeAs

Magnetometry measurements in high quality LiFeAs single-crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field $H_{c2}$, from a clear diamagnetic response dominated by the pinning of vortices, to a considerably smaller net hysteretic response of opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response at high fields results from pinned vortices and associated screening super-currents, this sign change must result from currents circulating in the opposite sense, which give rise to a small field-dependent magnetic moment \emph{below} $H_{c2}$. This behavior seems to be extremely sensitive to the sample quality or stoichiometry, as we have observed it only in a few fresh crystals, which also display the de Haas van Alphen-effect. We provide arguments against the surface superconductivity, the flux compression, and the random $π$ junction scenarios, which have been previously put forward to explain a paramagnetic Meissner effect, below the lower critical field $H_{c1}$. The observed anomalous hysteresis at high fields will be compatible with the existence of chiral gap wave-functions, which possess a field dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate how a $(d_{x^2 - y^2} + id_{xy})$ or a $(p_x+ip_y)$ chiral superconducting component can be stabilized in the mixed state of $s_{\pm}$ superconductor, due to the combined effects of the magnetic field and the presence of competing pairing channels. The realization of a particular chiral pairing depends on the microscopic details of the strengths of the competing pairing channels.

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arXiv2013

Experimental investigation of the electronic structure of Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$

We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca$_{0.83}$La$_{0.17}$Fe$_2$As$_2$. A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space, as well as the consequent band folding in the momentum space. Besides this band folding effect, the Fermi surface topology of this material is similar to that reported previously for BaFe$_{1.85}$Co$_{0.15}$As$_2$, with $Γ$-centred hole pockets quasi-nested to M-centred electron pockets by the antiferromagnetic wave vector. Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction, a gap-like density of states depression of $7.7\pm 2.9$ meV is determined by scanning tunneling microscopy.

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arXiv2012

Magnetic orders and spin-flop transitions in the cobalt doped multiferroic $\rm Mn_{1-x}Co_{x}WO_4$

We present a comprehensive single crystal neutron diffraction investigation of the $\rm Mn_{1-x}Co_{x}WO_4$ with $0.02\leq x \leq0.30$. At lower concentration $x \leq 0.05$, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For $x\sim 0.075$, the magnetic structure undergoes a spin flop transition that is characterized by a sudden rotation of the spin helix envelope into the $ac$ plane. This spin structure persists for concentration up to $x=0.15$, where additional competing magnetic orders appear at low temperature. For $0.17 \leq x \leq 0.30$, the system experiences another spin flop transition and recovers the low-$x$ spiral spin configuration. A simple commensurate spin structure with $\vec{q}=(0.5,0,0)$ is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped $\rm MnWO_4$ contrasts sharply with other transition metal ion doped $\rm Mn_{1-x}A_xWO_4$ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $\rm Mn_{1-x}Co_{x}WO_4 $ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.

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arXiv2012

Weak ferromagnetism and internal magnetoelectric effect in LiFeP$_2$O$_7$

The magnetic, thermodynamic, and pyroelectric properties of LiFeP$_2$O$_7$ single crystals are investigated with emphasis on the magnetoelectric interaction of the electrical polarization with the magnetic order parameter. The magnetic order below T$_N\simeq$ 27 K is found to be a canted antiferromagnet with a weak ferromagnetic component along the $b-$axis. A sharp peak of the pyroelectric current at T$_N$ proves the strong internal magnetoelectric interaction resulting in a sizable polarization decrease at the onset of magnetic order. The magnetoelectric effect in external magnetic fields combines a linear and a quadratic field dependence below T$_N$. Thermal expansion data show a large uniaxial magnetoelastic response and prove the existence of strong spin lattice coupling. LiFeP$_2$O$_7$ is a polar compound with a strong interaction of the magnetic order parameter with the electric polarization and the lattice.

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arXiv2012

Field-induced continuous rotation of the polarization in multiferroic Mn$_{0.95}$Co$_{0.05}$WO$_{4}$

We report the observation a continuous rotation of the polarization in Mn$_{0.95}$Co$_{0.05}$WO$_{4}$ under magnetic field. At zero field, this compound shows a transition into the spiral magnetic and ferroelectric phase at 12.2K, which is the ground state, with the polarization oriented along the b-axis. Increasing b-axis magnetic fields rotate the ferroelectric polarization continuously toward the a-axis, indicating a rotation of the spin spiral plane. This rotation extends over a large field and temperature range. At a constant magnetic field of 3 Tesla, the polarization also rotates from the a-axis at the ferroelectric transition toward the b-axis upon decreasing temperature.

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arXiv2012

The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.

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arXiv2012

Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2

We report specific-heat and resistivity experiments performed in parallel in a Bridgman-type of pressure cell in order to investigate the nature of pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of the superconducting state. The thermodynamic transition temperature differs dramatically from the onset of the resistive transition. Our data indicates that superconductivity occurs in the vicinity of a crystallographic phase transition. We discuss the discrepancy between the two methods as caused by strain-induced superconducting precursors formed above the bulk thermodynamic transition due to the vicinity of the structural instability.

