arXiv subjects

Niels Bech Christensen

Publications and source records attributed to Niels Bech Christensen.

6 recordsLinked to original sources

arXiv2026

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes.

cond-mat.str-el↗

arXiv2022

Gradual emergence of superconductivity in underdoped LSCO

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2022

Crystal Symmetry of Stripe Ordered La1.88Sr0.12CuO4

We present a combined x-ray and neutron diffraction study of the stripe ordered superconductor \lscox{0.12}. The average crystal structure is consistent with the orthorhombic $Bmab$ space group as commonly reported in the literature. This structure however is not symmetry compatible with a second order phase transition into the stripe order phase, and, as we report here numerous Bragg peaks forbidden in the $Bmab$ space group are observed. We have studied and analysed these $Bmab$-forbidden Bragg reflections. Fitting of the diffraction intensities yields monoclinic lattice distortions that are symmetry consistent with charge stripe order.

cond-mat.supr-con↗

arXiv2019

Magnetic structures and quadratic magnetoelectric effect in LiNiPO$_4$ beyond 30T

Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in LiNiPO$_4$ up to 25T and 42T, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to 56T applied along the easy $c$-axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4 and 54T. Ordering vectors are identified with ${\bf Q}_{\mathrm{VI}}$ = (0, 1/3, 0) in the interval 37.6--39.4T and ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) in the interval 39.4-54T. A quadratic magnetoelectric effect is discovered in the ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) phase and the field-dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in LiNiPO4.

cond-mat.str-el↗

arXiv2019

Dzyaloshinskii-Moriya interaction and the magnetic ground state in magnetoelectric LiCoPO$_4$

Magnetic structures are investigated by means of neutron diffraction to shine a light on the intricate details that are believed to be key to understanding the magnetoelectric effect in LiCoPO$_4$ . At zero field, a spontaneous spin canting of $φ= 7(1)^{\circ}$ is found. The spins tilt away from the easy $b$-axis toward $c$. Symmetry considerations lead to the magnetic point group $m'_z$, which is consistent with the previously observed magnetoelectric tensor form and weak ferromagnetic moment along $b$. For magnetic fields applied along $a$, the induced ferromagnetic moment couples via the Dzyaloshinskii-Moriya interaction to yield an additional field-induced spin canting. An upper limit to the size of the interaction is estimated from the canting angle.

cond-mat.str-el↗

arXiv2017

Magnetic order, hysteresis and phase coexistence in magnetoelectric LiCoPO$_4$

The magnetic phase diagram of magnetoelectric LiCoPO$_4$ is established using neutron diffraction and magnetometry in fields up to 25.9T applied along the crystallographic $b$-axis. For fields greater than 11.9T the magnetic unit cell triples in size with propagation vector Q = (0, 1/3, 0). A magnetized elliptic cycloid is formed with spins in the $(b,c)$-plane and the major axis oriented along $b$. Such a structure allows for the magnetoelectric effect with an electric polarization along $c$ induced by magnetic fields applied along $b$. Intriguingly, additional ordering vectors Q $\approx$ (0, 1/4, 0) and Q $\approx$ (0, 1/2, 0) appear for increasing fields in the hysteresis region below the transition field. Traces of this behavior are also observed in the magnetization. A simple model based on a mean-field approach is proposed to explain these additional ordering vectors. In the field interval 20.5-21.0T, the propagation vector Q = (0, 1/3, 0) remains but the spins orient differently compared to the cycloid phase. Above 21.0T and up until saturation a commensurate magnetic structure exists with a ferromagnetic component along $b$ and an antiferromagnetic component along $c$.

cond-mat.str-el↗