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In low–electron density materials, interactions can lead to highly correlated quantum states of matter. Ta 2 NiSe 5 , an excitonic insulator (EI) candidate, exists in a novel broken-symmetry phase below 327 K, characterized by robust exchange interaction and electron-lattice coupling. We study this phase of Ta 2 NiSe 5 using the quadrupole circular photogalvanic effect (QCPGE). Light-matter interaction in Ta 2 NiSe 5 mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC response even with centrosymmetry, making it particularly sensitive to certain other broken symmetries. We show that the exchange interaction in Ta 2 NiSe 5 can lead to a triclinic structure with a broken C 2 symmetry. Our results provide an incisive probe of the symmetries of the low-temperature phase of Ta 2 NiSe 5 and add new symmetry constraints to the identification of a strongly correlated EI phase. The high sensitivity of QCPGE to subtle symmetry breaking in centrosymmetric systems will enable its use in studying other complex crystalline systems.

We study pressure-induced structural evolution of vanadium diselenide (${\mathrm{VSe}}_{2}$), a $1T$ polymorphic member of the transition metal dichalcogenide (TMD) family, using synchrotron-based powder x-ray diffraction (XRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at $P\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ in the c/a ratio, V-Se bond length, and Se-V-Se bond angle, signaling an isostructural transition. This transition is followed by a first-order structural transition from the $1T$ (space group $P\overline{3}m1$) phase to a $3R$ (space group $R\overline{3}m)$ phase at $P\ensuremath{\sim}11\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ due to sliding of adjacent Se-V-Se layers. Both the transitions at $\ensuremath{\sim}4$ and 11 GPa are cognate with associated changes in the Debye-Waller factors not reported so far. We present various scenarios to understand the experimental results within DFT and find that the $1T$ to $3R$ transition is captured using spin-polarized calculations with Hubbard correction (${U}_{\mathrm{eff}}=U\ensuremath{-}J=8\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$), giving a transition pressure of $\ensuremath{\sim}9$ GPa, close to the experimental value.

Fermi surfaces, three-dimensional (3D) abstract interfaces that define the occupied energies of electrons in a solid, are important for characterizing and predicting the thermal, electrical, magnetic, and optical properties of crystalline metals and semiconductors [1]. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta ( kF ) from energy- and angular distribution of photoelectrons dislodged by monochromatic light [2]. Existing electron analyzers are able to determine a number of kF -vectors simultaneously, but current technical limitations prohibit a direct high-resolution 3D Fermi surface mapping. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces and restricting a detailed comparison with the widely available nowadays calculated 3D Fermi surfaces. Here we show that using a simpler instrumentation, based on the Fourier electron optics combined with a retardation field of the detector, it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface recorded in the full Brillouin zone along the kz-direction as well as other experimental results featuring multiple advantages of our technique. In combination with various light sources, including synchrotron radiation, our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences.

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature. The dominant mechanism of the excitonic insulator transition in Ta2NiSe5 and the nature of its high-temperature phase are debated. The authors report transient reflectivity measurements indicating a significant electronic contribution to the transition and a gapped state of preformed excitons at high temperatures.

In this paper, using low-temperature scanning tunneling microscopy, we focus on understanding the native defects in pristine $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$ at the atomic scale. We probe how they perturb charge density waves (CDWs) and lead to local CDW modulated region formation. These defects influence the correlation length of CDWs. We establish a connection between suppression of CDWs and Ti intercalation, and show how this supports the exciton condensation model of CDW formation in $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$.

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature., The dominant mechanism of the excitonic insulator transition in Ta2NiSe5 and the nature of its high-temperature phase are debated. The authors report transient reflectivity measurements indicating a significant electronic contribution to the transition and a gapped state of preformed excitons at high temperatures.

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBSs) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken ${C}_{4}$ rotational symmetry. One of the few stoichiometric IBSs, lithium iron arsenide superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission spectroscopy, that even the superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting ${T}_{c}$. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and $s\ensuremath{-}d$ mixing in the gap function.

