arXiv subjects

G. Campi

Publications and source records attributed to G. Campi.

16 recordsLinked to original sources

arXiv2026

Electronic Structure and Superconductivity in La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ Artificial High-$T_c$ Superlattices Probed by Hard and Soft X-ray Spectroscopy

In La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ (LSCO/LCO) artificial high-$T_c$ superlattices (AHTS), grown by quantum material design, the superconducting dome can be tuned by varying the geometric ratio $L/d$, where $L$ is the LCO layer thickness and $d$ is the superlattice nanoscale period. Here, we combine hard X-ray photoelectron spectroscopy (HAXPES) and polarization-dependent soft X-ray absorption spectroscopy (XAS) to probe how the electronic structure evolves across the $L/d$-tuned superconducting dome. We observe systematic chemical-potential evolution across the series, together with enhanced spectral weight near the Fermi level, enhanced local and non-local screening signatures, and increased in-plane orbital polarization near the top of the dome. Together, these spectroscopic signatures provide insight into the emergence of Fano-Feshbach resonant superconductivity in the LSCO/LCO AHTS series.

cond-mat.supr-con↗

arXiv2025

Upper critical magnetic field and multiband superconductivity in artificial high-Tc superlattices of nano quantum wells

Artificial high-Tc superlattices (AHTS) composed of quantum building blocks with tunable superconducting critical temperature have been synthesized by engineering their nanoscale geometry using the Bianconi-Perali-Valletta (BPV) two gaps superconductivity theory. These quantum heterostructures consist of quantum wells made of superconducting, modulation-doped Mott insulators (S), confined by a metallic (N) potential barrier. The lattice geometry has been carefully engineered to induce the predicted Fano-Feshbach shape resonance between the gaps, near a topological Lifshitz transition. Here, we validate the BPV theory by providing compelling experimental evidence that AHTS samples, at the peak of the superconducting dome, exhibit resonant two-band, two-gap superconductivity. This is demonstrated by measuring the temperature dependence of the upper critical magnetic field,Bc2, in samples with superlattice periods 3.3<d<5.28 nm and L/d ratios close to the magic value 2/3 (where L is the thickness of the superconducting La2CuO4 layer and d is the superlattice period). The data reveal the predicted upward concavity in Hc2(T) and a characteristic kink in the coherence length as a function of temperature, confirming the predicted two-band superconductivity with Fermi velocity ratio 0.25 and significant pair exchange term among the two condensates.

cond-mat.mes-hall↗

arXiv2020

Intermittent dynamics of antiferromagnetic phase in inhomogeneous iron-based chalcogenide superconductor

Coexistence of phases, characterized by different electronic degrees of freedom, commonly occurs in layered superconductors. Among them, alkaline intercalated chalcogenides are model systems showing microscale coexistence of paramagnetic (PAR) and antiferromagnetic (AFM) phases, however, temporal behavior of different phases is still unknown. Here, we report the first visualization of the atomic motion in the granular phase of K$_{x}$Fe$_{2-y}$Se$_2$ using X-ray photon correlation spectroscopy. Unlike the PAR phase, the AFM texture reveals an intermittent dynamics with avalanches as in martensites. When cooled down across the superconducting transition temperature T$_c$, the AFM phase goes through an anomalous slowing behavior suggesting a direct relationship between the atomic motions in the AFM phase and the superconductivity. In addition of providing a compelling evidence of avalanche-like dynamics in a layered superconductor, the results provide a basis for new theoretical models to describe quantum states in inhomogeneous solids.

cond-mat.supr-con↗

arXiv2017

Intrinsic dynamical fluctuations of PNS myelin

The ultrastructure fluctuations and complex dynamics of the multi-layered membrane structure of myelin are fundamental for understanding and control its formation process and its degeneration and repair in neurological diseases such as multiple sclerosis (MS). Myelin is considered a liquid-crystal but information are confined to its average structure due to limitations of the available standard techniques. To overcome this limitation in this work we have used Scanning micro X-ray Diffraction (SμXRD) which is a unique non-invasive probe of both k-space and real space allowing to visualize disorder in myelin with high spatial resolution in real space. We have used this method to examine the myelin sheath in the sciatic nerve of Xenopus laevis. Our results open could open new venues for understanding formation and degradation of myelin.

