Superconductivity of 30.4 K and its Reemergence under Pressure in Fe1.11Se Synthesized via Ion-exchange and De-intercalation Reaction

This study reports the successful synthesis of a non-stoichiometric Fe1.11Se single crystal with a record-high superconducting onset temperature of 30.4 K via hydrothermal ion-exchange, which exhibits a unique "V"-shaped pressure evolution and a reemergent superconducting state despite containing 11% interstitial iron that typically suppresses superconductivity.

The Gist

Imagine a material called Iron Selenide (FeSe) as a delicate, multi-layered sandwich. Scientists have long known that this sandwich can conduct electricity with zero resistance (a state called superconductivity) when cooled down, but usually, it only works at a very chilly -265°C (8.5 Kelvin).

The problem? This sandwich is incredibly sensitive. If you accidentally drop just a tiny crumb of extra iron into the filling (about 3%), the whole superconducting effect vanishes. It's like adding a single grain of sand to a perfect cake and ruining the texture.

The "Magic" Recipe

In this study, a team of scientists decided to break the rules. Instead of baking the sandwich at high heat (which usually creates that "bad" extra iron), they used a special hydrothermal ion-exchange recipe. Think of this as a chemical "swap meet" in a pressure cooker filled with hot water.

1. Step 1: They started with a different, pre-made sandwich structure.
2. Step 2: They swapped out the outer layers for something else.
3. Step 3: They carefully removed the "guest" ingredients they added in step 2.

The result? They created a new, slightly "over-stuffed" version of the sandwich, which they call Fe1.11Se. This version has 11% extra iron stuffed between the layers. According to the old rulebook, this should have killed the superconductivity. Instead, it did the opposite: the material started superconducting at -243°C (30.4 K). That is nearly four times hotter than the original version!

The "V" Shape Surprise

The most exciting part of the story happens when the scientists squeezed this new material with physical pressure (like using a giant, microscopic vice).

Usually, when you squeeze these materials, the superconducting temperature goes up in a smooth hill shape (a "dome"). But this new material did something weird:

• The Dip: As they started squeezing, the temperature dropped, hitting a low point at a specific pressure.
• The Rebound: As they squeezed even harder, the temperature shot back up, creating a second, even higher peak.

If you drew a graph of this, it looks like a "V" shape. This behavior is rare and reminds scientists of other complex iron-superconductors that have "guest" molecules trapped inside them. It's as if the material had a "dead zone" in the middle of its pressure range, but then woke up and became super-strong again.

The Mystery of the "Ghost" Magnet

While squeezing the material in that second, high-pressure zone, the scientists noticed a faint signal that looked like magnetism was appearing. This is interesting because, in the original simple version of the material, magnetism and superconductivity usually fight each other. Here, they seem to be hanging out together in a strange new state.

Why Does This Matter?

The scientists believe the extra iron atoms act like beneficial dopants. Instead of being the "bad crumbs" that ruin the cake, these extra iron atoms are actually helping the electrons move more freely, boosting the superconducting power.

They also found that this new material is metastable. Think of it like a snowflake: it's beautiful and strong, but if you warm it up too much (above 400°C), it melts back into the ordinary, weaker version. This tells us that by using clever, non-standard chemical tricks (like their hydrothermal recipe), we can create materials that exist in a "sweet spot" that nature doesn't usually allow.

The Bottom Line

This paper shows that by using a clever chemical "swap" method, scientists can force extra iron into a superconductor where it usually isn't allowed. This creates a material that superconducts at much higher temperatures and behaves in a unique "V-shaped" way when squeezed. It bridges the gap between simple iron-superconductors and complex, high-tech versions, offering a new map for how to build better superconductors in the future.

Technical Summary

Technical Summary: Superconductivity of 30.4 K and its Reemergence under Pressure in Fe1.11Se

Problem Statement
Stoichiometric FeSe (s-FeSe), synthesized via high-temperature equilibrium reactions, is a parent compound for high-temperature unconventional superconductors. However, its superconducting transition temperature ($T_c$) is modest (8.5 K) and extremely sensitive to interstitial iron (Fe). The presence of merely 3% interstitial Fe is sufficient to fully suppress superconductivity in s-FeSe. While non-equilibrium approaches such as chemical intercalation, monolayer deposition, and physical pressure have successfully enhanced $T_c$ in related systems, the specific role of high concentrations of interstitial Fe in non-stoichiometric bulk crystals remains underexplored. Specifically, there is a need to understand how interstitial Fe behaves when its concentration exceeds the equilibrium phase diagram limits and how such a system responds to physical pressure compared to both s-FeSe and FeSe-intercalates.

Methodology
The authors synthesized new non-stoichiometric Fe$_{1+\delta}$Se$_{1-x}$S$_x$ (where $x=0, 0.3, 0.6$) bulk single crystals using a two-step hydrothermal route involving ion-exchange and de-intercalation.