Pump-probe microscopy measurements reveal the presence of rapidly propagating phason-phonon modes in an excitonic condensate., Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta2NiSe5. Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~105 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a substantial contribution to the ordered phase in Ta2NiSe5. These results allow us to understand how the condensate’s collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials.

SnTe is a narrow band-gap topological crystalline insulator (TCI), whereas SnSe is a normal semiconductor. We report Raman study of SnTe and SnSe as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, isostructural transition is observed in SnTe, as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa, accompanied by an increase in the linewidth. Our first-principles calculations show that the mirror Chern number of SnTe does not change and the TCI phase remains unaffected by pressure. Raman signatures of its phase transition at 1.5 GPa are associated with phonon instability at the $\mathrm{\ensuremath{\Gamma}}$ point and inversion of the lowest-energy conduction bands. An anomaly in the electron-phonon coupling results in anomalous behavior of the Raman modes at this pressure. Further, SnTe undergoes structural transitions at $\ensuremath{\sim}5.8, \ensuremath{\sim}12$, and $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The 5.8-GPa transition is associated with a structural transition from the ambient cubic ( $Fm\overline{3}m$ ) to orthorhombic (Pnma) phase, which is no longer a topological insulator, resulting in a topological phase transition. Above the transition pressure of 12 GPa, another orthorhombic Pnma[GeS] phase is stabilized, coexisting with the Pnma phase. The reduction in the number of observed Raman modes above $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and enthalpy calculations show a transition from orthorhombic (Pnma) to a more symmetric cubic ($\mathrm{Pm}\overline{3}\mathrm{m}$) structure. Our high-pressure study of SnSe, on the other hand, reveals that it undergoes two phase transitions: one from the orthorhombic (Pnma) structure to the orthorhombic (Cmcm) structure at $\ensuremath{\sim}6.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and the other at $\ensuremath{\sim}12.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, in which the Cmcm phase undergoes a semimetal to metal transition. Density functional theory calculations capture the contrast in the pressure-dependent behavior of the topological crystalline insulator SnTe and the normal semiconductor SnSe.

The magnetic, thermodynamic and dielectric properties of the γ -Fe2WO6 system is reported. Crystallizing in the centrosymmetric Pbcn space group, this particular polymorph exhibits a number of different magnetic transitions, all of which are seen to exhibit a finite magneto-dielectric coupling. At the lowest measured temperatures, the magnetic ground state appears to be glass-like, as evidenced by the waiting time dependence of the magnetic relaxation. Also reflected in the frequency dependent dielectric measurements, these signatures possibly arise as a consequence of the oxygen non-stoichiometry, which promotes a disordered magnetic and electronic ground state.

We report pressure evolution of charge density wave (CDW) order and the emergence of superconductivity (SC) in $1\mathit{T}\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ single crystal by studying resistance and magnetoresistance behavior under high pressure. With increasing quasihydrostatic pressure the CDW order enhances with its ordering temperature ($\ensuremath{\sim}100\phantom{\rule{4pt}{0ex}}\mathrm{K}$ at ambient P) increasing marginally up to 5 GPa. At higher pressures, CDW-like resistance anomaly increases more rapidly with the characteristic temperature reaching $\ensuremath{\sim}290\phantom{\rule{4pt}{0ex}}\mathrm{K}$ at 14.2 GPa. Upon further increase of pressure, the resistance anomaly due to CDW order gets suppressed drastically with rapidly increased metallicity (as clearly evidenced from the increased RRR value) and superconductivity emerges at $\ensuremath{\sim}15$ GPa, with the onset critical temperature (${T}_{c}$) $\ensuremath{\sim}4$ K. The pressure dependence of ${T}_{c}$ is found negligible, different from an increase or a dome-shaped behavior seen in isostructural layered diselenide superconductors. The high-pressure magnetoresistance and Hall measurements suggest successive electronic structural changes with Fermi surface modifications at 5 and $\ensuremath{\sim}12$ GPa.