q-bio.NC↗

arXiv2016

2D nano-granularity of the oxygen chains in the YBa2Cu3O6.33 superconductor

The organization of dopants in high temperature superconductors provides complex topological geometries that controls superconducting properties. This makes the study of dopants spatial distribution of fundamental importance. The mobile oxygen ions, y, in the CuO2 plane of YBa2Cu3O6+y (0.33<y<0.67) form ordered chains which greatly affect the transport properties of the material. Here we visualize and characterize the 2D spatial organization of these oxygen chains using scanning micro X-ray diffraction measurements in transmission mode on a thin single crystal slab with y=0.33 (Tc=7 K) near the critical doping for the insulator-to-metal transition. We show the typical landscape of percolation made of a granular spatial pattern due the oxygen chains segregating in quasi-one-dimensional needles of Ortho II (O-II) phase embedded in an insulating matrix with low density of disordered oxygen interstitials

cond-mat.supr-con↗

arXiv2015

High Temperature superconductivity in a hyperbolic geometry of complex matter from nanoscale to mesoscopic scale

While it was known that High Temperature Superconductivity appears in cuprates showing complex multiscale phase separation due to inhomogeneous charge density wave (CDW) order, the spatial distribution of CDW domains remained an open question for a long time, because of the lack of experimental probes able to visualize their spatial distribution at mesoscale, between atomic and macroscopic scale. Recently scanning micro X-ray Diffraction (SmXRD) revealed CDW crystalline electronic puddles with a complex fat-tailed spatial distribution of their size. In this work we have determined and mapped the anisotropy of the Charge Density Waves (CDW) puddles in HgBa2CuO4+y (Hg1201) single crystal. We discuss the emergence of high temperature superconductivity in the interstitial space with hyperbolic geometry, that opens a new paradigm for quantum coherence at high temperature where negative dielectric function and interference between different pathways can help to raise the critical temperature.

cond-mat.supr-con↗

arXiv2015

Inhomogeneity of charge density wave order and quenched disorder in a high Tc superconductor

It has recently been established that the high temperature (high-Tc) superconducting state coexists with short-range charge-density-wave order and quenched disorder arising from dopants and strain. This complex, multiscale phase separation invites the development of theories of high temperature superconductivity that include complexity. The nature of the spatial interplay between charge and dopant order that provides a basis for nanoscale phase separation remains a key open question, because experiments have yet to probe the unknown spatial distribution at both the nanoscale and mescoscale (between atomic and macroscopic scale). Here we report micro X-ray diffraction imaging of the spatial distribution of both the charge-density-wave puddles (domains with only a few wavelengths) and quenched disorder in HgBa2CuO4+y, the single layer cuprate with the highest Tc, 95 kelvin. We found that the charge-density-wave puddles, like the steam bubbles in boiling water, have a fat-tailed size distribution that is typical of self-organization near a critical point. However, the quenched disorder, which arises from oxygen interstitials, has a distribution that is contrary to the usual assumed random, uncorrelated distribution. The interstitials-oxygen-rich domains are spatially anti-correlated with the charge-density-wave domains, leading to a complex emergent geometry of the spatial landscape for superconductivity.

cond-mat.supr-con↗

arXiv2015

Direct observation of nanoscale interface phase in the superconducting chalcogenide K$_{x}$Fe$_{2-y}$Se$_2$ with intrinsic phase separation

We have used scanning micro x-ray diffraction to characterize different phases in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T$_c\sim$32 K), phase separation temperature (T$_{ps}\sim$520 K) and iron-vacancy order temperature (T$_{vo}\sim$580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below $\sim$300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides a clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T$_{ps}$ and T$_c$ is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in K$_{x}$Fe$_{2-y}$Se$_2$.

cond-mat.supr-con↗

arXiv2015

CDW and similarity of the Mott-Insulator-to-Metal transition in cuprates with the gas to liquid-liquid transition in supercooled water

New advances in x-ray diffraction, extended x-ray absorption fine structure EXAFS and x-ray absorption near edge structure XANES using synchrotron radiation have now provided compelling evidence for a short range charge density wave phase (CDW) called striped phase in the CuO2 plane of all cuprate high temperature superconductors. The CDW is associated with a bond order wave (BOW) and an orbital density wave (ODW) forming nanoscale puddles which coexist with superconducting puddles below Tc. The electronic CDW crystalline phase occurs around the hole doping 0.125 between the Mott charge transfer insulator, and the 2D metal. The Van der Waals (VdW) theoretical model for a liquid of anisotropic extended objects proposed for supercooled water is used to describe : a) the underdoped regime as a first spinodal regime of a slightly doped charge transfer Mott insulator puddles coexisting with the striped polaronic CDW puddles; and b) the optimum doping regime as a second spinodal regime where striped polaronic CDW puddles coexist with the normal 2D metal puddles. This complex phase separation with 3 competing phases depends on the strength of the anisotropic electron-phonon interaction that favors the formation striped polaronic CDW phase.

