arXiv subjects

S. -H. Baek

Publications and source records attributed to S. -H. Baek.

At least 19 recordsLinked to original sources

arXiv2022

Possible quadrupole-order-driven commensurate-incommensurate phase transition in B20 CoGe

The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and $^{59}$Co nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of non-magnetic origin. In addition, we detected a crossover scale $T_0\sim30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the P2$_1$3 symmetry that is commensurate to the R3 symmetry with an incommensurate wavevector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family.

cond-mat.str-el↗

arXiv2018

Nematicity and magnetism in LaFeAsO single crystals probed by $^{75}$As nuclear magnetic resonance

We report a $^{75}$As nuclear magnetic resonance study in LaFeAsO single crystals, which undergoes nematic and antiferromagnetic transitions at $T_\text{nem}\sim 156$ K and $T_N \sim 138$ K, respectively. Below $T_\text{nem}$, the $^{75}$As spectrum splits sharply into two for an external magnetic field parallel to the orthorhombic $a$ or $b$ axis in the FeAs planes. Our analysis of the data demonstrates that the NMR line splitting arises from an electronically driven rotational symmetry breaking. The $^{75}$As spin-lattice relaxation rate as a function of temperature shows that spin fluctuations are strongly enhanced just below $T_\text{nem}$. These NMR findings indicate that nematic order promotes spin fluctuations in magnetically ordered LaFeAsO, as observed in non-magnetic and superconducting FeSe. We conclude that the origin of nematicity is identical in both FeSe and LaFeAsO regardless of whether or not a long range magnetic order develops in the nematic state.

cond-mat.str-el↗

arXiv2017

Low-energy spin dynamics and critical hole concentrations in La$_{2-x}$Sr$_x$CuO$_4$ ($0.07\leq x \leq 0.2$) revealed by $^{139}$La and $^{63}$Cu nuclear magnetic resonance

We report a comprehensive $^{139}$La and $^{63}$Cu nuclear magnetic resonance study on La$_{2-x}$Sr$_x$CuO$_4$ ($0.07\leq x \leq 0.2$) single crystals. The $^{139}$La spin-lattice relaxation rate $^{139}T_1^{-1}$ is drastically influenced by Sr doping $x$ at low temperatures. A detailed field dependence of $^{139}T_1^{-1}$ at $x=1/8$ suggests that charge ordering induces the critical slowing down of spin fluctuations toward glassy spin order and competes with superconductivity. On the other hand, the $^{63}$Cu relaxation rate $^{63}T_1^{-1}$ is well described by a Curie-Weiss law at high temperatures, yielding the Curie-Weiss temperature $Θ$ as a function of doping. $Θ$ changes sharply through a critical hole concentration $x_c\sim 0.09$. $x_c$ appears to correspond to the delocalization limit of doped holes, above which the bulk nature of superconductivity is established.

cond-mat.str-el↗

arXiv2017

Evidence for a Field-induced Quantum Spin Liquid in $α$-RuCl$_3$

We report a $^{35}$Cl nuclear magnetic resonance study in the honeycomb lattice, $α$-RuCl$_3$, a material that has been suggested to potentially realize a Kitaev quantum spin liquid (QSL) ground state. Our results provide direct evidence that $α$-RuCl$_3$ exhibits a magnetic field-induced QSL. For fields larger than $\sim 10$ T a spin-gap opens up while resonance lines remain sharp, evidencing that spins are quantum disordered and locally fluctuating. The spin gap increases linearly with increasing magnetic field, reaching $\sim50$ K at 15 T, and is nearly isotropic with respect to the field direction. The unusual rapid increase of the spin gap with increasing field and its isotropic nature are incompatible with conventional magnetic ordering and in particular exclude that the ground state is a fully polarized ferromagnet. The presence of such a field-induced, gapped QSL phase has indeed been predicted in the Kitaev model.

cond-mat.str-el↗

arXiv2017

Frustration-driven C4 symmetric orders in a hetero-structured iron-based superconductor

A subtle balance between competing interactions in strongly correlated systems can be easily tipped by additional interfacial interactions in a heterostructure. This often induces exotic phases with unprecedented properties, as recently exemplified by high-Tc superconductivity in FeSe monolayer on the nonmagnetic SrTiO3. When the proximity-coupled layer is magnetically active, even richer phase diagrams are expected in iron-based superconductors (FeSCs), which however has not been explored due to the lack of a proper material system. One promising candidate is Sr2VO3FeAs, a naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium oxide. Here, using high-quality single crystals and high-accuracy 75As and 51V nuclear magnetic resonance (NMR) measurements, we show that a novel electronic phase is emerging in the FeAs layer below T0 ~ 155 K without either static magnetism or a crystal symmetry change, which has never been observed in other FeSCs. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order with C4-symmetry in the FeAs layers, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling is effective to stabilize a hidden order in FeSCs and, more generally, in strongly correlated heterostructures.