We report on the low-energy electronic structure of Tantalum ditelluride (1$T'$-TaTe$_2$), one of the charge density wave (CDW) materials from the group V transition metal dichalcogenides using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that the Fermi surface topology of TaTe$_2$ is quite complicated compared to its isovalent compounds such as TaS$_2$, TaSe$_2$, and isostructural compound NbTe$_2$. More importantly, we discover that the surface electronic structure of 1$T'$-TaTe$_2$ has more resemblance to the 2$H$-TaTe$_2$, while the bulk electronic structure has more resemblance to the hypothetical 1$T$-TaTe$_2$. These experimental observations are thoroughly compared with our DFT calculations performed on 1$T$-, 2$H$- and 2$H$ (monolayer)/1$T$- TaTe$_2$. We further notice that the Fermi surface topology is temperature independent up to 180 K, confirming that the 2$H$ phase on the surface is stable up to 180 K and the CDW order is not due to the Fermi surface nesting., Comment: 5 figures, 22 pages, Accepted for publication in JPCC

Using angle resolved photoemission spectroscopy (ARPES) we studied the low energy electronic structure of transition metal dichalcogenide (TMDC), NiTe2, single crystal. Our ARPES data clearly show a surface Dirac point observed at a binding energy of 1.45 eV which is consistent with an earlier prediction. Thus, our results for the first time experimentally prove that NiTe2is a topological TMDC. Our ARPES data further suggest a metallic nature of NiTe2. We found that overall the electronic structure of NiTe2has similarities with the other topological TMDCs like PdTe2, PtTe2 and PtSe2.

Using angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations we studied the low-energy electronic structure of bulk ZrTe$_2$. ARPES studies on ZrTe$_2$ demonstrate free charge carriers at the Fermi level, which is further confirmed by the DFT calculations. An equal number of hole and electron carrier density estimated from the ARPES data, points ZrTe$_2$ to a semimetal. The DFT calculations further suggest a band inversion between Te $p$ and Zr $d$ states at the $\Gamma$ point, hinting at the non-trivial band topology in ZrTe$_2$. Thus, our studies for the first time unambiguously demonstrate that ZrTe$_2$ is a topological semimetal. Also, a comparative band structure study is done on ZrSe$_2$ which shows a semiconducting nature of the electronic structure with an indirect band gap of 0.9 eV between $\Gamma (A) $ and $M (L)$ high symmetry points. In the below we show that the metal-chalcogen bond-length plays a critical role in the electronic phase transition from semiconductor to a topological semimetal ingoing from ZrSe$_2$ to ZrTe$_2$., Comment: 10 pages, 7 figures

High-quality single crystals of CaTe are grown using the high temperature solution-growth method employing Te as the flux. The grown crystals were characterized using single crystal and powder X-ray diffraction, X-ray Laue diffraction, scanning electron microscope and the energy dispersive X-rays analysis for composition mapping. CaTe is an insulator, crystalizing with a cubic NaCl-type of structure. The low-temperature specific heat C p of CaTe exhibits a peak in C p / T 3 versus temperature T plot near T = 20 K with a conspicuous absence of the Debye T 3 regime down to T = 2 K.

We report magnetotransport measurements and their scaling analysis for the optimally electron-doped Sr(Fe