cond-mat.supr-con↗

arXiv2012

Local structure in Mg1-xAlxB2 system by high resolution neutron diffraction

The local structure in high temperature superconductors is nowadays considered a key point for understanding superconductivity mechanism. MgB2 has a well-known simple structure; but its local structure remains quite unexplored. This is due to the fact that typical x-ray local probes, such as EXAFS, fail when used to study local structure of light atoms, such as Mg and B. We used high resolution neutron diffraction with pair distribution function (PDF) analysis for investigating disorder on the atomic scale in the Al doped Mg1-xAlxB2 system. The results indicate an anisotropic structural inhomogeneity along the c-axis that could be related with the delocalized metallic-type bonding between Boron layers.

cond-mat.supr-con↗

arXiv2011

Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused x-ray diffraction

Advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the K0.8Fe1.6Se2 superconducting system using scanning nanofocus single-crystal X-ray diffraction. The results show an intrinsic phase separation in K0.8Fe1.6Se2 single crystals at T< 520 K, revealing coexistence of i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and ii) a non-magnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space-resolved imaging of the phase separation permitted us to provide a direct evidence of nanophase domains smaller than 300 nm and different micrometer-sized regions with percolating magnetic or nonmagnetic domains forming a multiscale complex network of the two phases.

cond-mat.supr-con↗

arXiv2009

A model for liquid-striped liquid phase separation in liquids of anisotropic polarons

The phase separation between a striped polaron liquid at the particular density and a high density polaron liquid is described by a modified Van der Waals scheme. The striped polaron liquid represents the pseudo gap matter or Wigner-like polaron phase at 1/8 doping in cuprate superconductors. The model includes the tendency of pseudo- Jahn-Teller polarons to form anisotropic directional bonds at a preferential volume with the formation of different liquid phases. The model gives the coexistence of a first low density polaron striped liquid and a second high density liquid that appears in cuprate superconductors for doping larger than 1/8. We discuss how the strength of anisotropic bonds controls the variation the phase separation scenarios for complex systems in the presence of a quantum critical point where the phase separation vanishes.

cond-mat.supr-con↗

arXiv2006

Study of temperature dependent atomic correlations in MgB$_{2}$

We have studied the evolution with temperature of the local as well as the average crystal structure of MgB$_2$ using the real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction. We have investigated the correlations of the B-B and B-Mg nearest neighbor pair motion by comparing, in the wide temperature range from T=10 K up to T=600 K, the mean-square displacements (MSD) of single atoms with the mean-square relative displacements (MSRD) obtained from the PDF peak linewidths. The results show that the single atom B and Mg vibrations are mostly decoupled from each other, with a small predominance of positive (in phase) correlation factor for both the B-B and B-Mg pairs. The small positive correlation is almost temperature independent, in contrast with our theoretical calculations; this can be a direct consequence of the strong decay processes of the $E_{2g}$ anharmonic phonons.

cond-mat.supr-con↗

arXiv2004

Sc substitution for Mg in MgB2: effects on Tc and Kohn anomaly

Here we report synthesis and characterization of Mg_{1-x}Sc_{x}B_{2} (0.12 T_{c}>6 K. We find that the Sc doping moves the chemical potential through the 2D/3D electronic topological transition (ETT) in the sigma band where the ``shape resonance" of interband pairing occurs. In the 3D regime beyond the ETT we observe a hardening of the E_{2g} Raman mode with a significant line-width narrowing due to suppression of the Kohn anomaly over the range 0<q<2k_{F}.

cond-mat.supr-con↗

arXiv2001

High Tc superconductivity at a critical strain and charge density in diborides

We report variation of lattice structure in diborides (AB2) with different A atoms to identify the special case of MgB2 showing high Tc superconductivity. High purity MgB2 (Tc~39 K) has been prepared by direct reaction of the elements and the lattice parameters are measured by x-ray diffraction determining the strain e of the B-B distance. The results show that the superconductivity in these intermetallics appear near a critical point in a two variables (strain and charge density) phase diagram at (ec, rc).

cond-mat.supr-con↗

arXiv2001

A Superconductor Made by a Metal Heterostructure at the Atomic Limit Tuned at the "Shape Resonance": MgB2

We have studied the variation of Tc with charge density and lattice parameters in Mg1-xAlxB2 superconducting samples at low Al doping x<8%. We show that high Tc occurs where the chemical potential is tuned at a "superconducting shape resonance" near the energy Ec of the quantum critical point (QCP) for the dimensional transition from 2D to 3D electronic structure in a particular subband of the natural superlattice of metallic atomic boron layers. At the "shape resonance" the electrons pairs see a 2D Fermi surface at EF-w0 and a 3D Fermi surface at EF+wo, where wo is the energy cut off of the pairing interaction. The resonant amplification occurs in a narrow energy range where EF-Ec is in the range of 2wo.

cond-mat.supr-con↗