We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in La_{1.675}Eu_{0.2}Sr_{0.125}CuO_{4}. By differentiating elastic from inelastic scattering, it is demonstrated ...that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as T^{-2} towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (La_{1.875}Ba_{0.125}CuO_{4}, La_{1.475}Nd_{0.4}Sr_{0.125}CuO_{4}, and La_{1.875}Sr_{0.125}CuO_{4}) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in La_{1.675}Eu_{0.2}Sr_{0.125}CuO_{4} extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.
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Sm$_2$Ti$_2$O$_7$, a member of rare-earth titanate pyrochlores, exhibits
dipolar-octupolar antiferromagnetism below $T_N$ = 0.35 K. We observed two
ferroelectric transitions at 182 ($T_{FE1}$) and 52 ...K ($T_{FE2}$),
significantly higher than $T_N$ for Sm$_2$Ti$_{2-x}$V$_x$O$_7$ ($x$ = 0, 0.1).
Although the ferroelectric transition temperatures remain unchanged, the
polarization value decreases considerably with V doping. A structural
transition to a polar $R3m$ rhombohedral phase from the cubic $Fd\bar{3}m$
structure occurs at $T_{FE1}$, involving a distortion in the pyochlore lattice.
Remarkably, significant linear magnetoelectric coupling is observed in both
compounds, with further enhancement of magnetoelectric coupling due to magnetic
V doping. The existence of magnetoelectric coupling without long-range magnetic
order in a frustrated pyrochlore system could enable the tailoring of
magnetoelectric coupling properties, which can be further fine-tuned through V
doping. The emergence of ferroelectricity in a frustrated magnetic system
introduces an intriguing aspect to these compounds and paves the way for
developing ferroelectric order driven by the alleviation of magnetic
frustration in pyrochlore systems.
A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in Ir Ir
Pt
Te
, as a function of Pt substitution and externally applied hydrostatic pressure, is ...presented. Experiments are focused on samples near the critical composition x
~ 0.045 where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in Ir
Pt
Te
is preempted - and hence triggered - by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1, 0, 1) domain axis with a (1/5, 0, 1/5) modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.
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The strongly correlated insulator Ca_{2}RuO_{4} is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective ...coexistence of a band and a Mott gap. We present a high resolution oxygen K-edge resonant inelastic x-ray scattering study of the antiferromagnetic Mott insulating state of Ca_{2}RuO_{4}. A set of low-energy (about 80 and 400 meV) and high-energy (about 1.3 and 2.2 eV) excitations are reported, which show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modeling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveils the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by Hund’s coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca_{2}RuO_{4}.
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Sm\(_2\)Ti\(_2\)O\(_7\), a member of rare-earth titanate pyrochlores, exhibits dipolar-octupolar antiferromagnetism below \(T_N\) = 0.35 K. We observed two ferroelectric transitions at 182 ...(\(T_{FE1}\)) and 52 K (\(T_{FE2}\)), significantly higher than \(T_N\) for Sm\(_2\)Ti\(_{2-x}\)V\(_x\)O\(_7\) (\(x\) = 0, 0.1). Although the ferroelectric transition temperatures remain unchanged, the polarization value decreases considerably with V doping. A structural transition to a polar \(R3m\) rhombohedral phase from the cubic \(Fd\bar{3}m\) structure occurs at \(T_{FE1}\), involving a distortion in the pyochlore lattice. Remarkably, significant linear magnetoelectric coupling is observed in both compounds, with further enhancement of magnetoelectric coupling due to magnetic V doping. The existence of magnetoelectric coupling without long-range magnetic order in a frustrated pyrochlore system could enable the tailoring of magnetoelectric coupling properties, which can be further fine-tuned through V doping. The emergence of ferroelectricity in a frustrated magnetic system introduces an intriguing aspect to these compounds and paves the way for developing ferroelectric order driven by the alleviation of magnetic frustration in pyrochlore systems.
We present a combined growth and transport study of superconducting
single-crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$. Evidence of two-dimensional
superconductivity with significant phase fluctuations ...of preformed Cooper pairs
preceding the superconducting transition is reported. This result is based on
three key observations. (1) The resistive superconducting transition
temperature $T_c$ (defined by resistivity $\rho \rightarrow 0$) increases with
increasing disorder. (2) As $T\rightarrow T_c$, the conductivity diverges
significantly faster than what is expected from Gaussian fluctuations in two
and three dimensions. (3) Non-Ohmic resistance behavior is observed in the
superconducting state. Altogether, our observations are consistent with a
temperature regime of phase-fluctuating superconductivity. The crystal
structure with magnetic ordering tendencies in the NdO$_{0.5}$F$_{0.5}$ layers
and (super)conductivity in the BiS$_2$ layers is likely responsible for the
two-dimensional phase fluctuations. As such, NdO$_{0.5}$F$_{0.5}$BiS$_2$ falls
into the class of unconventional ``laminar" bulk superconductors that include
cuprate materials and 4Hb-TaS$_2$.
Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and ...importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO\(_2\), single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La\(_2\)CuO\(_4\) indicate that quantum fluctuations are much stronger in SrCuO\(_2\) suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO\(_2\) - due to strong quantum fluctuations - is a unique starting point for the exploration of novel magnetic ground states.
Unconventional superconductivity often couples to other electronic orders in
a cooperative or competing fashion. Identifying external stimuli that tune
between these two limits is of fundamental ...interest. Here, we show that
$c$-axis strain couples directly to the competing interaction between charge
stripe order and superconductivity in La$_{1.88}$Sr$_{0.12}$CuO$_4$ (LSCO).
Compressive $c$-axis pressure amplifies stripe order within the superconducting
state, while having no impact on the normal state. By contrast, strain
dramatically diminishes the magnetic field enhancement of stripe order in the
superconducting state. This observation hints to the existence of a ground
state without phase competition. As such, c-axis pressure is a promising tuning
parameter in the search for ground states where superconductivity adopts broken
translational symmetry from charge ordering phenomena.
The kagome lattice is an intriguing and rich platform for discovering, tuning
and understanding the diverse phases of quantum matter, which is a necessary
premise for utilizing quantum materials in ...all areas of modern and future
electronics in a controlled and optimal way. The system LaRu$_{3}$Si$_{2}$ was
shown to exhibit typical kagome band structure features near the Fermi energy
formed by the Ru-$dz^{2}$ orbitals and the highest superconducting transition
temperature $T_{\rm c}$ ${\simeq}$ 7K among the kagome-lattice materials.
However, the effect of electronic correlations on the normal state properties
remains elusive. Here, we report the discovery of charge order in
La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$ ($x$ = 0, 0.01, 0.05) beyond
room-temperature. Namely, single crystal X-ray diffraction reveals charge order
with a propagation vector of ($\frac{1}{4}$,0,0) below $T_{\rm CO-I}$
${\simeq}$ 400K in all three compounds. At lower temperatures, we see the
appearance of a second set of charge order peaks with a propagation vector of
($\frac{1}{6}$,0,0). The introduction of Fe, which is known to quickly suppress
superconductivity, does not drastically alter the onset temperature for charge
order. Instead, it broadens the scattered intensity such that diffuse
scattering appears at the same onset temperature, however does not coalesce
into sharp Bragg diffraction peaks until much lower in temperature. Our results
present the first example of a charge ordered state at or above room
temperature in the correlated kagome lattice with bulk superconductivity.
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