Single crystals of Eu

We study magnetic pair breaking due to Mn impurities in the optimally electron-doped superconductor $\mathrm{Sr}{({\mathrm{Fe}}_{0.88}{\mathrm{Co}}_{0.12})}_{2}{\mathrm{As}}_{2}$. We found that the as-grown $\mathrm{Sr}{({\mathrm{Fe}}_{0.88\ensuremath{-}y}{\mathrm{Co}}_{0.12}{\mathrm{Mn}}_{y})}_{2}{\mathrm{As}}_{2}$ single crystals exhibit a ${T}_{c}$ suppression rate of $\ensuremath{\sim}30$ mK/$\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\text{cm}$. This rate is slow but in good agreement with the previous reports on various magnetic/nonmagnetic impurities doped in other structurally analogous iron-based superconductors. The slow ${T}_{c}$ suppression rate for magnetic impurities is often cited as an evidence for the nonvalidity of the ${s}_{++}$-wave symmetry, which should have suppressed ${T}_{c}$ in accordance with the Abrikosov-Gor'kov theory. Here, we show that the crystallographic defects are the main source of pair breaking in the as-grown crystals. Once these defects are healed by a low-temperature annealing, the true ${T}_{c}$ suppression rate due to Mn impurities is revealed. We thus estimate the actual ${T}_{c}$ suppression rate due to Mn alone to be $\ensuremath{\ge}325 \mathrm{mK}/\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\text{cm}$, and that due to the nonmagnetic crystallographic defects to be nearly 35 mK/$\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\text{cm}$. These findings can be reconciled with the fully gapped ${s}_{+\ensuremath{-}}$-wave symmetry provided the interband scattering is rather weak. On the other hand, the ${s}_{++}$-wave symmetry, which is resilient to the nonmagnetic defects and fragile against the magnetic impurities, can be a possible pairing symmetry in the optimally electron-doped ${\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$. The crucial information that we provide here is that the magnetic pair breaking in these superconductors is not as weak as is generally believed.

One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole $d$ band appears to be a hallmark of many families of unconventional superconductors.

A wet-chemistry technique, namely the citrate route, has been used to prepare high-quality polycrystalline samples of double perovskite Sr{sub 2}FeMoO{sub 6}. We report on the evolution of magnetic and magnetoresistive properties of the synthesized samples as a function of three parameters (i) the pH of the starting solution, (ii) the decomposition temperature of the citrate precursors and (iii) the sintering conditions. The low-field magnetoresistance (LFMR) value of our best samples is as high as 5% at room temperature for an applied magnetic field of 1 kOe. Additionally, the distinguishing feature of these samples is the persistence of LFMR, with a reasonably large value, up to 400 K which is a crucial parameter for any practical application. Our study indicates that the enhancement of LFMR observed is due to a good compromise between the grain size distribution and their magnetic polarization. -- Graphical abstract: The microstructure (left panel) and corresponding low-field magnetoresistance of one of the Sr{sub 2}FeMoO{sub 6} samples synthesized in the course of this work. Highlights: • Samples of Sr{sub 2}FeMoO{sub 6} are prepared using a citrate route under varying conditions. • Magnetoresistive properties are improved and optimized. • Low-field magnetoresitence values as large as 5% at 300 K/1more » kOe are reported. • Persistence of low-field magnetoresistance up to 400 K.« less

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe 0.944 Co 0.056 ) 2 As 2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave ( T SDW ) and superconducting ( T SC ) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be ∼1.8 k B T SC and an electron–phonon coupling constant λ of ∼0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between T SC and T SDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.

We report magnetotransport measurements and their scaling analysis for the optimally electron-doped Sr(Fe0.88Co[Formula: see text]As2 system. We observe that both the Kohler's and modified Kohler's scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence (cot[Formula: see text] [Formula: see text] T 2) similar to many cuprates and heavy fermion systems. The fact that this T 2 dependence is seen in spite of the violation of modified Kohler's scaling suggests that the Hall angle and magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetoresistance in this system, which does not harbor a spin density wave ground state. Implications of our observations are discussed in the context of existing models for the magnetotransport of these strongly correlated electron systems.

Single crystals of Eu1-x Ca x Fe2As2 ([Formula: see text]) are grown using the high-temperature solution-growth method employing FeAs self-flux. Structural and chemical analysis indicates that these crystals are homogeneous and their lattice parameters exhibit a gradual monotonic decrease with increasing Ca concentration. Detailed magnetic, specific heat and resistivity data were used to construct a phase diagram which depicts the evolution of the structural/spin-density-wave transition at T 0, and of the antiferromagnetic (AFM) ordering temperature of the Eu moments at T N. We found out that while T N decreases monotonically from 19.1 K (for x = 0) to below 2 K (for [Formula: see text]), T 0 remains almost constant up to x = x c and decreases steadily for higher values of x. Annealing at low temperatures for several days leads to enhancement of T N and T 0 by a few kelvin and sharpened the anomalies associated with these transitions. However, annealing did not change the variation of T N and T 0 across the series. The observation that T 0 is almost constant until the long-range AFM ordering of the Eu2+ moments gets destroyed, suggests a subtle interrelationship between the Eu2+ and Fe2+ magnetic sublattices.