1. Synthesis: High-purity tetragonal FeSe was converted to K$_{0.8}$Fe$_2$Se$_2$, which was then reacted with LiOH to form (Li$_{1-y}$Fe$_y$)OHFeSe. Finally, a de-intercalation reaction using KOH and Fe powder yielded the target Fe$_{1.11}$Se crystals.
2. Structural and Chemical Characterization: Single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) were used to determine crystal structure and phase purity. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES), energy-dispersive X-ray spectroscopy (EDX), and dynamic gas composition analysis (QMS) confirmed elemental ratios and the absence of intercalated species. $^{57}$Fe Mössbauer spectroscopy at 23 K was employed to probe the local environment and valence state of iron atoms.
3. Transport and Magnetic Measurements: Electrical resistivity ($\rho$) and Hall coefficient ($R_H$) were measured at ambient and high pressures (up to ~13.6 GPa) using a diamond anvil cell (DAC). Magnetic properties were measured via SQUID magnetometry.
4. Theoretical Calculations: Density functional theory (DFT) calculations were performed to analyze the Fermi surface evolution and density of states under pressure.

Key Results

• Synthesis and Structure: The study successfully synthesized Fe$_{1.11}$Se single crystals containing 11% interstitial Fe ions (Fe$_2$), a concentration significantly exceeding the solubility limit in the binary Fe-Se phase diagram. The crystals crystallize in the LiFeAs-type structure (space group $P4/nmm$), with interstitial Fe$_2$ ions randomly occupying the 2c site between FeSe layers. Mössbauer spectroscopy confirmed the presence of non-magnetic interstitial Fe$_2$ ions with an effective moment of 4.9 $\mu_B$.
• Ambient Pressure Superconductivity: Unlike s-FeSe, where interstitial Fe suppresses superconductivity, the 11% interstitial Fe in Fe$_{1.11}$Se acts as a benign dopant. The material exhibits a superconducting onset temperature ($T_{c,onset}$) of 30.4 K, comparable to K$_x$Fe$_2$Se$_2$, and a zero-field-cooled transition ($T_{c,zero}$) of 27.1 K. The material displays type-II superconductivity with strong flux pinning.
• Electronic Properties: Hall measurements indicate a single-band model dominated by electrons, with an electron concentration ($n_e$) of $1.28 \times 10^{21}$ cm$^{-3}$, significantly higher than in s-FeSe. The normal-state resistivity follows a non-Fermi-liquid behavior ($\rho \propto T^\alpha$ with $\alpha \approx 1$). The coherence length along the c-axis ($\xi_c$) is remarkably short (2.9 Å), indicating strong anisotropy.
• Pressure-Induced Evolution: Under physical pressure, Fe$_{1.11}$Se exhibits a unique "V"-shaped evolution of $T_c$:
  • SC-I Zone: $T_c$ decreases from 30.4 K to a minimum of 15.4 K at 2.6 GPa.
  • SC-II Zone: Upon further compression, $T_c$ re-enters a superconducting state, reaching a maximum of 31.1 K at 10.6 GPa.
  • Magnetic Transition: Above 5.5 GPa, a weak kink in the resistivity curve suggests a stripe-type antiferromagnetic (S-AFM) transition, which coexists with superconductivity in the SC-II zone. This transition peaks at 35.2 K at 10.6 GPa.
  • Phase Disappearance: Superconductivity and magnetic transitions vanish above 13.6 GPa, coinciding with a structural transition to a non-superconducting NiAs-type hexagonal phase.
• Comparison with Analogues: The pressure-dependent behavior of Fe$_{1.11}$Se resembles that of FeSe-intercalates (which also show V-shaped $T_c$ and re-entrant superconductivity) rather than s-FeSe (which shows a single dome and monotonic $T_c$ increase at low pressure). However, the emergence of a magnetic phase at high pressure in Fe$_{1.11}$Se mirrors the behavior seen in pressurized s-FeSe.

Significance and Claims
The paper claims that the synthesis of Fe$_{1.11}$Se demonstrates that interstitial Fe, typically detrimental in equilibrium s-FeSe, can act as a benign electron dopant that suppresses nematicity and enhances superconductivity when incorporated via non-equilibrium hydrothermal routes. The observation of a "V"-shaped $T_c$ evolution and a re-entrant superconducting phase in a bulk non-stoichiometric crystal bridges the gap between the behaviors of binary s-FeSe and FeSe-intercalates. Furthermore, the coexistence of a pressure-induced magnetic order with superconductivity in the SC-II zone provides new insights into the interplay between magnetism and superconductivity in Fe-based systems. The authors conclude that metastable synthesis strategies offer a viable pathway for discovering high-$T_c$ superconductors and inducing emergent physical properties in layered materials with weak interlayer forces.