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arXiv2012

Magnetoelectricity in the system $RAl_3(BO_3)_4$ ($R$ = Tb, Ho, Er, Tm)

The magnetoelectric effect in the system $RAl_3(BO_3)_4$ ($R$ = Tb, Ho, Er, Tm) is investigated between 3 K and room temperature and at magnetic fields up to 70 kOe. We show a systematic increase of the magnetoelectric effect with decreasing magnetic anisotropy of the rare earth moment. A giant magnetoelectric polarization is found in the magnetically (nearly) isotropic $HoAl_3(BO_3)_4$. The polarization value in transverse field geometry at 70 kOe reaches 3600 $μC/m^2$ which is significantly higher than reported values for the field-induced polarization of linear magnetoelectric or even multiferroic compounds. The results indicate a very strong coupling of the f-moments to the lattice. They further indicate the importance of the field-induced ionic displacements in the unit cell resulting in a polar distortion and a change in symmetry on a microscopic scale. The system $RAl_3(BO_3)_4$ could be interesting for the technological utilization of the high-field magnetoelectric effect.

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arXiv2011

High-pressure study of superconducting and non-superconducting single crystals of the same nominal composition Rb0.8Fe2Se2

Two single crystalline samples with the same nominal composition of Rb0.8Fe2Se2 prepared via slightly different precursor routes under the same thermal processing conditions were investigated at ambient and high pressures. One sample was found superconducting with a Tc of ~31 K without the previously reported resistivity-hump and the other was unexpectedly found to be a narrow-gap semiconductor. While the high pressure data can be understood in terms of pressure-induced variation in doping, the detailed doping effect on superconductivity is yet to be determined.

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arXiv2011

Pressure effects on strained FeSe0.5Te0.5 thin films

The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc's are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explained by a pressure-induced decrease of the anion height but other parameters have to be considered to explain the data for all films.

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arXiv2011

Incommensurate spin-density wave and multiband superconductivity in Na$_{x}$FeAs as revealed by nuclear magnetic resonance

We report a $^{23}$Na and $^{75}$As nuclear magnetic resonance (NMR) investigation of Na$_{x}$FeAs series ($x=1$, 0.9, 0.8) exhibiting a spin-density wave (SDW) order below $T_{\rm SDW}=45$, 50 and 43 K for $x=1$, 0.9, 0.8, respectively, and a bulk superconductivity below $T_c\approx 12$ K for x=0.9. Below $T_{\rm SDW}$, a spin-lattice relaxation reveals the presence of gapless particle-hole excitations in the whole $x$ range, meaning that a portion of the Fermi surface remains gapless. The superconducting fraction as deduced from the bulk susceptibility scales with this portion, while the SDW order parameter as deduced from the NMR linewidth scales inversely with it. The NMR lineshape can only be reproduced assuming an incommensurate (IC) SDW. These findings qualitatively correspond to the mean-field models of competing interband magnetism and intraband superconductivity, which lead to an IC SDW order coexisting with superconductivity in part of the phase diagram.

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arXiv2011

Raman scattering study of electron-doped Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$ superconductors

Temperature-dependent polarized Raman spectra of electron-doped superconducting Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$ ($x \approx 0.12$) single crystals are reported. All four allowed by symmetry even-parity phonons are identified. Phonon mode of B$_{1g}$ symmetry at 222 cm$^{-1}$, which is associated with the c-axis motion of Fe ions, is found to exhibit an anomalous frequency hardening at low temperatures, that signals non-vanishing electron-phonon coupling in the superconducting state and implies that the superconducting gap magnitude $2Δ_c < 27$meV.

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arXiv2011

The Unusual Superconducting State at 49 K in Electron-Doped CaFe2As2 at Ambient

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at ~ 49 K, which has a low critical field ~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.

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arXiv2011

Giant Magnetoelectric Effect in HoAl_3(BO_3)_4

A giant magnetoelectric polarization is found in HoAl$_3$(BO$_3$)$_4$. The polarization in transverse field geometry at 70 kOe reaches 3600 $μC/m^2$ which is significantly higher than reported values of linear magnetoelectric or even multiferroic compounds. The magnetostrictive effect is also measured and compared with the magnetoelectricity. The results show that spin-lattice coupling in HoAl$_3$(BO$_3$)$_4$ is extremely strong and that the magnetic field causes a large polar distortion of the ionic positions in the unit cell.

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arXiv2011

Long-Range Magnetic Interactions in the Multiferroic Antiferromagnet $\rm MnWO_4$

The spin-wave excitations of the multiferroic $\rm MnWO_4$ have been measured in its low-temperature collinear commensurate phase using high-resolution inelastic neutron scattering. These excitations can be well described by a Heisenberg model with competing long-range exchange interactions and a single-ion anisotropy term. The magnetic interactions are strongly frustrated within the zigzag spin chain along c axis and between chains along the a axis, while the coupling between spin along the b axis is much weaker. The balance of these interactions results in the noncollinear incommensurate spin structure associated with the magnetoelectric effect, and the perturbation of the magnetic interactions leads to the observed rich phase diagrams of the chemically-doped materials. This delicate balance can also be tuned by the application of external electric or magnetic fields to achieve magnetoelectric control of this type of materials.

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