Large and high quality single crystals of the new unconventional superconductor LiFeAs were grown by a new approach using the self-flux technique. Both energy dispersive X-ray spectroscopy and inductively coupled plasma mass spectroscopy revealed a stoichiometric Li/Fe/As composition. Measurements of the magnetic susceptibility reveal the superconducting transition at TC = 17 K with a very sharp ΔTC and a 100% shielding fraction and, thus, bulk superconductivity. This sharp transition is also found by measurements of the specific heat and by measurements of the temperature dependence of the resistivity. Nuclear quadrupole resonance (NQR) spectroscopy reveals a very sharp resonance line, with a much smaller line width than reported for all other FeAs superconductors, confirming the high ordering of the LiFeAs single crystals also on a local scale.

We present a detailed investigation of the magnetic and superconducting properties of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals with $x=0.00$, 0.35, 0.50, and 0.67 by means of the local probe techniques M\"ossbauer spectroscopy and muon spin relaxation experiments. With increasing Na-substitution level, the magnetic order parameter is suppressed. For $x=0.50$ we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature ${T}_{c}$. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures ${T}_{c}/{T}_{N}$, which can be understood in the framework of a Landau theory. In the optimally doped sample with ${T}_{c}\ensuremath{\approx}34$ K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting $s$-wave gaps.

We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $\mathbf{Q}=(0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},{q}_{l})$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with a longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor of 8 weaker than the resonance signal in nearly optimally Co-doped ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, although a factor of 2 is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin-space anisotropy with a slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic fields parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps., 9 pages, 6 figures

We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving displacement of Fe atoms, show sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400-1200 cm-1 are attributed to the electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which increases as temperature decreases below Tsm. A broad Raman band observed at ~ 3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet., Accepted for Publication in JPCM

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole-doped superconductor ${\mathrm{Ca}}_{0.32}{\mathrm{Na}}_{0.68}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. This compound exhibits bulk superconductivity with a transition temperature ${T}_{\mathrm{c}}\ensuremath{\approx}34$ K, which is evident from the magnetization, transport, and specific heat measurements. The zero-field data manifest a significant electronic specific heat in the normal state with a Sommerfeld coefficient $\ensuremath{\gamma}\ensuremath{\approx}53$ mJ/mol K${}^{2}$. Using a multiband Eliashberg analysis, we demonstrate that the dependence of the zero-field specific heat in the superconducting state is well described by a three-band model with an unconventional s${}_{\ifmmode\pm\else\textpm\fi{}}$ pairing symmetry and gap magnitudes ${\ensuremath{\Delta}}_{i}$ of approximately 2.35, 7.48, and $\ensuremath{-}$7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling, which contributes significantly to the relatively high value of ${T}_{c}$. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-${T}_{c}$ iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions.

Iron-based high-temperature superconductors form an in-creasingly growing subject for investigation. Unlike othertypes of high-temperature superconductors, iron-based com-pounds can be synthesized in the form of various crystals, ex-hibiting a large variety of electronic band structures, magneticproperties, superconducting order parameters, and electronicproperties in general.

In this article, we show possibilities and limitations of the Fe-57-Mossbauer spectroscopy applied to iron-based pnictides and chalcogenides. The principle structure of these materials are FePn- or FeCh-layers with tetragonally coordinated Fe2+ ions. These layers are the electronically active structures for the competing magnetic and superconducting ground states. Mossbauer spectroscopy is frequently applied to investigate zero field local magnetic properties of both powder and single crystal materials. However, the absolute size of the hyperfine parameters often is small and the study can benefit from the application of an external field. With the help of an applied field it is possible to study magnetic interactions, ferromagnetic impurities, the sign of the electric field gradient (EFG) and magnetoelastic coupling. We show different aspects of applied magnetic field measurements and discuss results in iron-based pnictides and chalcogenides. (C) 2016 Wiley-Blackwell- VCH Verlag GmbH & Co. KGaA, Weinheim