cond-mat.supr-con↗

arXiv2016

Nematicity and in-plane anisotropy of superconductivity in $β$-FeSe detected by $^{77}$Se nuclear magnetic resonance

The recent study of $^{77}$Se nuclear magnetic resonance (NMR) in a $β$-FeSe single crystal proposed that ferro-orbital order breaks the $90^\circ$ $C_4$ rotational symmetry, driving nematic ordering. Here, we report an NMR study of the impact of small strains generated by gluing on nematic state and spin fluctuations. We observe that the local strains strongly affect the nematic transition, considerably enhancing its onset temperature. On the contrary, no effect on low-energy spin fluctuations was found. Furthermore we investigate the interplay of the nematic phase and superconductivity. Our study demonstrates that the twinned nematic domains respond unequivalently to superconductivity, evidencing the twofold $C_2$ symmetry of superconductivity in this material. The obtained results are well understood in terms of the proposed ferro-orbital order.

cond-mat.supr-con↗

arXiv2015

Influence of hydrostatic pressure on the bulk magnetic properties of Eu$_{2}$Ir$_{2}$O$_{7}$

We report on the magnetic properties of Eu$_{2}$Ir$_{2}$O$_{7}$ upon the application of hydrostatic pressure $P$ by means of macroscopic and local-probe techniques. In contrast to previously reported resistivity measurements, our dc magnetization data unambiguously demonstrate a non-monotonic $P$-dependence for $T_{N}$, i.~e., the critical transition temperature to the magnetic phase. More strikingly, we closely reproduce the recently calculated behaviour for $T_{N}$ under the assumption that $P$ lowers the $U/W$ ratio (i.~e., Coulomb repulsion energy over electronic bandwidth). Zero-field muon-spin spectroscopy measurements confirm that the local magnetic configuration is only weakly perturbed by low $P$ values, in agreement with theoretical predictions. The current results experimentally support the preservation of a $4$-in/$4$-out ground state and, simultaneously, a departure from the single-band $j_{\text{eff}} = 1/2$ model across the accessed region of the phase diagram.

cond-mat.str-el↗

arXiv2015

Magnetic field induced anisotropy of $^{139}\mathrm{La}$ spin-lattice relaxation rates in stripe ordered ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_{4}$

We report $^{139}$La nuclear magnetic resonance studies performed on a La$_{1.875}$Ba$_{0.125}$CuO$_4$ single crystal. The data show that the structural phase transitions (high-temperature tetragonal $\rightarrow$ low-temperature orthorhombic $\rightarrow$ low-temperature tetragonal phase) are of the displacive type in this material. The $^{139}$La spin-lattice relaxation rate $T_1^{-1}$ sharply upturns at the charge-ordering temperature $T_\text{CO}$ = 54 K, indicating that charge order triggers the slowing down of spin fluctuations. Detailed temperature and field dependencies of the $T_1^{-1}$ below the spin-ordering temperature $T_\text{SO}$ = 40 K reveal the development of enhanced spin fluctuations in the spin-ordered state for $H \parallel [001]$, which are completely suppressed for large fields along the CuO$_2$ planes. Our results shed light on the unusual spin fluctuations in the charge and spin stripe ordered lanthanum cuprates.

cond-mat.str-el↗

arXiv2014

Orbital-driven nematicity in FeSe

A very fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of {\it two} other instabilities. Apart from a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here we report a very clear splitting of NMR resonance lines in FeSe at $T_{nem}$ = 91K, far above superconducting $T_c$ of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe-planes and has the temperature dependence of a Landau-type order-parameter. Spin-lattice relaxation rates are not affected at $T_{nem}$, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.

cond-mat.supr-con↗

arXiv2014

Inhomogeneous slowing down of spin fluctuations induced by charge stripe order in 1/8-doped lanthanum cuprates

We report $^{139}$La nuclear magnetic resonance (NMR) measurements on La$_{2-x}$Sr$_x$CuO$_4$ ($0.07\leq x \leq 0.15$) and La$_{2-x}$Ba$_x$CuO$_4$ ($x=1/8$) single crystals, focusing on the spin freezing observed in 1/8-doped lanthanum cuprates. Charge stripe order seems to induce the inhomogeneous slowing down of spin fluctuations toward spin order and compete with superconductivity.

cond-mat.supr-con↗

arXiv2014

Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO$_4$ detected by $^{31}$P and $^7$Li nuclear magnetic resonance

We report $^{31}$P and $^{7}$Li nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO$_4$-type LiCoPO$_4^\text{tetra}$ microcrystals, where the Co$^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO$_4$, which was directly transformed from LiCoPO$_4^\text{tetra}$ by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the $^{31}$P Knight shift obey the Curie-Weiss law for both materials with a high spin Co$^{2+}$ ($3d^7$, $S=3/2$), but the Weiss temperature $Θ$ and the effective magnetic moment $μ_\text{eff}$ are considerably smaller in LiCoPO$_4^\text{tetra}$. The spin-lattice relaxation rate $T_1^{-1}$ reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO$_4$, which may lead to the incommensurate magnetic ordering.