$^{75}$As NMR investigation of a single crystal of superconducting LiFeAs is presented. The Knight shift and the \textit{in situ} ac susceptibility measurements as a function of temperature and external field are indicative of two superconducting (SC) transition temperatures, each of which is associated with its own upper critical field. Strikingly, the Knight shift maintains its normal state value over a temperature range in the SC state before it drops abruptly being consistent with spin-singlet pairing. Together with our previous NMR study, the anomalous SC state featured by the constant Knight shift is attributed to the extremely sensitive SC properties of LiFeAs, probably stemming from its proximity to a critical instability., The title and text were modified; accepted in J. Physics: Condensed matter

Ca(Fe1-xCox)(2)As-2 single crystals with concentration x = 0.032, 0.051, and 0.063 were investigated by means of neutron diffraction under hydrostatic pressure. The region of existence of the high-temperature tetragonal, low-temperature orthorhombic, and pressure-induced collapsed tetragonal phases have been determined and the corresponding p-T phase diagrams for the three compositions have been constructed. Structural details of the tetragonal phases at various conditions have been determined. The anomaly marking the orthorhombic-to-tetragonal structural phase transition temperature T-S decreases with increasing Co substitution. Similarly to that, the critical pressure of the tetragonal (or orthorhombic)-to-collapsed-tetragonal transition also decreases with increasing Co doping level. The behavior is highly hysteretic and the differences in critical pressures with increasing and decreasing branches eventually makes the collapsed tetragonal phase stable down to ambient pressure and enables structural studies under identical conditions. Within the error bars on the fourth digit, no modification of the As structural positional parameter is found. Such a result suggests that the change of the As-As bond is caused solely by the reduction of the c-axis lattice parameter. (Less)

The role of Co substitution in the low-energy electronic structure of Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant photoemission spectroscopy and density functional theory. The Co 3d-state center-of-mass is observed at 250 meV higher binding energy than Fe's, indicating that Co posses one extra valence electron, and that Fe and Co are in the same 2+ oxidation state. Yet, significant Co character is detected for the Bloch wavefunctions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2, and the inadequacy of a rigid-band shift description., 4 pages, 4 figures

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector ($0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},0$) with $\ensuremath{\delta}\ensuremath{\sim}0.07$ slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)${}_{2}$As${}_{2}$ and Ba(Fe,TM)${}_{2}$As${}_{2}$ with transition metal TM = Ni or Cu. The valence band photoemission data show directly that the TM states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the $3{d}^{8}$ final state of the $LVV$ Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is rather robust against impurity scattering.

Several angle resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within hole-like bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the hole-like bands. The resolution follows from an excellent agreement between experimental quasiparticle scattering data and T-matrix QPI calculations (based on experimental band structure data), which allows disentangling interband and intraband scattering processes.

We report ${}^{75}$As NQR measurements on single-crystalline Ca(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.09$). The nuclear spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ as a function of temperature $T$ and Co dopant concentration $x$ reveals a gradual decrease of ${({T}_{1}T)}^{\ensuremath{-}1}$ below a crossover temperature ${T}^{*}$ in the under- and optimally-doped region. Since there is no hint of a thermodynamic phase transition near ${T}^{*}$, this spin-gap behavior is attributed to the presence of a pseudogap-like novel state of electrons above ${T}_{c}$. The resulting $x$-$T$ phase diagram shows that, after suppression of the spin-density-wave order, ${T}^{*}$ intersects ${T}_{c}$ falling to zero rapidly near the optimal doping regime. Possible origins of the pseudogap behavior are discussed.

LiFeAs is unique among the broad family of FeAs-based superconductors, because it is superconducting with a rather large $T_c\simeq 18$ K under ambient conditions although it is a stoichiometric compound. We studied the electrical transport on a high-quality single crystal. The resistivity shows quadratic temperature dependence at low temperature giving evidence for strong electron-electron scattering and a tendency towards saturation around room temperature. The Hall constant is negative and changes with temperature, what most probably arises from a van Hove singularity close to the Fermi energy in one of the hole-like bands. Using band structure calculations based on angular resolved photoemission spectra we are able to reproduce all the basic features of both the resistivity as well as the Hall effect data., Comment: 6 pages, 3 figures included; V2 has been considerably revised and contain a more detailed analysis of the Hall effect data

Single crystals of the new unconventional superconductor LiFe${}_{1\ensuremath{-}x}$Co${}_{x}$As with $x$$=$ 0, 0.025, 0.05 were grown by a new approach using the self-flux technique. The superconducting transition temperature was found to decrease upon Co doping at the Fe site. Apparently, in LiFeAs this doping scheme suppresses superconductivity, in contrast to the effects of Co doping in other Fe-As compounds, where it suppresses the spin-density wave and establishes superconductivity. Angle-resolved photoemission spectroscopy shows that the bottom of electron-like bands sinks by about 17 meV upon $5%$ Co doping, which indicates that the chemical substitution of Co for Fe in LiFeAs results in charge doping.

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high ${T}_{c}\ensuremath{\sim}18\text{ }\text{ }\mathrm{K}$. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and ${s}_{\ifmmode\pm\else\textpm\fi{}}$ superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries ${s}_{++}$, ${s}_{\ifmmode\pm\else\textpm\fi{}}$, and $d$ wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary $p$ wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., $s+id$ wave symmetry).

Plateletlike single crystals of the Ca(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ series having lateral dimensions up to 15 mm and thicknesses up to 0.5 mm were obtained from the high-temperature solution-growth technique using Sn flux. Upon Co doping, the $c$ axis of the tetragonal unit cell decreases, while the $a$ axis shows a less significant variation. Pristine CaFe${}_{2}$As${}_{2}$ shows a combined spin-density wave and structural transition near $T=$ 166 K, which gradually shifts to lower temperatures and splits with increasing Co doping. Both transitions terminate abruptly at a critical Co concentration of ${x}_{c}=$ 0.075. For $x$ \ensuremath{\geqslant} 0.05, superconductivity (SC) appears at low temperatures with a maximum transition temperature ${T}_{C}$ of around 20 K. The SC volume fraction increases with Co concentration up to $x=$ 0.09 followed by a gradual decrease with a further increase of the doping level. The electronic phase diagram of the Ca(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ (0 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature SC properties of Co-doped CaFe${}_{2}$As${}_{2}$ differ considerably from those of BaFe${}_{2}$As${}_{2}$ reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe${}_{2}$As${}_{2}$ relative to its Ba counterpart.

We investigate the fishtail effect, critical current density ($J_c$) and vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak (SP) in the magnetization loop only with the field $||$ c-axis. We calculate a reasonably high $J_c$, however, values are lower than in 'Ba-122' and '1111'-type FeAs-compounds. Magnetic relaxation data imply a strong pinning which appears not to be due to conventional defects. Instead, its behavior is similar to that of the triplet superconductor Sr$_2$RuO$_4$. Our data suggest that the origin of the SP may be related to a vortex lattice phase transition. We have constructed the vortex phase diagram for LiFeAs on the field-temperature plane., Comment: 5 pages, 5 figures

We present a calorimetric study on single crystals of Ca(Fe1-xCox)2As2 (x = 0, 0.032, 0.051, 0.056, 0.063, and 0.146). The combined first order spin-density wave/structural transition occurs in the parent CaFe2As2 compound at 168 K and gradually shifts to lower temperature for low doping levels (x = 0.032 and x = 0.051). It is completely suppressed upon higher doping x = 0.056. Simultaneously, superconductivity appears at lower temperature with a transition temperature around Tc = 14.1 K for Ca(Fe0.937Co0.063)2As2. The phase diagram of Ca(Fe0.937Co0.063)2As2 has been derived and the upper critical field is found to be H(c) c2 = 11.5, Comment: 6 pages, 4 figures