cond-mat.str-el↗

arXiv2013

Detection of shorter-than-skin-depth acoustic pulses in a metal film via transient reflectivity

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO$_{3}$ layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.

cond-mat.mtrl-sci↗

arXiv2013

Structural transitions in a doped lanthanum cuprate

139La NMR and relaxation measurements have been performed on La{1.8-x}Eu{0.2}Sr{x}CuO{4} (x = 0.13 and 0.2) single crystals. The temperature dependence of the 139La NMR spectra in all the structural phases (HTT -> LTO -> LTT) reveals the non-vanishing tilt angle of the CuO6 octahedra in the HTT phase, opposed to the case of La{2-x}Sr{x}CuO{4} where the tilt angle disappears immediately above the transition. Since 139La relaxation data provide evidence of the thermodynamic critical fluctuations associated with the structural phase transitions, HTT -> LTO and LTO -> LTT, we conclude that the structural transitions in Eu-doped La{2-x}Sr{x}CuO{4} should be of the order-disorder type rather than of the displacive type observed in La{2-x}Sr{x}CuO{4}. The change of the nature of the structural transitions caused by doping Eu appears to be consistent with the LTO -> LTT transition that is absent in La{2-x}Sr{x}CuO{4}.

cond-mat.str-el↗

arXiv2013

Anomalous superconducting state in LiFeAs implied by the $^{75}$As Knight shift measurement

$^{75}$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.

cond-mat.supr-con↗

arXiv2012

Evidence of a critical hole concentration in underdoped YBa$_2$Cu$_3$O$_y$ single crystals revealed by $^{63}$Cu NMR

We report a $^{63}$Cu NMR investigation in detwinned YBa$_2$Cu$_3$O$_y$ single crystals, focusing on the highly underdoped regime (y=6.35-6.6). Measurements of both the spectra and the spin-lattice relaxation rates of $^{63}$Cu uncover the emergence of static order at a well-defined onset temperature $T_0$ without a known order parameter as yet. While $T_0$ is rapidly suppressed with increasing hole doping concentration $p$, the spin pseudogap was identified only near and above the doping content at which $T_0\rightarrow 0$. Our data indicate the presence of a critical hole doping $p_c\sim 0.1$, which may control both the static order at $p p_c$.

cond-mat.str-el↗

arXiv2012

Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te NMR

We present high-field magnetization and $^{79}$Br nuclear quadrupole resonance (NQR) and $^{125}$Te nuclear magnetic resonance (NMR) studies in the weakly coupled Cu$^{2+}$ ($S=1/2$) tetrahedral system Cu$_2$Te$_2$O$_5$Br$_2$. The field-induced level crossing effects were observed by the magnetization measurements in a long-ranged magnetically ordered state which was confirmed by a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much larger than that observed by static bulk measurements. Our results imply that the inter- and the intra-tetrahedral interactions compete, but at the same time they cooperate strengthening effectively the local intratetrahedral exchange couplings. We discuss that the unusual feature originates from the frustrated intertetrahedral interactions.

cond-mat.str-el↗

arXiv2012

$^{139}$La NMR investigation in underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$

We report $^{139}$La and $^{63}$Cu nuclear magnetic and quadrupole resonance (NMR/NQR) studies in an underdoped La$_{1.93}$Sr$_{0.07}$CuO$_4$ single crystal, focusing on the $^{139}$La NMR in the normal state. We demonstrate that the local structural distortions in the low-temperature orthorhombic structure cause the tilting of the direction of the electric field gradient (EFG) at the nuclei from the c axis, resulting in two NMR central transition spectra at both the $^{139}$La and $^{63}$Cu nuclei in an external field. Taking into account the tilt angle of the EFG, the temperature dependence of the $^{139}$La spectra allowed us to determine the $^{139}$La Knight shift and the structural order parameter. The angle and temperature dependence of the $^{139}$La spectrum is in perfect agreement with the macroscopic average structure and proves a displacive transition. The $^{139}$La nuclear spin-lattice relaxation rates, $T_1^{-1}$, suggest that La$_{1.93}$Sr$_{0.07}$CuO$_4$ undergoes a gradual change to a temperature-independent paramagnetic regime in the high temperature region. Both the spectra and $T_1^{-1}$ of the $^{139}$La as a function of temperature reveal a sharp anomaly around $T_S=387(1)$ K, implying a first-order-like structural transition, and a dramatic change below ~70 K arising from collective glassy spin freezing.

cond-mat.supr-con↗

arXiv2011

Pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by $^{75}$As NQR

We report $^{75}$As NQR measurements on single crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x \leq 0.09$). The nuclear spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant concentration $x$ reveals a normal-state pseudogap-like phase below a crossover temperature $T^*$ in the under- and optimally-doped region. 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.

cond-mat.supr-con↗